Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Display Method:
Three-dimensional characteristics of mesoscale eddies simulated by a regional model in the northwestern Pacific Ocean during 2000–2008
Guijing Yang, Xiayan Lin, Guoqing Han, Yu Liu, Gengxin Chen, Jianhui Wang
 doi: 10.1007/s13131-022-2006-4
[Abstract](0) [FullText HTML](0)
Mesoscale eddies play vital roles in ocean processes. Although previous studies focused on eddy surface features and individual three-dimensional (3D) eddy cases in the northwestern Pacific Ocean, the analysis of unique eddy 3D regional characteristics is still lacking. A 3D eddy detection scheme is applied to 9 years (2000–2008) of eddy-resolving Regional Ocean Modeling System (ROMS) output to obtain a 3D eddy dataset from the surface to a depth of 1 000 m in the northwestern Pacific Ocean (15–35°N and 120–145°E). The 3D characteristics of mesoscale eddies are analyzed in two regions, namely, Box1 (Subtropical countercurrent, 15–25°N, and 120–145°E) and Box2 (Southern Kuroshio Extension, 25–35°N, and 120–145°E). In Box1, the current is characterized by strong vertical shear and weak horizontal shear. In Box2, the current is characterized by the strong Kuroshio, topographic effect, and the westward propagation of Rossby waves. The results indicate the importance of baroclinic instability in Box1, whereas in Box2, both the barotropic and baroclinic instability are important. Moreover, the mesoscale eddies’ properties in Box1 and Box2 are distinct. The eddies in Box1 have larger number and radius but a shorter lifetime. By contrast, Box2 has fewer eddies, which have smaller radius but longer lifetime. Vertically, more eddies are detected at the subsurface than at the surface in both regions; the depth of 650 m is the turning point in Box1. Above this depth, the number of cyclonic eddy (CE)s is larger than that of anticyclonic eddy (AE)s. In Box2, the number of CEs is dominant vertically. Eddy kinetic energy (EKE) and mean normalized relative vorticity in Box2 are significantly higher than those in Box1. With increasing depth, the attenuation trend of EKE and relative vorticity of Box1 become greater than those of Box2. Furthermore, the upper ocean (about 300 m in depth) contains 68.6% of the eddies (instantaneous eddy). Only 16.6% of the eddies extend to 1 000 m. In addition, more than 87% of the eddies are bowl-shaped eddies in the two regions. Only about 3% are cone-shaped eddies. With increasing depth of the eddies, the proportion of bowl-shaped eddies gradually decreases. Conversely, the cone- and lens-shaped eddies are equal in number at 700–1 000 m, accounting for about 30% each. Studying the 3D characteristics of eddies in two different regions of the northwestern Pacific Ocean is an important stepping stone for discussing the different eddy generation mechanisms.
Tide-induced Lagrangian residual velocity and dynamic analysis based on field observations in the inner Xiangshan Bay
Xiaoxuan Sheng, Qi Quan, Jinzhen Yu, Xinyan Mao, Wensheng Jiang
 doi: 10.1007/s13131-022-2007-3
[Abstract](11) [FullText HTML](1)
In the Xiangshan Bay at the east coast of China, coastal marine pollution is conspicuous and severe in recent years. As transport of the pollutants is closely related to the coastal circulation, there is a great practical significance to investigate the circulation in this area. In this work, the surface pattern and vertical profiles of Lagrangian residual velocity (LRV) were studied based on field observation data from the inner Xiangshan Bay. By tracking GPS-GPRS drifters’ trajectories, the surface LRV pattern is going out in the central deep trough and flowing inwards near the shoreside. Combined with data from two mooring stations, vertical profiles of LRV is flowing out at surface and flowing in at the bottom, consistent with the gravitational circulation induced by baroclinic effects at the estuary. However, according to the diagnostic analysis, the main mechanism driving the residual current is barotropic rather than baroclinic. The LRV equation is controlled by the tidally-averaged barotropic pressure gradient force, tidal body force and tidally-averaged turbulent stress, while the tidally-averaged baroclinic pressure gradient force is one order of magnitude less than other forces. Additionally, the tidally mean eddy viscosity coefficient which is used in the expression of tidally-averaged turbulent stress might be not adequate and requires further studies.
Long-term nutrient variation trends and their potential impact on phytoplankton in the Southern Yellow Sea, China
Yan Wang, Yongjian Liu, Hao Guo, Haibo Zhang, Dongmei Li, Ziwei Yao, Xiaocheng Wang, Chuan Jia
 doi: 10.1007/s13131-022-2031-3
[Abstract](15) [FullText HTML](2)
The concentration and composition of nutrients, such as N, P, and Si, respond to biogeochemical processes and in turn, impact the phytoplanktons’ community structure and primary production. In this study, historical data was systematically analyzed to identify long-term variations in nutrient trends, red tide frequency, phytoplankton community abundance, and dominant species succession in the Southern Yellow Sea (SYS). Results showed that N/P ratios dramatically increased as a function of increasing dissolved inorganic nitrogen (DIN) concentrations, and Si/N ratios were generally > 1, indicating that N limitation morphed to P limitation and potentially to Si limitation, which impacted the phytoplankton community. Furthermore, inter-annual trends over the past 50 years show that phytoplankton community abundance has been higher in spring and summer, relative to autumn and winter. Moreover, with respect to red tide frequency, diatom abundance gradually decreased, while that of dinoflagellates gradually increased. Dominant species succession showed that the phytoplankton community exhibited an evident tendency to transform from diatoms to dinoflagellates. These research results clearly depict the presence of an important correlation between the phytoplankton community and nutrient structure in the SYS.
Vertical multiple-layer structure of temperature and turbulent diffusivity in the South China Sea
Xin He, Changrong Liang, Yang Yang, Guiying Chen, Xiaodong Shang, Xiaozhou He, Penger Tong
 doi: 10.1007/s13131-022-2005-5
[Abstract](16) [FullText HTML](2)
We report field measurements of vertical profiles of the turbulent diffusivity and temperature at different stations in the South China Sea (SCS). Our study shows that the measured turbulent diffusivity follows a power-law distribution with a varying exponent in water layers. Similar multiple-layer scaling regimes were also observed from the temperature fluctuations. Combining turbulent diffusivity and temperature fluctuations, the vertical structure of temperature was revealed. Furthermore, we discuss the temperature profiles in each layer. A constant function of a dimensionless temperature profile was found in water layers that have identical turbulence conditions. Our results reveal the multiple-layer structure of temperature in the SCS. This study contributes to the understanding of the vertical structure of multiple layers in the SCS and provides clues for exploring the physical mechanism for maintaining the temperature structure.
Rapid environmental assessment in the South China Sea: Improved inversion of sound speed profile using remote sensing data
Ke Qu, Binbin Zou, Jianbo Zhou
 doi: 10.1007/s13131-022-2032-2
[Abstract](26) [FullText HTML](5)
Complex perturbations in the profile and the sparsity of samples often limit the validity of rapid environmental assessment (REA) in the South China Sea (SCS). In this paper, the remote sensing data were used to estimated sound speed profile (SSP) with the self-organizing map (SOM) method in the SCS. First, the consistency of the empirical orthogonal functions was examined by using k-means clustering. The clustering results indicated that SSPs in the SCS have a similar perturbation nature, which means the inverted grid could be expanded to the entire SCS to deal with the problem of sparsity of the samples without statistical improbability. Second, a machine learning method was proposed that took advantage of the topological structure of SOM to significantly improve their accuracy. Validation revealed promising results, with a mean reconstruction error of 1.26 m/s, which is 1.16 m/s smaller than the traditional single empirical orthogonal function regression (sEOF-r) method. By violating the constraints of linear inversion, the topological structure of the SOM method showed a smaller error and better robustness in the SSP estimation. The improvements to enhance the accuracy and robustness of REA in the SCS were offered. These results suggested a potential utilization of REA in the SCS based on satellite data and provided a new approach for SSP estimation derived from sea surface data.
Circulation in the South China Sea is in a state of forced oscillation: Results from a simple reduced gravity model with a closed boundary
Ruixin Huang, Hui Zhou
 doi: 10.1007/s13131-022-2013-5
[Abstract](56) [FullText HTML](11)
The South China Sea (SCS) is a narrow semi-enclosed basin, ranging from 4–6°N to 21–22°N meridionally. It is forced by a strong annual cycle of monsoon-related wind stress. The Coriolis parameter f increases at least three times from the southern basin to the northern basin. As a result, the basin-cross time for the first baroclinic Rossby wave in the southern part of the basin is about 10-times faster than that in the northern part, which plays the most vitally important role in setting the circulation. At the northernmost edge of SCS, the first baroclinic Rossby wave takes slightly less than one year to move across the basin, however, it takes only 1–2 months in the southernmost part. Therefore, circulation properties for a station in the model ocean are not solely determined by the forcing at that time instance only; instead, they depend on the information over the past months. The combination of a strong annual cycle of wind forcing and large difference of basin-cross time for the first baroclinic Rossby wave leads to a strong seasonal cycle of the circulation in the SCS, hence, the circulation is dominated by the forced oscillations, rather than the quasi-steady state discussed in many textbooks.The circulation in SCS is explored in detail by using a simple reduced gravity model forced by seasonally varying zonal wind stress. In particular, for a given time snap the western boundary current in the SCS cannot play the role of balancing mass transport across each latitude nor balancing mechanical energy and vorticity in the whole basin. In a departure from the steady wind-driven circulation discussed in many existing textbooks, the circulation in SCS is characterized by the imbalance of mechanical energy and vorticity for the whole basin at any part of the seasonal cycle. In particular, the western boundary current in SCS cannot balance the mass, mechanical energy, and vorticity in the seasonal cycle of the basin. Consequently, the circulation near the western boundary cannot be interpreted in terms of the wind stress and thermohaline forcing at the same time. Instead, circulation properties near the western boundary should be interpreted in terms of the contributions due to the delayed wind stress and the eastern boundary layer thickness. In fact, there is a clear annual cycle of net imbalance of mechanical energy and vorticity source/sink. Results from such a simple model may have important implications for our understanding of the complicated phenomena in SCS, either from in-situ observations or numerical simulations.
Spatial patterns and environmental associations of deep scattering layers in the northwestern subtropical Pacific Ocean
Yuhang Song, Juan Yang, Chunsheng Wang, Dong Sun
 doi: 10.1007/s13131-021-1973-1
[Abstract](121) [FullText HTML](40) [PDF 17385KB](4)
The mesopelagic communities are important for food web and carbon pump in ocean, but the large-scale studies of them are still limited until now because of the difficulties on sampling and analyzing of mesopelagic organisms. Mesopelagic organisms, especially micronekton, can form acoustic deep scattering layers (DSLs) and DSLs are widely observed. To explore the spatial patterns of DSLs and their possible influencing factors, the DSLs during daytime (10:00–14:00) were investigated in the subtropical northwestern Pacific Ocean (13°–23.5°N, 153°–163°E) using a shipboard acoustic Doppler current profiler at 38 kHz. The study area was divided into three parts using k-means cluster analysis: the northern part (NP, 22°–24°N), the central part (CP, 17°–22°N), and the southern part (SP, 12°–17°N). The characteristics of DSLs varied widely with latitudinal gradient. Deepest core DSLs (523.5 m±17.4 m), largest nautical area scattering coefficient (NASC) (130.8 m2/nmi2±41.0 m2/n mile2), and most concentrated DSLs (MGL, 6.7%±0.7%) were observed in NP. The proportion of migration was also stronger in NP (39.7%) than those in other parts (18.6% in CP and 21.5% in SP) for mesopelagic organisms. The latitudinal variation of DSLs was probably caused by changes in oxygen concentration and light intensity of mesopelagic zones. A positive relationship between NASC and primary productivity was identified. A four-months lag was seemed to exist. This study provides the first basin-scale baselines information of mesopelagic communities in the northwest Pacific with acoustic approach. Further researches are suggested to gain understandings of seasonal and annual variations of DSLs in the region.
Characteristics and influencing factors of frontal upwelling in the Yellow Sea in summer
Fan Sun, Fei Yu, Guangcheng Si, Jianfeng Wang, Anqi Xu, Jun Pan, Ying Tang
 doi: 10.1007/s13131-021-1967-z
[Abstract](47) [FullText HTML](10) [PDF 10433KB](2)
Frontal upwelling is an important phenomenon in summer in the Yellow Sea (YS) and plays an essential role in the distribution of nutrients and biological species. In this paper, a three-dimensional hydrodynamic model is applied to investigate the characteristics and influencing factors of frontal upwelling in the YS. The results show that the strength and distribution of frontal upwelling are largely dependent on the topography and bottom temperature fronts. The frontal upwelling in the YS is stronger and narrower near the eastern coast than near the western coast due to the steeper shelf slope. Moreover, external forcings, such as the meridional wind speed and air temperature in summer and the air temperature in the preceding winter and spring, have certain influences on the strength of frontal upwelling. An increase in air temperature in the previous winter and spring weakens the frontal upwelling in summer; in contrast, an increase in air temperature in summer strengthens the frontal upwelling. When the southerly wind in summer increases, the upwelling intensifies in the western YS and weakens in the eastern YS. The air temperature influences the strength of upwelling by changing the baroclinicity in the frontal region. Furthermore, the meridional wind speed in summer affects frontal upwelling via Ekman pumping.
Automated multi-scale classification of the terrain units of the Jiaxie Guyots and their mineral resource characteristics
Yong Yang, Gaowen He, Yonggang Liu, Jinfeng Ma, Zhenquan Wei, Binbin Guo
 doi: 10.1007/s13131-021-1981-1
[Abstract](42) [FullText HTML](6)
Given the advances in satellite altimetry and multibeam bathymetry, benthic terrain classification based on digital bathymetric models (DBMs) has been widely used for the mapping of benthic topographies. For instance, Cobalt-rich crusts (CRCs) are important mineral resources found on seamounts and guyots in the western Pacific Ocean. Thick, plate-like CRCs are known to form on the summit and slopes of seamounts at the 1 000–3 000 m depth, while the relationship between seamount topography and spatial distribution of CRCs remains unclear. The benthic terrain classification of seamounts can solve this problem, thereby, facilitating the rapid exploration of seamount CRCs. Our study used an EM122 multibeam echosounder to retrieve high-resolution bathymetry parameters in the CRCs contract license area of China, i.e., the Jiaxie Guyots in 2015 and 2016. The DBM, broad- and fine-scale bathymetric position indices were utilized for quantitative classification of the terrain units of the Jiaxie Guyots on multiple scales. The classification revealed four first-order terrain units (e.g., flat, crest, slope, and depression) and eleven second-order terrain units (e.g., local crests, depressions on crests, gentle slopes, crests on slopes, and local depressions, etc.). Furthermore, the classification of the terrain and geological analysis indicated that the Weijia Guyot has a large flat summit, with local crests at the southern summit, whereas most of the guyot flanks were covered by gentle slopes. “Radial” mountain ridges have developed on the eastern side, while large-scale gravitational landslides have developed on the western and southern flanks. Additionally, landslide masses can be observed at the bottom of these slopes. The coverage of local crests on the seamount is ~1 000 km2, and the local crests on the peak and flanks of the guyots may be the areas where thick and continuous plate-like CRCs are likely to occur.
Statistics of underwater ambient noise at high sea states arisen from typhoon out zones in the Philippine Sea and South China Sea
Qiulong Yang, Kunde Yang, Shunli Duan, Yuanliang Ma
 doi: 10.1007/s13131-022-1991-7
[Abstract](81) [FullText HTML](22)
Oceanic noise is the background interference in sonar performance prediction and evaluation at high sea states. Statistics of underwater ambient noise during typhoon Soulik and Nida were analyzed on the basis of experimental measurements conducted in a deep area of the Philippine Sea and the South China Sea. Generated linear regression, frequency correlation matrix (FCM), Burr distribution and Gumbel distribution were described for the analysis of correlation with environmental parameters including wind speed (WS), significant wave height (SWH), and the inter-frequency relationship and probability density function of noise levels (NLs). When the typhoons were quite close to the receivers, the increment of NLs exceeded 10 dB. Whilst ambient noise was completely dominated by wind agitation, NLs were proportional to the cubic and quintic functions of WS and SWH, respectively. The fitted results between NLs and oceanic parameters were different for “before typhoon” and “after typhoon”. The fitted slopes of linear regression showed a linear relationship with the logarithm of frequency. The average observed typhoon-generated NLs were 5 dB lower than the Wenz curve at the same wind force due to the insufficiently developed sea state or the delay between NLs and WS. The cross-correlation coefficient of FCM, which can be utilized in the identification of noise sources in different bands, exceeded 0.8 at frequencies higher than 250 Hz. Furthermore, standard deviation increased with frequency. The kurtosis was equal to 3 at >400 Hz approximately. The characteristics of NLs showed good agreement with the results of FCM.
FIO-ESM v2.0 CORE2-forced experiment for the CMIP6 Ocean Model Intercomparison Project
Qi Shu, Zhenya Song, Ying Bao, Xiaodan Yang, Yajuan Song, Xinfang Li, Meng Wei, Fangli Qiao
 doi: 10.1007/s13131-022-2000-x
[Abstract](11) [FullText HTML](2)
In this study, we introduced the Coupled Model Intercomparison Project Phase 6 (CMIP6) Ocean Model Intercomparison Project CORE2-forced (OMIP-1) experiment by using the First Institute of Oceanography Earth System Model version 2.0 (FIO-ESM v2.0), and comprehensively evaluated the simulation results. Unlike other OMIP models, FIO-ESM v2.0 includes a coupled ocean surface wave component model that takes into account non-breaking surface wave-induced vertical mixing in the ocean and effect of surface wave Stokes drift on air-sea momentum and heat fluxes in the climate system. A sub-layer sea surface temperature (SST) diurnal cycle parameterization was also employed to take into account effect of SST diurnal cycle on air-sea heat fluxes to improve simulations of air-sea interactions. Evaluations show that mean values and long-term trends of significant wave height were adequately reproduced in the FIO-ESM v2.0 OMIP-1 simulations, and there is a reasonable fit between the SST diurnal cycle obtained from in situ observations and that parameterized by FIO-ESM v2.0. Evaluation of model drift, temperature, salinity, mixed layer depth, and the Atlantic Meridional Overturning Circulation shows that the model performs well in the FIO-ESM v2.0 OMIP-1 simulation. However, the summer sea ice extent of the Arctic and Antarctic is underestimated.
Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea
Ruixi Zheng, Zhiyou Jing
 doi: 10.1007/s13131-021-1971-3
[Abstract](20) [FullText HTML](5)
Submesoscale activity in the upper ocean has received intense study through simulations and observations in the last decade, but in the eddy-active South China Sea (SCS) the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood. This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution (~500 m) simulation. The satellite images show that the elongated highly productive stripes with a typical lateral scale of ~25 km and associated filaments are frequently observed at the periphery of mesoscale eddies. The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching (100 m/d) due to submesoscale instabilities. The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m2. This result implies a significant submesoscale-enhanced vertical exchange between the surface and ocean interior in the filaments. Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity. The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy, but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence. In this context, these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy, as well as enhanced vertical heat transport.
Objective array design for three-dimensional temperature and salinity observation: Application to the South China Sea
Mengxue Qu, Zexun Wei, Yanfeng Wang, Yonggang Wang, Tengfei Xu
 doi: 10.1007/s13131-021-1975-z
[Abstract](14) [FullText HTML](2)
In this study, a moored array optimization tool (MAOT) was developed and applied to the South China Sea (SCS) with a focus on three-dimensional temperature and salinity observations. Application of the MAOT involves two steps: (1) deriving a set of optimal arrays that are independent of each other for different variables at different depths based on an empirical orthogonal function method, and (2) consolidating these arrays using a K-center clustering algorithm. Compared with the assumed initial array consisting of 17 mooring sites located on a 3°×3° horizontal grid, the consolidated array improved the observing ability for three-dimensional temperature and salinity in the SCS with optimization efficiencies of 19.03% and 21.38%, respectively. Experiments with an increased number of moored sites showed that the most cost-effective option is a total of 20 moorings, improving the observing ability with optimization efficiencies up to 26.54% for temperature and 27.25% for salinity. The design of an objective array relies on the ocean phenomenon of interest and its spatial and temporal scales. In this study, we focus on basin-scale variations in temperature and salinity in the SCS, and thus our consolidated array may not well resolve mesoscale processes. The MAOT can be extended to include other variables and multi-scale variability and can be applied to other regions.
Distribution and invasion of Spartina alterniflora within the Jiaozhou Bay monitored by remote sensing image
Jianbu Wang, Zhaoyang Lin, Yuanqing Ma, Guangbo Ren, Zijun Xu, Xiukai Song, Yi Ma, Andong Wang, Yajie Zhao
 doi: 10.1007/s13131-021-1907-y
[Abstract](32) [FullText HTML](5)
Spartina alterniflora as an alien invasive plant, poses a serious threat to the ecological functions of the coastal wetland of the Jiaozhou Bay. As of 2019, the distribution area of S. alterniflora in the Jiaozhou Bay has reached more than 500 hm2. For this reason, combined with field surveys, remote sensing monitoring of the invasion S. alterniflora in the Jiaozhou Bay has been carried out. To accurately identify S. alterniflora within the Jiaozhou Bay coastal wetland, we used a new method which is an implement of deep convolutional neural network, and by which we got a higher accuracy than the traditional method. Based on distribution of S. alterniflora extracted by the proposed method, the temporal and spatial distribution characteristics of S. alterniflora were analyzed. And then combined with environmental factors, the invasion mechanism of S. alterniflora in the Jiaozhou Bay was analyzed in detail. From the monitoring results, it can be seen that S. alterniflora in Jiaozhou Bay is mainly distributed in the beaches near the Yang River Estuary and its southern side, the Dagu River Estuary and the Nvgukou. S. alterniflora first broke out near the Yang River Estuary and gradually spread to the tidal flats near the Nvgukou. The Dagu River Estuary is dominated by Spartina anglica, whose area has not changed much over the years, and a small amount of S. alterniflora has invaded later.
Estimating seasonal habitat suitability for migratory species in the Bohai Sea and Yellow Sea: a case study of Tanaka's snailfish (Liparis tanakae)
Yunlong Chen, Xiujuan Shan, Dingyong Zeng, Harry Gorfine, Yinfeng Xu, Qiang Wu, Tao Yang, Xianshi Jin
 doi: 10.1007/s13131-021-1912-1
[Abstract](15) [FullText HTML](3)
Acquiring a comprehensive and accurate understanding of habitat preference is essential for species conservation and fishery management, especially for mobile species that migrate seasonally. Presence and absence data from field surveys are recommended when available due to their high reliability. Using field survey data, we investigated seasonal habitat suitability requirements for Tanaka's snailfish (Liparis tanakae) in the Bohai Sea and Yellow Sea (BSYS) via a machine-learning, random forests (RFs) method. Five environmental, biologically relevant variables (bottom temperature, bottom salinity, current velocity, depth and primary productivity) were used to build the ecological niches between the presence/absence data and suitable habitat. In addition, the degree to which false absence data might impact model performance was evaluated. Our results indicated that RFs provided accurate predictions, with seasonal habitat suitability maps of L. tanakae differing substantially. Bottom temperature and salinity were identified as important factors influencing the distribution of L. tanakae. False absence data were found to have negative effects on model performance and the decrease in evaluation metrics was usually significant (P<0.05) after 30% or more errors were added to the absence data. Through identifying highly suitable areas within its geographic range, our study provides a baseline for L. tanakae that can be further applied in ecosystem modelling and fishery management in the BSYS.
Distribution characteristics of wave energy in the Zhe-Min coastal area
Qin Ye, Zhongliang Yang, Min Bao, Weiyong Shi, Hongyuan Shi, Zaijin You, Wenyan Zhang
 doi: 10.1007/s13131-021-1859-2
[Abstract](41) [FullText HTML](9) [PDF 1201KB](5)
A 10-year (2003–2012) hindcast was conducted to study the wave field in the Zhe-Min coastal area (Key Area OE-W2) located off Zhejiang and Fujian provinces of China. Forced by the wind field from a weather research and forecasting model (WRF), high-resolution wave modelling using the SWAN was carried out in the study area. The simulated wave fields show a good agreement with observations. Using the simulation results, we conducted statistical analysis of wave power density in terms of spatial distribution and temporal variation. The effective duration of wave energy in the sea area was discussed, and the stability of wave energy was evaluated using the coefficient of variation of wave power density. Results indicate that the wave energy resource in the study area was about 4.11×106 kW. The distribution of wave energy tends to increase from the north (off Zhejiang coast) to the south (off Fujian coast), and from near-shore area to the open sea. The sea areas with wave power density greater than 2 kW/m are mostly distributed seaward of the 10-m isobath, and the contours of the wave power density are almost parallel to the shoreline. The sea areas around the islands that are far from the mainland are rich in wave energy, usually more than 6 kW/m, and therefore are of obvious advantages in planning wave energy development and utilization. The effective duration of wave energy in the offshore area shows an increasing trend from north (off Zhejiang coast) to south (off Fujian coast), with values of ~3 500 h in the north and ~4 450 h in the south. The coefficient of variation of wave energy in this region is mostly in the range of 1.5–3.0, and gradually decreases from the north to the south, suggesting that the wave energy in the south is more stable than that in the north.
Detection of oil spill based on CBF-CNN using HY-1C CZI multispectral images
Kai Du, Yi Ma, Zongchen Jiang, Xiaoqing Lu, Junfang Yang
 doi: 10.1007/s13131-021-1977-x
[Abstract](130) [FullText HTML](41) [PDF 2075KB](7)
Accurate detection of an oil spill is of great significance for rapid response to oil spill accidents. Multispectral images have the advantages of high spatial resolution, short revisit period, and wide imaging width, which is suitable for large-scale oil spill monitoring. However, in wide remote sensing images, the number of oil spill samples is generally far less than that of seawater samples. Moreover, the sea surface state tends to be heterogeneous over a large area, which makes the identification of oil spills more difficult because of various sea conditions and sunglint. To address this problem, we used the F-Score as a measure of the distance between forecast value and true value, proposed the Class-Balanced F loss function (CBF loss function) that comprehensively considers the precision and recall, and rebalances the loss according to the actual sample numbers of various classes. Using the CBF loss function, we constructed convolution neural networks (CBF-CNN) for oil spill detection. Based on the image acquired by the Coastal Zone Imager (CZI) of the Haiyang-1C (HY-1C) satellite in the Andaman Sea (study area 1), we carried out parameter adjustment experiments. In contrast to experiments of different loss functions, the F1-Score of the detection result of oil emulsions is 0.87, which is 0.03–0.07 higher than cross-entropy, hinge, and focal loss functions, and the F1-Score of the detection result of oil slicks is 0.94, which is 0.01–0.09 higher than those three loss functions. In comparison with the experiment of different methods, the F1-Score of CBF-CNN for the detection result of oil emulsions is 0.05–0.12 higher than that of the deep neural networks, support vector machine and random forests models, and the F1-Score of the detection result of oil slicks is 0.15–0.22 higher than that of the three methods. To verify the applicability of the CBF-CNN model in different observation scenes, we used the image obtained by HY-1C CZI in the Kalimata Strait to carry out experiments, which include two studies areas (study area 2 and study area 3). The experimental results show that the F1-Score of CBF-CNN for the detection result of oil emulsions is 0.88, which is 0.16–0.24 higher than that of other methods, and the F1-Score of the detection result of oil slicks is 0.96–0.97, which is 0.06–0.23 higher than that of other methods. Based on all the above experiments, we come to the conclusions that the CBF loss function can restrain the influence of oil spill and seawater sample imbalance on oil spill detection of CNN model thus improving the detection accuracy of oil spills, and our CBF-CNN model is suitable for the detection of oil spills in an area with weak sunglint and can be applied to different scenarios of CZI images.
Temperature coefficient of seawater pH as a function of temperature, pH, DIC and salinity
Yubin Hu
 doi: 10.1007/s13131-021-1955-3
[Abstract](96) [FullText HTML](39)
pH is a measure of the hydrogen ion activity in solution, which is a function of temperature. Under normal seawater conditions, it is well constrained. Nowadays, with an increasing interest in complex environments (e.g., sea ice), a better understanding of the temperature change on pH under extreme conditions is needed. The objective of this paper was to investigate the temperature coefficient of the seawater pH (∆pH/∆T) over a wide range of temperature, pH, dissolved inorganic carbon (DIC) and salinity by a method of continuous pH measurement with the temperature change, and to verify the application of CO2SYS for pH conversion under extreme conditions (on the National Bureau of Standards (NBS) scale and the total proton scale). Both experimental results and CO2SYS calculations showed that ∆pH/∆T was slightly affected by temperature over the range of 0°C to 40°C and by pH (at 25°C) from 7.8 to 8.5. However, when pH was out of this range, ∆pH/∆T varied greatly with pH value. According to the experimental results, changes in DIC from 1 mmol/kg to 5 mmol/kg and salinity from 20 to 105 had no significant effect on ∆pH/∆T. CO2SYS calculations showed a slight increase in ∆pH/∆T with DIC on both the NBS scale and the total proton scale; and underestimated ∆pH/∆T at high salinity (i.e., beyond the oceanographic range) on the NBS scale. Nevertheless, CO2SYS is still suitable for pH conversion even under extreme conditions by simply setting the input values of DIC and salinity in CO2SYS within the oceanographic range (e.g., DIC=2 mmol/kg and S=35).
Seasonal variation of Caligus rotundigenitalis infestation on the host fish Etroplus suratensis from the Cochin Backwaters, southwest coast of India
Jemi Job N, A A Mohamed Hatha, C K Radhakrishnan
 doi: 10.1007/s13131-021-1904-1
[Abstract](52) [FullText HTML](14) [PDF 320KB](9)
Investigation of four edible fishes (Etroplus suratensis, Oreochromis mossambicus, Lates calcarifer, Chanos chanos) collected from a fish landing center along Cochin Backwaters has done for caligid infestation. Of these, caligids were detected only from Etroplus suratensis and was identified to be Caligus rotundigenitalis. Parasites were attached to the inner side of operculum of host which indicates strong site preference. The annual observation of prevalence, mean intensity and abundance were found to be 19.21%, 1.051 and 0.202 respectively. Variation of prevalence with respect to the different length groups of E. suratensis has discussed in this paper. Damages of the inner side of operculum and increase in mucous production were noticed as a result of copepod attachment.
Molecular insights into the circadian clock in marine diatoms
Shufeng Zhang, Yue Wu, Lin Lin, Dazhi Wang
 doi: 10.1007/s13131-021-1962-4
[Abstract](22) [FullText HTML](5)
The circadian clock is a fundamental endogenous mechanism of adaptation that coordinates the physiology and behavior of most organisms with diel variations in the external environment to maintain temporal homeostasis. Diatoms are the major primary producers in the ocean. However, little is known about the circadian clock in marine diatoms compared with other organisms. Here, we investigated circadian clock genes, their expression patterns, and responses to environmental stimuli such as light, nitrogen and phosphorus in two marine diatoms, Skeletonema costatum and Phaeodactylum tricornutum, using a combination of qRT-PCR and bioinformatic analysis. We identified 17 and 18 circadian clock genes in P. tricornutum and S. costatum, respectively. Despite significant evolutionary differences, these genes were similar to those of the higher plant Arabidopsis. We also established a molecular model for the marine diatom circadian clock comprising an input pathway, core oscillator, output pathway, and valve effector. Notably, the expression patterns of core clock genes (CCA1, LHY and TOC1) in both species differed from those of terrestrial plants. Furthermore, the expression of these genes was influenced by variations in ambient light, nitrogen and phosphorus availability. Although marine diatoms and higher plants share common circadian clock components, their clock genes have diverged throughout evolution, likely as a result of adapting to contrasting environments.
Descriptions of four new species of the genus Reticunassa Iredale, 1936 (Gastropoda: Nassariidae) from the northeastern coast of Taiwan and the South China Sea
Shuqian Zhang, Suping Zhang, Chih-Yeh Lee
 doi: 10.1007/s13131-021-1910-3
[Abstract](27) [FullText HTML](7)
In the present study, four new species of the genus Reticunassa Iredale, 1936 collected from the adjacent sea areas of China are described and illustrated. Reticunassa hugokooli sp. nov., Reticunassa jungi sp. nov. and Reticunassa aureolineata sp. nov. were collected from the northeastern coast of Taiwan, whereas Reticunassa fuscofasciata sp. nov. was collected from the northeastern coast of Taiwan and the South China Sea. The four new species can be distinguished conchologically from other congeners mainly in protoconch, shell shape, sculpture, and coloration. These findings demonstrate that the biodiversity of this group in China might be largely underestimated.
Particle size shapes the prokaryotic microbial communities in mangrove sediments: a case study of Sanya, China
Wu Qu, Michael Vicent Shija, Jie Wu, Min Jin, Jianxin Wang, Runying Zeng, Zhuhua Chan
 doi: 10.1007/s13131-021-1915-y
[Abstract](32) [FullText HTML](6)
The prokaryotic microbial communities in the sediments play crucial roles in the ecological functions of mangrove ecosystems. Therefore, the environmental factors that affect the structures of these prokaryotic microbial communities could indirectly participate in the regulation of mangrove functions, which is of great value for mangrove studies. The particle size (PS) of soils is recently demonstrated as a key environmental factor for shaping the microbial communities; however, this hypothesis has rarely been tested for mangrove environments. A case study of three tropical mangroves from Sanya City, China was performed in this work to assess the influence of PS on the prokaryotic microbial community structures of bacteria, archaea, diazotrophs, and denitrifiers in the sediments. Results showed the variability in the spatial scale and the stability in the temporal scale for the prokaryotic communities, indicating that the tropical mangrove sediments could be a versatile but stable environment. Among the collected environmental factors, PS, salinity, and humidity had the greatest impacts, and PS mostly affected the structures of these prokaryotic communities based on its highest R2 values of canonical correspondence analysis, Mental test, and linear fitting (p≤0.05). Furthermore, PS was positively correlated with the diversity and abundance of diazotrophic communities and negatively correlated with the abundances of methanogenic communities including Methanobacteriaceae, Methanospirillaceae, Methanoregulaceae, and Methanosaetaceae. Former studies show the increasing trend of PS caused by the rise of sea level and the intensification of human activities. Therefore, our findings indicate that PS could be a potential intermediate that links climate change and human activities with the possible ecological function migration of mangroves; meanwhile, the increase of PS could in turn release the stress of these environmental changes by increasing the abundance and diversity of the diazotrophic community and decreasing the abundances of methanogens.
Contrasting behaviors of 210Po, 210Pb and 234Th in the East China Sea during a severe red tide: enhanced scavenging and promoted fractionation
Qiangqiang Zhong, Linwei Li, Viena Puigcorbé, Dekun Huang, Tao Yu, Jinzhou Du
 doi: 10.1007/s13131-021-1958-0
[Abstract](49) [FullText HTML](18)
210Po, 210Pb, and 234Th were determined in water columns of the East China Sea (ECS) to investigate their biogeochemical behaviors during a severe red tide event. Dissolved 210Po, 210Pb, and 234Th accounted for large fractions of the total phases. The abnormally high concentrations of dissolved 210Pb were observed. Partition behaviors of these radionuclides were influenced by particle content effect and particle composition based on distribution coefficient (Kd) vs. total suspended matter (TSM) content and Kd vs. ratios of particulate organic carbon and total suspended matter content (POC/TSM ratios). The peaks of mass specific activities of 210Po, 210Pb, and 234Th indicated that degraded particles could have an intensified enrichment ability for radionuclides compared with the surficial suspended matters. Fractionation factor of 210Po and 210Pb (FPo/Pb) (>1) and fractionation factor of 210Po and 234Th (FPo/Th) (>1) were much higher at algal blooming regions than that at non-blooming stations, indicating that algal blooms promoted the fractionation of 210Po against 210Pb and 234Th, and proving that 210Po exhibited a stronger affinity for biogenic particles than 210Pb and 234Th when POC content increased in the sea. POC/210Po, POC/210Pb and POC/234Th content ratios sharply decreased with depth in both algal bloom and non-bloom stations. The outbreak of algal bloom promoted the complexity of suspended particles and increased the variability of POC/tracer ratios in the different depth of the shallow seas. More considerations should be taken to the difficulty of the selection of export interface and the suitable tracers when algal blooming occurs.
Winter distribution of diatom assemblages along the coastline of Korea in 2020
Joon Sang Park, Kyun-Woo Lee, Seung Won Jung, Taek-Kyun Lee, Hyoung Min Joo
 doi: 10.1007/s13131-021-1929-5
[Abstract](33) [FullText HTML](10)
Diatoms are a globally successful and eukaryotic photosynthetic organism with an ornamented silica external wall. The relationship between their valve morphology and habitat means that diatoms can be used as bioindicators to characterize the aquatic environment. To estimate the differential distribution and diversity of diatom assemblages along the coastal line, we collected phytoplankton samples from 114 coastal sites of South Korean waters. We applied the unweighted pair-group technique using the arithmetic averages clustering method to cluster the sampling sites—apart from those where the biota consisted of other groups, such as dinoflagellates—into four regions: Yellow Sea, South Sea, Southern East Sea, and Northern East Sea. Indicator species analysis in each region led to the selection of tychoplanktonic, chain-forming, attached species to substrates, and psychrophilic indicator diatoms respectively, each of which represented a planktonic lifestyle associated with one of the four regions. This study shows the diatom assemblages to serve as bioindicators of Korean coastal water in winter, and the subsequent seasonal survey will provide a starting point for the improved understanding of Korean diatom-based ecoregions, in both time and space.
Fluxes of riverine nutrient to the Pearl River Estuary and its potential eutrophication effect
Li Zhang, Yumin Yang, Weihong He, Jie Xu, Ruihuan Li
 doi: 10.1007/s13131-021-1919-7
[Abstract](37) [FullText HTML](8)
The Pearl River Estuary is becoming eutrophic due to the impact of anthropogenic activities in the past decades. To understand nutrient dynamics and fluxes to Lingdingyang via four outlets (Humen, Jiaomen, Hongqimen and Hengmen), we investigated the spatial distribution and seasonal variation of dissolved nutrients in the Pearl River Estuary, based on fourteen cruises conducted from March 2015 to October 2017, covering both wet (April to September) and dry (October to March next year) seasons. Our results showed that riverine fluxes of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) into the Lingdingyang through four outlets varied seasonally due to the influence of river discharge, with the highest in spring and the lowest in winter. However, riverine flux of phosphate exhibited little significant seasonal variability. Riverine nutrients into Lingdingyang most resulted through Humen Outlet. The estuarine export fluxes of DIN out of the Pearl River Estuary derived from a box model were higher than fluxes of riverine nutrients in May, likely due to the influence of local sewage, while lower than riverine flux in August. The export fluxes of phosphate were higher than the fluxes of riverine phosphate in May and August. In contrast, large amounts of DSi were buried in the estuary in May and August. Although excess DIN was delivered into the Pearl River Estuary, eutrophication effect was not as severe as expected in the Pearl River Estuary, since the light limitation restricted the utilization of nutrients by phytoplankton.
Investigating the ENSO prediction skills of the Beijing Climate Center climate prediction system version 2
Yanjie Cheng, Youmin Tang, Tongwen Wu, Xiaoge Xin, Xiangwen Liu, Jianglong Li, Xiaoyun Liang, Qiaoping Li, Junchen Yao, Jinghui Yan
 doi: 10.1007/s13131-021-1951-7
[Abstract](48) [FullText HTML](7) [PDF 927KB](5)
The El Niño-Southern Oscillation (ENSO) ensemble prediction skills of the Beijing Climate Center (BCC) climate prediction system version 2 (BCC-CPS2) are examined for the period from 1991 to 2018. The upper-limit ENSO predictability of this system is quantified by measuring its “potential” predictability using information-based metrics, whereas the actual prediction skill is evaluated using deterministic and probabilistic skill measures. Results show that: (1) In general, the current operational BCC model achieves an effective 10-month lead predictability for ENSO. Moreover, prediction skills are up to 10–11 months for the warm and cold ENSO phases, while the normal phase has a prediction skill of just 6 months. (2) Similar to previous results of the intermediate coupled models, the relative entropy (RE) with a dominating ENSO signal component can more effectively quantify correlation-based prediction skills compared to the predictive information (PI) and the predictive power (PP). (3) An evaluation of the signal-dependent feature of the prediction skill scores suggests the relationship between the “Spring predictability barrier (SPB)” of ENSO prediction and the weak ENSO signal phase during boreal spring and early summer.
Effects of daily nitrogen and phosphorus input on planktonic community metabolism in a semi-enclosed bay by mesocosm experiment
Chenhui Xiang, Yao Li, Zhixin Ke, Gang Li, Yadong Huang, Xinying Su, Liangmin Huang, Xinyu Song
 doi: 10.1007/s13131-022-1986-4
[Abstract](28) [FullText HTML](5)
Planktonic metabolism plays an important role in affecting the energy transportation and carbon cycle of the marine ecosystem. However, its regulation mechanism remains unclear under the continuously exogenous nutrient inputs in nearshore waters. In this study, a mesocosm experiment was conducted in a semi-enclosed bay, the Daya Bay, to explore the responses of plankton metabolic balance and community structure to a concentration gradient of daily nitrogen and phosphorus inputs. The results showed that nutrient enrichments promoted phytoplankton biomass, total primary production, and community respiration, and the promoting effect enhanced alongwith the increase of nutrient concentration. However, the net community production fluctuated more violently between autotrophic and heterotrophic with the increase of nutrient inputs and tended to be more heterotrophic from the 5th day to the 10th day of the experiment. In addition, the daily flux of nitrogen and phosphorus, 2 μmol/(L·d) and 0.066 μmol/(L·d) respectively, could be regarded as a potential threshold for ecosystem stability and health, since most of the ecological characteristics related to plankton structure and function have undergone significant changes when the nutrient level is higher than that. In the nearshore enclosed or semi-enclosed waters, nutrient from continuous terrigenous input is likely to be concentrated and exceed this level, indicating the ecological risks due to the metabolic unbalance and the deterioration of plankton community structure.
Arctic summer sea ice phenology including ponding from 1982 to 2017
Xiaoli Chen, Chunxia Zhou, Lei Zheng, Mingci Li, Yong Liu, Tingting Liu
[Abstract](18) [FullText HTML](3)
Information on the Arctic sea ice climate indicators is crucial to business strategic planning and climate monitoring. Data on the evolvement of the Arctic sea ice and decadal trends of phenology factors during melt season are necessary for climate prediction under global warming. Previous studies on Arctic sea ice phenology did not involve melt ponds that dramatically lower the ice surface albedo and tremendously affect the process of sea ice surface melt. Temporal means and trends of the Arctic sea ice phenology from 1982 to 2017 were examined based on satellite-derived sea ice concentration and albedo measurements. Moreover, the timing of ice ponding and two periods corresponding to it were newly proposed as key stages in the melt season. Therefore, four timings, i.e., date of snow and ice surface melt onset (MO), date of pond onset (PO), date of sea ice opening (DOO), and date of sea ice retreat (DOR), and three durations, i.e., melt pond formation period (MPFP, i.e., MO–PO), melt pond extension period (MPEP, i.e., PO–DOR), and seasonal loss of ice period (SLIP, i.e., DOO–DOR), were used. PO ranged from late April in the peripheral seas to late June in the central Arctic Ocean in Bootstrap results, whereas the pan-Arctic was observed nearly 4 days later in NASA Team results. Significant negative trends were presented in the MPEP in the Hudson Bay, Baffin Bay, the Greenland Sea, and Kara and Barents Seas in both results, indicating that the Arctic sea ice undergoes a quick transition from ice to open water, thereby extending the melt season year to year. The high correlation coefficient between MO and PO, MPFP illustrated that MO predominates the process of pond formation.
Distribution of phytoplankton in the East China Sea and the southern Yellow Sea in spring in relation to environmental variables and dimethylsulfide compounds
Jiawei Zhang, Yanghang Chen, Xueyan Ren, Vishal Patil, Lin Sun, Xuesong Li, Junrong Liang, Jun Zhang, Yahui Gao, Changping Chen
 doi: 10.1007/s13131-021-1913-0
[Abstract](57) [FullText HTML](14)
The coastal ecosystems are highly sensitive to climate change and are usually influenced by variations in phytoplankton communities and water physiochemical factors. In the present study, the phytoplankton community, chlorophyll a and their relationships with environmental variables and dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were investigated in spring 2017 (March 24 to April 16) in the East China Sea (26.0°-33.0° N, 120.0°-128.0° E) and southern Yellow Sea (31.0°-36.0°N, 120.0°-125.0°E). The spatial distributions of phytoplankton species composition and cell density were investigated by qualitative and quantitative methods and were compared with historical data to study phytoplankton species succession in the survey area. The results showed that there were 275 phytoplankton species belonging to 90 genera and 6 phyla in the survey area, of which 208 species belonged to 62 genera of Bacillariophyta and 56 species belonged to 20 genera of Pyrrophyta. The dominant phytoplankton species were Skeletonema dohrnii, Chaetoceros vanheurckii and Prorocentrum donghaiense. The phytoplankton cell densities ranged from 0.06×104 cells/L to 418.73×104 cells/L, with an average value of 21.46×104 cells/L. In spring, the average ratio of Bacillariophyta/Pyrrophyta was 41.13 for the entire study area. The areas with high phytoplankton cell density were mainly distributed in the northern South Yellow Sea and offshore waters of the East China Sea. According to a canonical correspondence analysis (CCA) among phytoplankton and environmental parameters, the water Chl a concentrations were notably consistent with phytoplankton cell density (P<0.001), and both showed significant negative correlations with salinity and nitrite (P<0.05) and significant positive correlations with dissolved oxygen and pH (P<0.001). There was a significant positive correlation between diatom (both in cell density and in dominant species) and DMS (P<0.05), which indicated that diatoms play a greater role in DMS production in this investigated area.
Temporal and spatial changes of the basal channel of the Getz ice shelf in Antarctica derived from multi-source data
Zemin Wang, Mingliang Liu, Baojun Zhang, Xiangyu Song, Jiachun An
[Abstract](38) [FullText HTML](10)
Basal melting is an important factor affecting the stability of the ice shelf. The basal channel is formed from uneven melting, which also has an important impact on the stability of the ice shelf. Therefore, it has important scientific value to study the basal channel changes. This study combined datasets of Mosaics of Antarctica, Reference Elevation Model of Antarctica (REMA) and Operation IceBridge to study the temporal and Spatial changes of basal channels at the Getz ice shelf in Antarctica. The relationships between the cross-sectional area and width of basal channel and those of its corresponding surface depression were statistically analyzed. Then, the changes of the basal channels of Getz ice shelf were derived from the ICESat observations and REMA digital elevation models (DEMs). After a detailed analysis of the factors affecting the basal channel changes, we found that the basal channels of Getz ice shelf were mainly concentrated in the eastern of the ice shelf, and most of them belonged to the ocean-sourced basal channel. From 2009 to 2016, the total length of the basal channel has increased by approximately 60 km. Affected by the warm Circumpolar Deep Water (CDW), significant changes in the basal channel occurred in the middle reaches of the Getz ice shelf. The change of the basal channels at the edge of the Getz ice shelf is significantly weaker than that in its middle and upper reaches. Especially in 2005–2012, the eastward wind on the ocean wind field and the westward wind around the continental shelf caused the invasion and upwelling of CDW. Meanwhile, the continuous warming of deep seawater also caused the deepening of the basal channel. During from 2012 to 2020, the fluctuations of the basal channels seem to be caused by the changes in temperature of CDW.
Relationships between daily growth of different groups of swordtip squid (Uroteuthis edulis) and environmental variables in the East China Sea
Nan Li, Yan Wang, Zhou Fang, Xinjun Chen, Zhiping Feng
 doi: 10.1007/s13131-021-1905-0
[Abstract](41) [FullText HTML](10)
Swordtip squid (Uroteuthis edulis) is one of the important economical fishing target species in the East China Sea. U. edulis is characterized by rapid growth, extensive migration, and long spawning period and sensitive to surrounding environment. In order to assess its stock status, it is necessary to explore its spawning season, growth patterns of different populations and their relationship with the environment in advance. In this paper, based on the samples of U. edulis collected in the East China Sea from September 2017 to March 2018, we explored the relationships between daily growth of statolith microstructure and environmental variables by Gradient forest method (GFM) and generalized additive model (GAM). The spawning season of U. edulis was found to be nearly one year and two dominant season groups were found: the spring group with the peak period of April and the summer group with the peak period of August. Water temperature in the depth of 25 m (Temp_25), sea surface temperature (SST) and zonal velocity (ZV) were the key environmental variables for the daily growth of April-spawning group. The most suitable environmental conditions for the growth of April-spawning group were water temperature (24-27°C) and velocity (0.1-0.3 m/s). SST, Temp_25 and mixed layer depth (MLD) were the key environment variables for the daily growth of August-spawning group. The most suitable environmental conditions for the growth of August-spawning group were water temperature (21-28°C) and water depth (0-50 m). Key environmental variables of different groups suggested that early growth was significantly affected by seasonal changes of water temperature, current velocity and prey abundance. This study explored the relationships between early growth and environmental variables and provided the scientific guidance for the management and conservation of U. edulis.
Effect of seasonal barrier layer on mixed-layer heat budget in the Bay of Bengal
Gayan Pathirana, Dongxiao Wang, Gengxin Chen, M. K. Abeyratne, Tilak Priyadarshana
[Abstract](25) [FullText HTML](6)
Time series measurements (2010–2017) from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) moorings at 15°N, 90°E and 12°N, 90°E are used to investigate the effect of the seasonal barrier layer (BL) on the mixed-layer heat budget in the Bay of Bengal (BoB). The mixed-layer temperature tendency (\begin{document}$\partial T/\partial t $\end{document}) is primarily controlled by the net surface heat flux that remains in the mixed layer (\begin{document}$ {Q}^{\text{'}} $\end{document}) from March to October, while both \begin{document}$ {Q}^{\text{'}} $\end{document} and the vertical heat flux at the base of the mixed layer (\begin{document}$ {Q}_{h} $\end{document}), estimated as the residual of the mixed-layer heat budget, dominate during winter (November–February). An inverse relation is observed between the BL thickness and the mixed-layer temperature (\begin{document}$ \mathrm{M}\mathrm{L}\mathrm{T} $\end{document}). Based on the estimations at the moorings, it is suggested that when the BL thickness is ≥25 m, it exerts a considerable influence on \begin{document}$\partial T/\partial t$\end{document} through the modulation of \begin{document}$ {Q}_{h} $\end{document} (warming) in the BoB. The cooling associated with \begin{document}$ {Q}_{h} $\end{document} is strongest when the BL thickness is ≤10 m with the \begin{document}$ \mathrm{M}\mathrm{L}\mathrm{T} $\end{document} exceeding 29°C, while the contribution from \begin{document}$ {Q}_{h} $\end{document} remains nearly zero when the BL thickness varies between 10 m and 25 m. Temperature inversion is evident in the BoB during winter when the BL thickness remains ≥25 m with an average \begin{document}$ \mathrm{M}\mathrm{L}\mathrm{T} $\end{document} <28.5°C. Furthermore, \begin{document}$ {Q}_{h} $\end{document} follows the seasonal cycle of the BL at these RAMA mooring locations, with \begin{document}$ r > 0.72 $\end{document} at the 95% significance level.
Applications of EMD to analyses of high-frequency beachface responses to Storm Bebinca in Qing’an Bay, Guangdong Province, China
Taihuan Hu, Zhiqiang Li, Chunhua Zeng, Gaocong Li, Huiling Zhang
 doi: 10.1007/s13131-021-1948-2
[Abstract](175) [FullText HTML](55) [PDF 2702KB](16)
On average, five to six storms occur in the Qiongzhou Strait every year, causing significant damage to coastal geomorphology and several property losses. Tropical Storm Bebinca is the most unusual and complex storm event that has occurred in this region over the last 10 years. To detect the high-frequency beachface responses to the storm, a pressure sensor was deployed in the surf zone to record the free sea surface height, and the heights of grid pile points on the beachface were measured manually to determine beach elevation changes during this storm. Empirical Mode Decomposition and related analysis techniques were used to analyze the high-frequency topography and wave data. The results showed that: (1) the beachface response process occurred in three stages. The first stage was the rapid response stage, wherein the spring tide berm began to erode significantly, and the front edge of the beach berm reacted closely. The two beach sections resisted the harmful energy of the main storm. In the second stage, the beach slope increased after a large sediment loss on the beach berm and its front edge. To adapt to the storm energy, the beach at the low tide line began to erode, and the beach slope decreased. In the third stage, after the storm turned, the wave energy was significantly attenuated, and the beach berm eroded to resist the residual wave energy. The beachface began to oscillate and recover. (2) The main wave surface was the superimposed product of a few internal mode functions. Similar results were observed in beachface changes. High-frequency driving factors determine the local characteristics of beach evolution, and low-frequency driving factors determine the beach evolution trend. (3) The response of sediment to the storm was not a single sea-transportation, but a single- or two-way conversion driven by factors such as wave energy, swash flow, and secondary wave breaking. (4) The Ω-RTR model is not completely applicable to beach states that undergo rapid changes during storms. Therefore, it is necessary to carry out further research on beach state identification during storms.
Spreading-rate dependence of hydroacoustic and teleseismic seismicity of ridge-transform systems: East Pacific Rise, Galapagos Ridge, and Mid-Atlantic Ridge
Tingting Zheng, Jian Lin, Qiu Zhong
 doi: 10.1007/s13131-021-1936-6
[Abstract](82) [FullText HTML](16) [PDF 5543KB](10)
Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges, while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events. From 1996 to 2003, the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration (NOAA/PMEL) deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean. These hydrophones recorded earthquakes with small magnitudes, providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences. This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise (EPR, 10°S to 12°N), intermediate-spreading Galapagos Ridge (GR, 2°W to 6°W), and slow-spreading Mid-Atlantic Ridge (MAR, 15°N to 37°N). We present a systematic study of the spatial and temporal distribution of events, aftershock seismicity, and possible triggering mechanisms of aftershock sequences. Our analysis yields the following conclusions. (1) From the hydroacoustic data, the EPR transform faults had the highest average seismicity rate among the three regions. (2) Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR, while they became dispersed on the GR and fewer and more scattered on the MAR, reflecting that the different tectonic origins were closely correlated with the spreading rate. (3) Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions. (4) Multiple types of aftershock triggering models were inferred from Coulomb stress changes: strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks, and normal faulting mainshocks triggered normal faulting aftershocks. Although these results are case studies, they may be applicable to other ocean ridge-transform systems in future investigations. Our results provide important new insights into the seismicity of global ocean ridge-transform systems.
Top-down control on major groups of global marine diazotrophs
Hua Wang, Ya-Wei Luo
 doi: 10.1007/s13131-021-1956-2
[Abstract](103) [FullText HTML](39)
Di-nitrogen (N2) fixed by a group of prokaryotes (diazotrophs) is the dominant process adding bioavailable nitrogen into the ocean. Although it has been intensively studied how N2 fixation is controlled by resources (bottom-up factors), it is unclear whether the grazing (top-down control) effectively impact growth and distribution of different diazotroph groups. In this study, we evaluate this question by conducting log-log regression of diazotroph biomass onto corresponding N2 fixation rates in the global ocean. The slope of regression for Trichodesmium is ~0.8, indicating that a small portion of the increase of N2 fixation does not accumulate as its biomass. That leads to a conclusion that Trichodesmium is under a substantial top-down control, although bottom-up control still dominates. We also analyze the residuals of the regression in the North Atlantic, concluding that free trichomes of Trichodesmium is subject to stronger top-down control than its colonies. The weak correlation between the biomass and N2 fixation of unicellular cyanobacterial diazotrophs indicates that the degree of top-down control on this type of diazotrophs varies greatly. The analyses obtain unrealistic results for diatom-diazotroph assemblages due to complicate nitrogen sources of these symbioses. Our study reveals the variability of top-down control among different diazotroph groups across time and space, suggesting its importance in improving our understandings of ecology of diazotrophs and predictions of N2 fixation in biogeochemical models. Measurements of size-specific N2 fixation rates and growth rates of different diazotroph groups can be useful to more reliably analyze the top-down control on these key organisms in the global ocean.
Long-term trend analysis of wave characteristics in the Bohai Sea based on interpolated ERA5 wave reanalysis from 1950 to 2020
Jichao Wang, Peidong Sun, Zhihong Liao, Fan Bi, Guiyan Liu
[Abstract](27) [FullText HTML](3)
Reasonably understanding of the long-term wave characteristics is very crucial for the ocean engineering. A feedforward neural network is operated for interpolating ERA5 wave reanalysis in this study, which embodies a detailed record from 1950 onwards. The spatiotemporal variability of wave parameters in Bohai Sea, especially the significant wave height (SWH), is presented in terms of combined wave, wind wave and swell by employing the 71 years (1950–2020) of interpolated ERA5 reanalysis. Annual mean SWH decreases at −0.12 cm/a estimated by Theil-Sen Estimator and 95th percentile SWH reflecting serve sea states decreases at −0.20 cm/a. Inter-seasonal analysis shows SWH of wind wave has steeper decreasing trend with higher slopes than that of swell, especially in summer and winter, showing the major decrease may attribute to the weakening of monsoon. The inner Bohai Sea reveals a general decreasing trend while the intersection connecting with the Yellow Sea has the lower significance derived by Mann-Kendall Test. Meanwhile, 95th percentile SWH decreases at a higher rate while with a lower significance in comparison with the mean state. The frequencies of mean wave directions in sub-sector are statistically calculated to find the seasonal prevailing directions. Generally, the dominant direction in summer and winter is south and north. A similar variation concerning to SWH, the trend of the mean wave period is provided, which also shows a decrease for decades.
Semisupervised heterogeneous ensemble for ship target discrimination in synthetic aperture radar images
Yongxu Li, Xudong Lai, Mingwei Wang
 doi: 10.1007/s13131-021-1980-2
[Abstract](131) [FullText HTML](38)
Ship detection using synthetic aperture radar (SAR) plays an important role in marine applications. The existing methods are capable of quickly obtaining many candidate targets, but numerous non-ship objects may be wrongly detected in complex backgrounds. These non-ship false alarms can be excluded by training discriminators, and the desired accuracy is obtained with enough verified samples. However, the reliable verification of targets in large-scene SAR images still inevitably requires manual interpretation, which is difficult and time consuming. To address this issue, a semisupervised heterogeneous ensemble ship target discrimination method based on a tri-training scheme is proposed to take advantage of the plentiful candidate targets. Specifically, various features commonly used in SAR image target discrimination are extracted, and several acknowledged classification models and their classic variants are investigated. Multiple discriminators are constructed by dividing these features into different groups and pairing them with each model. Then, the performance of all the discriminators is tested, and better discriminators are selected for implementing the semisupervised training process. These strategies enhance the diversity and reliability of the discriminators, and their heterogeneous ensemble makes more correct judgments on candidate targets, which facilitates further positive training. Experimental results demonstrate that the proposed method outperforms traditional tri-training.
CPT-Based estimation of undrained shear strength of fine-grained soils in the Huanghe River Delta
Zhongnian Yang, Xuesen Liu, Lei Guo, Yuxue Cui, Xiuting Su, Chao Jia, Xianzhang Ling
 doi: 10.1007/s13131-021-1946-4
[Abstract](55) [FullText HTML](8) [PDF 1666KB](4)
The Huanghe River (Yellow River) Delta has a wide distribution of fine-grained soils. Fluvial alluviation, erosion, and wave loads affect the shoal area, resulting complex physical and mechanical properties to sensitive fine-grained soil located at the river-sea boundary. The cone penetration test (CPT) is a convenient and effective in situ testing method which can accurately identify various soil parameters. Studies on undrained shear strength only roughly determine the fine content (FC) without making the FC effect clear. We studied four stations formed in different the Huanghe River Delta periods. We conducted in situ CPT and corresponding laboratory tests, examined the fine content influence on undrained shear strength, and determined the cone coefficient (Nk). The conclusions are as follows. (1) The fine content in the area exceeded 90%, and the silt content was high, accounting for more than 70% of all fine particle compositions. (2) The undrained shear strength gradually increased with depth with a maximum of approximately 250 kPa. When the silt content was lower than 60%–70%, the undrained shear strength decreased. (3) The silt and clay content influenced undrained shear strength, and the fitted fsh/qt function model was established, which could be applied to strata with a high fine content. The cone coefficients were between 20 and 25, and the overconsolidated soil layer had a greater cone coefficient.
Unveiling controls of the latitudinal gradient of surface pCO2 in the Kuroshio Extension and its recirculation regions (northwestern North Pacific) in late spring
Chenglong Li, Weidong Zhai, Di Qi
 doi: 10.1007/s13131-021-1949-1
[Abstract](129) [FullText HTML](51) [PDF 15376KB](29)
In the northwestern North Pacific, annual net air-sea CO2 flux is greatest in the Kuroshio Extension (KE) zone, owing to its low annual mean partial pressure of CO2 (pCO2), and it decreases southward across the basin. To quantify the influences of factors controlling the latitudinal gradient in CO2 uptake, sea surface pCO2 and related parameters were investigated in late spring 2018 in a study spanning the KE, Kuroshio Recirculation (KR), and subtropical zones. We found that the sea-to-air pCO2 difference (ΔpCO2) was negative and at its lowest in the KE zone. ΔpCO2 gradually increased southward across the KR zone, and the sea surface was nearly in air-equilibrium with atmospheric CO2 in the subtropical zone. We found that northward cooling and vertical mixing were the two major processes governing the latitudinal gradient in surface pCO2 and ΔpCO2, while biological influences were relatively minor. In the KE zone affected by upwelling, the vertical-mixing-induced increase in surface pCO2 likely canceled out approximately 61% of the decrease in surface pCO2 caused by cooling and biological activities. Moreover, the prolonged air-sea equilibration for CO2 and relatively short hydraulic retention time jointly led to the low surface pCO2 in the KE zone in spring. Ultimately, the cooling KE current flows out of the region before it can be re-equilibrated with atmospheric CO2.
Tropical cyclone genesis over the western North Pacific simulated by Coupled Model Intercomparison Project Phase 6 models
Cong Gao, Lei Zhou
 doi: 10.1007/s13131-021-1860-9
[Abstract](219) [FullText HTML](85) [PDF 2078KB](16)
Threatening millions of people and causing billions of dollars in losses, tropical cyclones (TCs) are among the most severe natural hazards in the world, especially over the western North Pacific. However, the response of TCs to a warming or changing climate has been the subject of considerable research, often with conflicting results. In this study, the abilities of Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) models to simulate TC genesis are assessed through historical simulations. The results indicate that a systematic humidity bias persists in most CMIP6 models from corresponding CMIP Phase 5 models, which leads to an overestimation of climatological TC genesis. However, the annual cycle of TC genesis is well captured by CMIP6 models. The abilities of 25 models to simulate the geographical patterns of TC genesis vary significantly. In addition, seven models are identified as well simulated models, but seven models are identified as poorly simulated ones. A comparison of the environmental variables for TC genesis in the well-simulated group and the poorly simulated group identifies moisture in the mid-troposphere as a key factor in the realistic simulation of El Niño-Southern Oscillation (ENSO) impacts on TC genesis. In contrast with the observations, the poorly simulated group does not reproduce the suppressing effect of negative moisture anomalies on TC genesis in the northwestern region (20°–30°N, 120°–145°E) during El Niño years. Given the interaction between TC and ENSO, these results provide a guidance for future TC projections under climate change by CMIP6 models.
An efficient algorithm for generating a spherical multiple-cell grid
Fang Hou, Zhiyi Gao, Jianguo Li, Fujiang Yu
 doi: 10.1007/s13131-021-1947-3
[Abstract](117) [FullText HTML](24) [PDF 1442KB](5)
This paper presents an efficient algorithm for generating a spherical multiple-cell (SMC) grid. The algorithm adopts a recursive loop structure and provides two refinement methods: (1) an arbitrary area refinement method and (2) a nearshore refinement method. Numerical experiments are carried out, and the results show that compared with the existing grid generation algorithm, this algorithm is more flexible and operable.
Short-term variations and influencing factors of suspended sediment concentrations at Heisha Beach, Guangdong, China
Jintang Ou, Haoyan Dong, Liangwen Jia, Xiangxin Luo, Zixiao He, Kanglin Chen, Jing Liu, Yitong Lin, Mingdong Yu, Mingen Liang
 doi: 10.1007/s13131-021-1874-3
[Abstract](96) [FullText HTML](23) [PDF 10652KB](1)
Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes. Thus, a field measurement was conducted at Heisha Beach, located on the west coast of the Zhujiang River (Pearl River) Estuary, to investigate the short-term variation in suspended sediment concentrations (SSCs) and the relationship between the SSC and turbulent kinetic energy, bottom shear stress (BSS), and relative wave height. Based on extreme event analysis results, extreme events have a greater influence on turbulent kinetic energy than SSC. Although a portion of the turbulent kinetic energy dissipates directly into the water column, it plays an important role in suspended sediment motion. Most of the time, the wave-current interaction is strong enough to drive sediment incipience and resuspension. When combined, the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone. The relative wave height also has a strong correlation with SSC, indicating that the combined effect of water depth and wave height significantly impacts SSC variation. Water depth is mainly controlled by the tide on the beaches; thus, the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.
Numerical investigation of the South China Sea deep circulation
Shengquan Tang, Xueen Chen, Zhi Zeng, Xin Liu
 doi: 10.1007/s13131-021-1879-y
[Abstract](89) [FullText HTML](25) [PDF 3744KB](6)
Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea (SCS), the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer (July), a northeast-southwest through-flow pattern across the deep basin without a western boundary current in winter (January), and a transitional pattern in spring and autumn. The sensitivity model experiments illustrate that the Luzon Strait deep overflow is the main factor controlling the seasonal variation in the SCS deep circulation. The SCS surface wind can significantly influence the SCS deep circulation in winter. The Luzon Strait deep overflow transport from the Pacific into the SCS ranges from 0.68×106 m3/s to 1.83×106 m3/s, reaching its maximum in summer (July, up to 1.83×106 m3/s), less in autumn and winter, and the minimum in spring (May, 0.68×106 m3/s). In summer, the strong Luzon Strait deep overflow dominates the SCS deep circulation when the role of the SCS surface wind is small. In winter, the weaker Luzon Strait deep overflow and SCS surface wind jointly drive the SCS deep circulation into a northeast-southwest through-flow pattern. The potential vorticity (PV) dissipation in the SCS deep basin reaches its maximum (−0.122 m2/s2) in May and its minimum (−0.380 m2/s2) in July.
Numerical study of power production from tidal energy in the Khuran Channel and its feedback on background hydrodynamics
Davood Shariatmadari, Seyed Mostafa Siadatmousavi, Cyrus Ershadi
 doi: org/10.1007/s13131-021-1968-y
[Abstract](15) [FullText HTML](1)
This study focuses on the development of a farm of tidal turbines in the Khuran Channel. The important factors include the location of turbines and their hydrodynamic effects on the environment. A three-dimensional circulation model for the Persian Gulf is employed for the comprehensive evaluation of the tidal energy potential throughout the study area. The model is validated by using in situ observations of water level and current data. The appropriate potential points for extracting the tidal energy were identified in the Persian Gulf using the model results. The Khuran Channel, located in the north of Qeshm Island, was found to be the best place to extract tidal energy inside the Persian Gulf. By adding the term of momentum losses to the governing equations, the feedback of extracting energy on the hydrodynamic around Qeshm Island was studied. The simulation results show that the average daily power production of a tidal farm with 99 turbines for one month is approximately 1.3 MW. This tidal farm also has a significant impact on the water level inside the Khuran Channel, especially near the tidal farm where these fluctuations exceed 4 cm. The change in the current speed caused by wake reaches 0.4 m/s. Wake effects were active up to 7 km downstream of the turbines. The current velocity was also estimated to be 1.6 m/s and 2.1 m/s during the spring and ebb tides within the channel, respectively.
Satellite observed shape-based overall rotation—a new aspect in eddy kinematics
Xueqing Yang, Guiyan Han, Chunyong Ma, Chuanchuan Cao, Jie Yang, Ge Chen
 doi: 10.1007/s13131-021-1970-4
[Abstract](44) [FullText HTML](10)
With the development of satellite altimetry technology, the resolution of sea-level anomaly (SLA) datasets is constantly improving. Current spatial resolution levels can reach a grid size of (1/4)° × (1/4)°, with daily measurements that span from 1993 to 2018, allowing for the precise identification and tracking of individual eddies. In the current study, in addition to the internal circulation and migration of eddies, a new aspect in eddy kinematics is revealed and investigated for the first time: shape-based overall eddy rotation (SOER), based on the intrinsic elliptical shape of eddies identified from a high-resolution SLA dataset. We found that eddies can maintain an elliptical shape and a slow and stable SOER during their migration process. The SOER speed was observed to be negatively correlated to eddy lifetime, and exhibited a dependence on latitude, decreasing from low- and high- to mid-latitudes. The SOER direction tended to be consistent with the direction of internal circulation, particularly for long-lived eddies. In addition, we identified a negative relationship between internal circulation speed and SOER speed while the migration speed was positively related to SOER speed. These findings further expand and improve eddy kinematics, which is of great significance for the future study of eddy dynamics.
Statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean
Chunjian Sun, Xidong Wang, Anmin Zhang, Lianxin Zhang, Caixia Shao, Guosong Wang
 doi: 10.1007/s13131-021-1969-x
[Abstract](58) [FullText HTML](11)
The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019. In the Arabian Sea (AS), seasonal variation of eddy characteristics is remarkable, while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal (BOB). Seasonal variation of the eddy kinetic energy (EKE) is distinct along the west boundary of AS, especially in the Somali Current region. In the BOB, larger EKE occurs at the northwest basin from March to May, to the east of Sri Lanka from June to September, and along the east coast of India from November to December. The wind stress work (WW) is further studied to figure out the direct influence of wind forcing on EKE. The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons. Besides, the WW also has impact on EKE along the east coast of India in November and December. Eventually, we investigate the characteristics and the driving mechanisms of long lifespan eddies. In the AS, long lifespan anti-cyclonic eddies (AEs) mainly generate in the Socotra, the West Indian Coastal Current and the East Arabian Current regions, while cyclonic eddies (CEs) are concentrated in the northwest region. In the BOB, long lifespan AEs mostly form near the west of Myanmar, while CEs are accumulated at the north and northwest basin. The instabilities caused by Rossby waves, coastal Kelvin waves, seasonal currents, together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.
Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea
Shuo Chen, Renhai Pu, Huiqiong Li, Hongjun Qu, Tianyu Ji, Siyu Su, Yunwen Guan, Hui Zhang
[Abstract](55) [FullText HTML](12)
The Dongfang1-1 gas field (DF1-1) in the Yinggehai Basin (YGHB) is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources. The second member of the Pliocene Yinggehai Formation (YGHF) is the main gas-producing formation and is composed of various sedimentary types; however, a clear understanding of the sedimentary types and development patterns is lacking. Here, typical lithofacies, logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling, logging, analysis and testing data. Based on 3D seismic interpretation and attribute analysis, the origin of high-amplitude reflections is clarified, and the main types and evolution characteristics of sedimentary facies are identified. Taking gas formation IIU as an example, the plane distribution of the delta front and bottom current channel is determined; finally, a comprehensive sedimentary model of the YGHF second member is established. This second member is a shallowly buried “bright spot” gas reservoir with weak compaction. The velocity of sandstone is slightly lower than that of mudstone, and the reflection has medium amplitude when there is no gas. The velocity of sandstone decreases considerably after gas accumulation, resulting in an increase in the wave impedance difference and high-amplitude (bright spot) reflection between sandstone and mudstone; the range of high amplitudes is consistent with that of gas-bearing traps. The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps, and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps. The study area is a delta front deposit developed on a shallow sea shelf. The lithologies of the reservoir are mainly composed of very fine sand and coarse silt, and a variety of sedimentary structural types reflect a shallow sea delta environment; upward thickening funnel type, strong toothed bell type and toothed funnel type logging facies are developed. In total, 4 stages of delta front sand bodies (corresponding to progradational reflection seismic facies) derived from the Red and Blue Rivers in Vietnam have developed in the second member of the YGHF; these sand bodies are dated to 1.5 Ma and correspond to four gas formations. During sedimentation, many bottom current channels (corresponding to channel fill seismic facies) formed, which interacted with the superposed progradational reflections. When the provenance supply was strong in the northwest, the area was dominated by a large set of delta front deposits. In the period of relative sea level rise, surface bottom currents parallel to the coastline were dominant, and undercutting erosion was obvious, forming multistage superimposed erosion troughs. Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical.
Responses of nutrient biogeochemistry and nitrogen cycle to seasonal upwelling in coastal waters of the eastern Hainan Island
Nan Zhou, Sumei Liu, Guodong Song, Yunyan Zhang, Lingyan Wang, Xiaoyan Ning
 doi: 10.1007/s13131-021-1934-8
[Abstract](98) [FullText HTML](29) [PDF 13879KB](8)
The coastal upwelling has profound influence on the surrounding ecosystem by supplying the nutrient-replete water to the euphotic zone. Nutrient biogeochemistry was investigated in coastal waters of the eastern Hainan Island in summer 2015 and autumn 2016. From perspectives of nutrient dynamics and physical transport, the nutrient fluxes entered the upper 50 m water depth (between the mixed layer and the euphotic zone) arisen from the upwelling were estimated to be 2.5−5.4 mmol/(m2·d), 0.15−0.28 mmol/(m2·d), and 2.2−7.2 mmol/(m2·d) for dissolved inorganic nitrogen (DIN), phosphate (DIP), and dissolved silicate (DSi), respectively, which were around 6- to 12-fold those in the background area. The upwelled nutrients supported an additional plankton growth of (14.70±8.95) mg/m2 for chlorophyll a (Chl a). The distributions of nitrate δ15N and δ18O above the 300 m water depth (top of the North Pacific Intermediate Water) were different among the upwelling area, background area in summer, and the stations in autumn, and the difference of environmental and biogeochemical conditions between seasons should be the reason. The higher DIN/DIP concentration ratio, nitrate concentration anomaly, and lower nitrate isotope anomaly (Δ(15, 18)) in the upper ocean in summer than in autumn indicated the stronger nitrogen fixation and atmospheric deposition, and the following fixed nitrogen regeneration in summer. The higher values of Chl a and nitrate δ15N and δ18O within the euphotic zone in autumn than the background area in summer suggested the stronger nitrate assimilation in autumn. The differences in relatively strength of the assimilation, nitrogen fixation and atmospheric deposition, and the following remineralization and nitrification between the two seasons made the higher δ18O:δ15N and larger difference of enzymatic isotope fractionation factors 15ε and 18ε for nitrate assimilation in summer than in autumn above the North Pacific Tropical Water.
Long-term trend of oceanic surface carbon in the Northwest Pacific from 1958 to 2017
Xuanliang Ji, Fei Chai, Peng Xiu, Guimei Liu
 doi: 10.1007/s13131-021-1953-5
[Abstract](127) [FullText HTML](42)
Contrasting decrease and increase trends of sea surface temperature (SST) have been documented in the western Subarctic (WSA) and the rest of the Northwest Pacific (NWP) from 1958 to 2017, respectively. Consequently, more (less) total carbon dioxide (TCO2) due to ocean cooling (warming) is transported to the surface, which leads to increase (decrease) of oceanic surface partial pressure of carbon dioxide (pCO2). With the combined influence of the rising atmospheric carbon dioxide (CO2) level and changing ocean conditions, a prominent increase in oceanic surface pCO2 occurred with different rates of increase in summer and winter in the NWP. The oceanic surface pCO2 is mainly controlled by the variation of TCO2 at the interdecadal timescale and by SST at the seasonal timescale. Our results also indicate that increasing SST tends to strengthen the capability of ocean in absorbing anthropogenic CO2 in the NWP, while ocean’s uptaking ability is weakened in the cooling area of the WSA.
A hybrid forecasting model for depth-averaged current velocities of underwater gliders
Yaojian Zhou, Yonglai Zhang, Wenai Song, Shijie Liu, Baoqiang Tian
[Abstract](28) [FullText HTML](6)
In this paper, we propose a hybrid forecasting model to improve the forecasting accuracy for depth-averaged current velocities (DACVs) of underwater gliders. The hybrid model is based on a discrete wavelet transform (DWT), a deep belief network (DBN), and a least squares support vector machine (LSSVM). The original DACV series are first decomposed into several high- and one low-frequency subseries by DWT. Then, DBN is used for high-frequency component forecasting, and the LSSVM model is adopted for low-frequency subseries. The effectiveness of the proposed model is verified by two groups of DACV data from sea trials in the South China Sea. Based on four general error criteria, the forecast performance of the proposed model is demonstrated. The comparison models include some well-recognized single models and some related hybrid models. The performance of the proposed model outperformed those of the other methods indicated above.
Characteristics of sea ice kinematics from the marginal ice zone to the packed ice zone observed by buoys deployed during the 9th Chinese Arctic Expedition
Xiaomin Chang, Longchun Ye, Guangyu Zuo, Jingyue Li, Keyu Wei, Yinke Dou
[Abstract](95) [FullText HTML](43)
Sea ice growth and consolidation play a significant role in heat and momentum exchange between the atmosphere and the ocean. However, few in situ observations of sea ice kinematics have been reported owing to difficulties of deployment of buoys in the marginal ice zone (MIZ). To investigate the characteristics of sea ice kinematics from MIZ to packed ice zone (PIZ), eight drifting buoys designed by Taiyuan University of Technology (TUT) were deployed in the open water at the ice edge of the Canadian Basin. Sea ice near the buoy constantly increased as the buoy drifted, and the kinematics of the buoy changed as the buoy was frozen into the ice. This process can be determined using sea ice concentration, sea skin temperature, and drift speed of buoy together. Sea ice concentration data showed that buoys entered the PIZ in mid-October as the ice grew and consolidated around the buoys, with high amplitude, high frequency buoy motions almost ceasing. Our results confirmed that good correlation coefficient in monthly scale between buoy drift and the wind only happened in the ice zone. The correlation coefficient between buoys and wind was below 0.3 while the buoys were in open water. As buoys entered the ice zone, the buoy speed was normally distributed at wind speeds above 6 m/s. The buoy drifted mainly to the right of the wind within 45 ° at wind speeds above 8 m/s. During further consolidation of the ice in MIZ, the direct forcing on the ice through winds will be lessened. The correlation coefficient value increased to 0.9 in November, and gradually decreased to 0.7 in April.
Assessment of theoretical approaches to derivation of internal solitary wave parameters from multi-satellite images near the Dongsha Atoll of the South China Sea
Huarong Xie, Qing Xu, Quanan Zheng, Xuejun Xiong, Xiaomin Ye, Yongcun Cheng
 doi: 10.1007/s13131-022-2015-3
[Abstract](108) [FullText HTML](38) [PDF 991KB](16)
This study assesses the accuracy and the applicability of the Korteweg-de Vries (KdV) and the nonlinear Schrödinger (NLS) equation solutions to derivation of dynamic parameters of internal solitary waves (ISWs) from satellite images. Visible band images taken by five satellite sensors with spatial resolutions from 5 m to 250 m near the Dongsha Atoll of the northern South China Sea (NSCS) are used as a baseline. From the baseline, the amplitudes of ISWs occurring from July 10 to 13, 2017 are estimated by the two approaches and compared with concurrent mooring observations for assessments. Using the ratio of the dimensionless dispersive parameter to the square of dimensionless nonlinear parameter as a criterion, the best appliable ranges of the two approaches are clearly separated. The statistics of total 18 cases indicate that in each 50% of cases, the KdV and the NLS approaches give more accurate estimates of ISW amplitudes. It is found that the relative errors of ISW amplitudes derived from two theoretical approaches are closely associated with the logarithmic bottom slopes. This may be attributed to the nonlinear growth of ISW amplitudes as propagating along a shoaling thermocline or topography. The test results using three consecutive satellite images to retrieve the ISW propagation speeds indicate that the use of multiple satellite images (>2) may improve the accuracy of retrieved phase speeds. Meanwhile, repeated multi-satellite images of ISWs can help to determine the types of ISWs if mooring data are available nearby.
Intraseasonal variability of the surface zonal current in the equatorial Indian Ocean: Seasonal differences and causes
Qingwen Zhong, Gengxin Chen, Yuanlong Li, Qihua Peng, Xiaoqing Chu
 doi: 10.1007/s13131-021-1935-7
[Abstract](245) [FullText HTML](84) [PDF 3582KB](16)
Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current (u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80°–90°E and near the western boundary at 45°–55°E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations (ISOs), which explains 91% of the standard deviation of the total u ISV. Further analysis suggests that it takes less than ten days for the intraseasonal zonal wind stress to generate the u ISV through the directly forced Kelvin and Rossby waves. Driven by the stronger zonal wind stress associated with the Indian summer monsoon ISO (MISO), the eastern u ISV in boreal summer (May to October) is about 1.5 times larger than that in boreal winter (November to April). In the western basin, both the atmospheric ISOs and the oceanic internal instabilities contribute substantially to the u ISV, and induce stronger u ISV in boreal summer. Energy budget analysis suggests that the mean flow converts energy to the intraseasonal current mainly through barotropic instabilities.
Comparative mitochondrial genome analysis of Varunidae and its phylogenetic implications
Ying Zhang, Li Gong, Xinting Lu, Zengliang Miao, Lihua Jiang, Bingjian Liu, Liqin Liu, Pengfei Li, Xu Zhang, Zhenming Lü
 doi: 10.1007/s13131-021-1927-7
[Abstract](89) [FullText HTML](23)
Complete mitochondrial genomes (mitogenomes) can indicate phylogenetic relationships, as well as useful information for gene rearrangement mechanisms and molecular evolution. Currently, the phylogenetic location of the genus Varuna (Brachyura; Varunidae) has not been well resolved mainly because of limited representatives (only two extant species). Here, we determined a new mitogenome of this genus (Varuna litterata) and added the published mitogenomes to reconstruct the phylogeny of Varunidae. The 16,368-bp mitogenome contains the entire set of 37 genes and a putative control region. The characteristics of this newly sequenced mitogenome were described and compared with the other 15 Varunidae mitogenomes. All 16 analyzed mitogenomes have identical gene order and similar molecular features. The sliding window and genetic distance analyses demonstrate highly variable nucleotide diversity, with comparatively low variability of COI and COII, and high variability of ND6. The dN/dS ratio analysis shows that all 13 PCGs are under purifying selection and ATP8 gene evolves under the least selective pressure. Twelve tRNA genes, two rRNAs, one PCG, and the putative CR are found to be rearranged with respect to the pancrustacean ground pattern gene order. Tandem duplication/random loss (TDRL) model is adopted to explain the large-scale gene rearrangement events occurring in Varunidae mitogenomes. Phylogenetic analyses show that all Varunidae species are placed into one group, and form a sister clade with Macrophthalmidae. Nevertheless, the phylogenetic relationships within Varunidae are not completely consistent based on the two different datasets used in this study. These findings will contribute to a better understanding of gene rearrangement and molecular evolution in Varunidae mitogenomes, as well as provide insights into the phylogenetic studies of Brachyura.
Environmental effects of mariculture in China: An overall study of nitrogen and phosphorus loads
Jihong Zhang, Wenguang Wu, Yuchen Li, Yi Liu, Xinmeng Wang
 doi: 10.1007/s13131-021-1909-9
[Abstract](59) [FullText HTML](13)
Eutrophication in coastal area has become more and more serious and mariculture potential is a main cause. Although there are some quantitative research on nutrient loads in national and global perspective, the calculation method problems make the results controversial. In this paper, the farming activities are divided into fed culture types (include cage culture and pond culture) and extractive culture types (e.g. seaweed, filter-feeding shellfish culture). Based on the annual yield of China in 2019 and feed coefficient of fed culture types and carbon (C), nitrogen (N), and phosphorus (P) content of extractive culture types, the annual nutrient loads was estimated. The results showed that to coastal region of China (1) annual nutrient released by fed culture types were about 58 451 t of N, 9 081 t of P, and annual nutrient removed by harvest of extractive culture types were 109 245 t of N, 11 980 t of P and 1.86×106 t of C. Overall, the net amount of nutrient removed annually by mariculture industry were 50 793 t of N and 2 899 t of P. (2) The nutrient released from mariculture industry influenced nutrient stoichiometry. Pond farming and seaweed farming had the potential of increasing the molar concentration ratio of N and P (N:P), while cage farming and bivalve farming decreased the N:P. (3) Due to different mariculture types and layouts in the coastal regions in China, N and P loading were regional different. Among the coastal regions in China, net release of nutrient from mariculture occurred only in Hainan and Guangxi regions, while in the other regions, N and P were completely removed by harvest. We suggest decrease the amount of fed culture types and increase the amount of integrated culture with extractive culture types. This study will help to adjust mariculture structure and layout at the national level to reduce the environmental impact.
Reclamation-oriented spatiotemporal evolution of coastal wetland along Bohai Rim, China
Fan Wei, Mei Han, Guangxuan Han, Min Wang, Lixin Tian, Jiqian Zhu, Xianglun Kong
 doi: 10.1007/s13131-022-1987-3
[Abstract](74) [FullText HTML](15)
Coastal wetlands are located in the ecotone of interaction between the land surface and sea, and anthropogenic activities extensively interfere with these wetlands through the reclamation of large tidal wetlands and destruction of the function of the ecosystems. In this study, we investigated the dynamic evolutionary characteristics of the Bohai Rim coastal area over the past 40 years using the Modified Normalized Difference Water Index, the fractal dimension, object-oriented classification, the land-use transfer trajectory, and regression analysis. Additionally, we quantified and monitored the evolution of reclamation and analyzed the correlation between reclamation and coastal wetlands based on 99 Landsat-2, -5, and -8 images (at 60 m and 30 m spatial resolution) over the period 1980–2019. The results showed that (1) the coastline of the Bohai Rim increased by 1 631.2 km from 1980 to 2019 with a zigzag variation. The artificial coastline increased by 2 946.1 km, whereas the natural coastline decreased by 90%. (2) The area of man-made wetlands increased by 3 736.9 km2, the area of construction land increased by 1 008.4 km2, and the natural wetland area decreased by 66%. The decrease of tidal flats is the main contributor to the decrease of natural wetland area (takes account for 91.1%). Coastal areas are affected by intense human disturbance, which was taken place across a large area of tidal flats and caused the landscape to fragment and be more heterogeneous. The coastal zone development activities were primarily concentrated in the southern Laizhou Bay, the Yellow River Delta, the Bohai Bay, the northern Liaodong Bay, and the Pulandian Bay. The solidified shorelines and increase in sea level have resulted in intertidal wetlands decreasing and impaired wetland ecology. (3) There is a good agreement between reclamation and the size of the coastal wetlands. Both land reclamation and the reduction in coastal wetland areas are significantly related to the population size, fishery output value, and urbanization rate. In summary, human activities, such as the construction of aquaculture ponds and salt pans, industrialization, and urbanization, are the primary forces that influence the environmental changes in the coastal region. This study is beneficial for establishing and improving the systems for the rational development and utilization of natural resources and provided theoretical references for restoring wetland ecology and managing future reclamation activities in other coastal zone-related areas.
Radium-traced nutrient outwelling from the Subei Shoal to the Yellow Sea: Fluxes and environmental implication
Jian’an Liu, Dongyan Liu, Jinzhou Du
[Abstract](87) [FullText HTML](30)
The Subei Shoal is the largest sandy ridge in the southern Yellow Sea and important source for nutrient loading to the sea. Here, the nutrient fluxes in the Subei Shoal associated with eddy diffusion and submarine groundwater discharge (SGD) were assessed to understand their impacts on the nutrient budget in the Yellow Sea. Based on the analysis of 223Ra and 224Ra in the field observation, the offshore eddy diffusivity mixing coefficient and SGD were estimated to be 2.3×108 cm2/s and 2.6×109 m3/d (16 cm/d), respectively, in the Subei Shoal. Combined the significant offshore decreasing gradients of nutrient in seawater of the Subei Shoal, the spatially integrated nutrient outwelling fluxes to the Yellow Sea were 262−1 465 μmol/(m2·d) for DIN, 5.2−21 μmol/(m2·d) for DIP and 711−913 μmol/(m2·d) for DSi. Compared to the riverine input, atmospheric deposition and mariculture, nutrient outwelling from the Subei Shoal might play an important role in nutrient budget of the Yellow Sea. These nutrient fluxes could provide 4.1%−23% N and 1.3%−5.3% P requirements for the primary productivity, and the deviated DIN/DIP ratios have the potential to affect the growth of phytoplankton in the marine ecosystem of the Yellow Sea.
Response of Japanese anchovy (Engraulis japonicus) to the Pacific Decadal Oscillation in the Yellow Sea over the past 400 a
Haoyu Li, Qisheng Tang, Yao Sun
 doi: 10.1007/s13131-021-1914-z
[Abstract](83) [FullText HTML](16) [PDF 1235KB](15)
Quantitative identification of long-term changes in the abundance of Japanese anchovy (Engraulis japonicus) in the Yellow Sea is particularly important for understanding evolutionary processes of the Yellow Sea ecosystem. Unfortunately, the driving mechanisms of climate variability on the anchovy are still unclear due to the lack of long-term observational data. In this study, we used the fish scale deposition rate in the central Yellow Sea to reconstruct the time series of the anchovy stock over the past 400 a. On this basis, we further explored the impacts of the Pacific Decadal Oscillation (PDO) on the anchovy. Our results show that the anchovy stock is positively correlated with the PDO on a decadal time scale. In addition, anchovy abundance was relatively high during 1620–1860 AD (the Little Ice Age, LIA), though in a state of constant fluctuation; anchovy abundance maintained at a relatively low level after ~1860 AD. In particular, followed by overfishing since the 1980s, the anchovy stock has declined sharply. Based on these findings, we infer that fluctuations of the anchovy stock may be regulated by basin-scale “atmosphere–ocean” interactions. Nevertheless, the role of overfishing should not be ignored.
Dynamics of seasonal and interannual variability of the ocean bottom pressure in the Southern Ocean
Xiaoqin Xiong, Xuhua Cheng, Niansen Ou, Tao Feng, Jianhuang Qin, Xiao Chen, Rui Xin Huang
 doi: 10.1007/s13131-021-1878-z
[Abstract](112) [FullText HTML](36)
Seasonal and interannual variability of ocean bottom pressure (OBP) in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment (GRACE) data and a Pressure Coordinate Ocean Model (PCOM) based on mass conservation. By comparing OBP, steric sea level, and sea level, it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales. The diagnostic OBP based on barotropic vorticity equation has a good correlation with the observations, indicating that wind forcing plays an important role in the variability of the OBP in the Southern Ocean. The unique interannual patterns of OBP in the Southern Ocean are closely associated with El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM). Regression analysis indicates that ENSO and SAM influence the OBP through altering the Ekman transport driven by surface wind. The leading pattern of OBP from PCOM are very similar to observations. Sensitive experiments of PCOM show that surface wind forcing explains the observed OBP variability quite well, confirming the importance of wind forcing and related oceanic processes. In the eastern South Pacific, the averaged OBP shows a decrease (increase) trend before (after) 2011, reflecting the reverse trend in westerly wind. In the South Indo-Atlantic Ocean, the averaged OBP has a weak increase trend during 2003–2016.
Vertical velocity and transport in the South China Sea
Yaohua Zhu, Dingqi Wang, Yonggang Wang, Shujiang Li, Tengfei Xu, Zexun Wei
 doi: 10.1007/s13131-021-1954-4
[Abstract](296) [FullText HTML](110) [PDF 1878KB](29)
Deep water in the South China Sea is renewed by the cold and dense Luzon Strait overflow. However, from where and how the deep water upwells is poorly understood yet. Based on the Hybrid Coordinate Ocean Model reanalysis data, vertical velocity is derived to answer these questions. Domain-integrated vertical velocity is of two maxima, one in the shallow water and the other at depth, and separated by a layer of minimum at the bottom of the thermocline. Further analysis shows that this two-segmented vertical transport is attributed to the vertical compensation of subsurface water to the excessive outflow of shallow water and upward push of the dense Luzon Strait overflow, respectively. In the abyssal basin, the vertical transport increases upward from zero at the depth of 3 500–4 000 m and reaches a maximum of 1.5×106 m3/s at about 1 500 m. Deep water upwells mainly from the northeastern and southwestern ends of the abyssal basin and off the continental slopes. To explain the upward velocity arising from slope breaks, a possible mechanism is proposed that an onshore velocity component can be derived from the deep western boundary current above steep slopes under bottom friction.
Vertical structure of tidal currents in the Xuliujing Section of Changjiang River Estuary
Zhigao Chen, Ya Ban, Xiaoye Chen, Dajun Li, Shengping Wang
 doi: 10.1007/s13131-021-1976-y
[Abstract](211) [FullText HTML](55)
Three long-term fixed acoustic Doppler current profilers (ADCPs) were first used for investigating the vertical structure of tidal currents in Xuliujing Section of Changjiang River Estuary. Moreover, three different periods (spring, summer and fall) were also considered for investigating seasonal variations. The semi-diurnal tides were the most energetic, with along-channel speed of up to 80 cm/s for M2 constituent, which dominates at all stations with percent energy up to 65%–75% during seasons. The shape of tidal ellipses of the most energetic semi-diurnal constituent M2 showed obvious polarization of the flow paralleling to the riverbank, with the minor semi-axis being generally general less than 20% of the major one. The maximum velocity of mean current is appeared in top layers at all the three stations, and the velocity decreased with the depth. The seasonal variations of direction are also observed, which is probably caused by complex local topography since the erosion and deposition in riverbed. Observed vertical variation of four parameters of M2 ellipses, agreed well with the optimally fit frictional solutions in top and middle layers. However, there was an obvious difference between frictional model and observed data in the lower water column. Discrepancies are probably on account of stratification, which strengthens in summer and fall due to the freshening influence of the Changjiang River Estuary outflow.
Variability of the deep South China Sea circulation derived from HYCOM reanalysis data
Yaohua Zhu, Guojiao Cao, Yonggang Wang, Shujiang Li, Tengfei Xu, Dingqi Wang, Fei Teng, Zexun Wei
 doi: 10.1007/s13131-021-1952-6
[Abstract](108) [FullText HTML](39)
This study aims to investigate variability of the deep South China Sea (SCS) circulation using the Hybrid Coordinate Ocean Model (HYCOM) global reanalysis product. The results reveal that annual cycle is a dominant component in the deep SCS circulation. Meanwhile, the boundary circulation strength is the weakest in January and peaks between June and September. The eastern and southern boundary currents strengthen and weaken one to three months earlier than that of the western and northern boundaries. Vector Empirical Orthogonal Functions (VEOF) analysis results reveal that semiannual and intraseasonal fluctuations are significant components, of which the spatial patterns are mainly confined in the northern and western boundary areas as well as the southwestern sub-basin. Wavelet analysis results show the strength of significant fluctuation varies year to year. Trend analysis results indicate a decadal weakening in the deep SCS circulation. An anomalous anticyclonic circulation, 50–70 km apart from the slope break, tends to weaken the cyclonic boundary circulation in the western and northern boundaries as well as the southwestern sub-basin. This trend is similar to the observed decadal weakening in the Northern Atlantic deep circulation. Thus, the findings of this study reveal that the variability of the deep SCS circulation has a remarkable response to the climate change. The mechanisms responsible for the variability are worth pursuing if more observations are available.
Two drifting paths of Sargassum bloom in the Yellow Sea and East China Sea during 2019−2020
Chao Yuan, Jie Xiao, Xuelei Zhang, Mingzhu Fu, Zongling Wang
 doi: 10.1007/s13131-021-1894-z
[Abstract](551) [FullText HTML](234)
The macroalgal blooms of floating brown algae Sargassum horneri are increasing in the Yellow and East China Seas during the past few years. However, the annual pattern of Sargassum bloom is not well characterized. To study the developing pattern and explore the impacts from hydro-meteorologic environment, high resolution satellite imageries were used to monitor the distribution, coverage and drifting of the pelagic Sargassum rafts in the Yellow and East China Seas from September 2019 to August 2020. Sargassum blooms were detected from October 2019 to June 2020 and presented two successive drifting paths that both initiated from around 37°N. The first path spanned smaller spatial scale and shorter period, starting with a bloom of 3 km2 distribution area near the eastern tip of Shandong Peninsular in late October 2019 and drifted southwards, hit the Pyropia aquaculture area in early January 2020, then vanished in the northwest of East China Sea (ca. 32°N) around end of January. The second path began with a large distribution area of 23 000 km2 east of 123°E in late January 2020, firstly moved southwards in the central Yellow and northern East China Seas (north of 29°N) till late April, then turned northwards with monsoon wind and vanished in mid-June. The mean sea surface temperature of 8°C to 20°C in the Sargassum bloom areas corresponded to in situ observed temperature range for vegetative growth and floating of S. horneri. The surface temperature higher than 22°C in July-September prohibits Sargassum blooming or completing life cycle. The results provide insights to the future management of Sargassum blooms. Further studies are needed to validate the pattern and source of annual Sargassum bloom in the Yellow and East China Seas.
Diversity of protease-producing bacteria in the Bohai Bay sediment and their extracellular enzymatic properties
Zhenpeng Zhang, Chaoya Wu, Shuai Shao, Wei Liu, En-Tao Wang, Yan Li
 doi: 10.1007/s13131-020-1589-x
[Abstract](275) [FullText HTML](85)
Protease-producing bacteria play key roles in the degradation of organic nitrogen materials in marine sediments. However, their diversity, production of proteases and other extracellular enzymes, even in situ ecological functions remain largely unknown. In this study, we investigated the diversity of cultivable extracellular protease-producing bacteria in the sediments of the Bohai Bay. A total of 109 bacterial isolates were obtained from the sediments of 7 stations. The abundance of cultivable protease-producing bacteria was about 104 CFU/g of sediment in all the samples. Phylogenetic analysis based on 16S rRNA gene sequences classified all the isolates into 14 genera from phyla Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria, with Pseudoalteromonas (63/109, 57.8%), Bacillus (9/109, 8.2%), Sulfitobacter (8/109, 7.3%) and Salegentibacter (6/109, 5.5%) as the dominant taxa. Enzymatic inhibition tests indicated that all the tested isolates produced serine and/or metalloprotease, with only a small proportion producing cysteine and/or aspartic proteases. Several extracellular enzyme activities, including alginase, lipase, amylase and cellulose, and nitrate reduction were also detected for strains with higher protease activities. According the results, the protease-producing bacteria could also be participate in many biogeochemical processes in marine sediments. Our study broadened understanding and knowledge on the potential ecological functions of protease-producing bacteria in marine sediments.
2022, 41(4).  
[Abstract](62) [PDF 6643KB](13)
2022, 41(4): .  
[Abstract](93) [FullText HTML](44) [PDF 0KB](0)
Article$Physical Oceanography, Marine Meteorology and Marine Physics
Submesoscale motions and their seasonality in the northern Bay of Bengal
Lanman Li, Xuhua Cheng, Zhiyou Jing, Haijin Cao, Tao Feng
2022, 41(4): 1-13.   doi: 10.1007/s13131-021/1847-6
[Abstract](190) [FullText HTML](79) [PDF 2036KB](55)
The unbalanced submesoscale motions and their seasonality in the northern Bay of Bengal (BoB) are investigated using outputs of the high resolution regional oceanic modeling system. Submesoscale motions in the forms of filaments and eddies are present in the upper mixed layer during the whole annual cycle. Submesoscale motions show an obvious seasonality, in which they are active during the winter and spring but weak during the summer and fall. Their seasonality is associated with the mixed layer instability that depends on the mixed layer depth (MLD). During the winter, the MLD provides a much greater reservoir of the available potential energy, which promotes mixed layer instability to develop active submesoscale motions. The variations of MLD are likely modulated by the larger scale motions and the influxes of freshwater. Further investigations imply that the MLD and the stratified barrier layer are combined to determine the vertical structure of the submesoscale motions. The shallow MLD and strong stratification below during the summer and fall seem to prevent the downward extension of submesoscale motions. But in spring when the weak stratification exists, the penetration depth exceeds the base of the barrier layer.
Seasonal and spatial variations of kinematic parameters of internal solitary waves in the Andaman Sea
Yuqi Wu, Jieshuo Xie, Jiexin Xu, Zhiwu Chen, Yinghui He, Shuqun Cai
2022, 41(4): 14-22.   doi: 10.1007/s13131-021-1854-7
[Abstract](181) [FullText HTML](58) [PDF 10378KB](22)
The horizontally variable density stratification and background currents are taken into the variable-coefficent extended Korteweg-de Vries (evKdV) theory to obtain the geographical and seasonal distribution of kinematic parameters of internal solitary waves in the Andaman Sea (AS). The kinematic parameters include phase speed, dispersion parameter, quadratic and cubic nonlinear parameters. It shows that the phase speed and dispersion parameter are mainly determined by the topographic feature and have limited seasonal variation. The maximum phase speed is 2.6 m/s, which occurs in the cool season (November) in the middle of the AS, while the phase speed in the cool season is slightly larger than those in other seasons, up to 11.4% larger than that in the rainy season (July) in the southern AS. The dispersion parameter in the cool season can be 22.3% larger than that in the hot season. The nonlinear parameters have significant seasonal variation, and they can even change their signs at the continental slope in the north of the AS, from season to season. Meanwhile, the algebraic solitons dominate in the AS with minimum amplitudes (aal) ranging from 0.1 m to 102 m, and the maximum aal occurs in the cool season in the southern AS. The effect of the background flow on the parameters is also studied. The background flow has a great influence on the nonlinear parameters, e.g., the value of cubic nonlinear parameter can be reduced by 1/3 when the background flow is not considered.
Spatiotemporal variation and mechanisms of temperature inversion in the Bay of Bengal and the eastern equatorial Indian Ocean
K M Azam Chowdhury, Wensheng Jiang, Guimei Liu, Md Kawser Ahmed, Shaila Akhter
2022, 41(4): 23-39.   doi: 10.1007/s13131-021-1873-4
[Abstract](344) [FullText HTML](125) [PDF 2302KB](47)
In the northern Bay of Bengal, the existence of intense temperature inversion during winter is a widely accepted phenomenon. However, occurrences of temperature inversion during other seasons and the spatial distribution within and adjacent to the Bay of Bengal are not well understood. In this study, a higher resolution spatiotemporal variation of temperature inversion and its mechanisms are examined with mixed layer heat and salt budget analysis utilizing long-term Argo (2004 to 2020) and RAMA (2007 to 2020) profiles data in the Bay of Bengal and eastern equatorial Indian Ocean (EEIO). Temperature inversion exists (17.5% of the total 39 293 Argo and 51.6% of the 28 894 RAMA profiles) throughout the year in the entire study area. It shows strong seasonal variation, with the highest occurrences in winter and the lowest in spring. Besides winter inversion in the northern Bay of Bengal, two other regions with frequent temperature inversion are identified in this study for the first time: the northeastern part of the Bay of Bengal and the eastern part of the EEIO during summer and autumn. Driving processes of temperature inversion for different subregions are revealed in the current study. Penetration of heat (mean ~25 W/m2) below the haline-stratified shallow mixed layer leads to a relatively warmer subsurface layer along with the simultaneous cooling tendency in mixed layer, which controls more occurrence of temperature inversion in the northern Bay of Bengal throughout the year. Comparatively lower cooling tendency due to net surface heat loss and higher mixed layer salinity leaves the southern part of the bay less supportive to the formation of temperature inversion than the northern bay. In the EEIO, slightly cooling tendency in the mixed layer along with the subduction of warm-salty Arabian Sea water beneath the cold-fresher Bay of Bengal water, and downwelling of thermocline creates a favorable environment for forming temperature inversion mainly during summer and autumn. Deeper isothermal layer depth, and thicker barrier layer thickness intensify the temperature inversion both in the Bay of Bengal and EEIO.
Surface available gravitational potential energy in the world oceans
Ruixin Huang, Bo Qiu, Zhiyou Jing
2022, 41(4): 40-56.   doi: 10.1007/s13131-021-1852-9
[Abstract](166) [FullText HTML](54) [PDF 19430KB](21)
Satellite altimetry observations, including the upcoming Surface Water and Ocean Topography mission, provide snapshots of the global sea surface high anomaly field. The common practice in analyzing these surface elevation data is to convert them into surface velocity based on the geostrophic approximation. With increasing horizontal resolution in satellite observations, sea surface elevation data will contain many dynamical signals other than the geostrophic velocity. A new physical quantity, the available surface potential energy, is conceptually introduced in this study defined as the density multiplied by half of the squared deviation from the local mean reference surface elevation. This gravitational potential energy is an intrinsic property of the sea surface height field and it is an important component of ocean circulation energetics, especially near the sea surface. In connection with other energetic terms, this new variable may help us better understand the dynamics of oceanic circulation, in particular the processes in connection with the free surface data collected through satellite altimetry. The preliminary application of this concept to the numerically generated monthly mean Global Ocean Data Assimilation System data and Archiving, Validation, and Interpretation of Satellite Oceanographic altimeter data shows that the available surface potential energy is potentially linked to other dynamic variables, such as the total kinetic energy, eddy kinetic energy and available potential energy.
Prediction of visibility in the Arctic based on dynamic Bayesian network analysis
Shijun Zhao, Yulong Shan, Ismail Gultepe
2022, 41(4): 57-67.   doi: 10.1007/s13131-021-1826-z
[Abstract](98) [FullText HTML](27) [PDF 1020KB](7)
With the accelerated warming of the world, the safety and use of Arctic passages is receiving more attention. Predicting visibility in the Arctic has been a hot topic in recent years because of navigation risks and opening of ice-free northern passages. Numerical weather prediction and statistical prediction are two methods for predicting visibility. As microphysical parameterization schemes for visibility are so sophisticated, visibility prediction using numerical weather prediction models includes large uncertainties. With the development of artificial intelligence, statistical prediction methods have received increasing attention. In this study, we constructed a statistical model with a physical basis, to predict visibility in the Arctic based on a dynamic Bayesian network, and tested visibility prediction over a 1°×1° grid area averaged daily. The results show that the mean relative error of the predicted visibility from the dynamic Bayesian network is approximately 14.6% compared with the inferred visibility from the artificial neural network. However, dynamic Bayesian network can predict visibility for only 3 days. Moreover, with an increase in predicted area and period, the uncertainty of the predicted visibility becomes larger. At the same time, the accuracy of the predicted visibility is positively correlated with the time period of the input evidence data. It is concluded that using a dynamic Bayesian network to predict visibility can be useful over Arctic regions for projected climatic changes.
Contraction and warming of Antarctic Bottom Water in the Amundsen Sea
Yu Bai, Liang Zhao, Jingen Xiao, Shiying Lin
2022, 41(4): 68-79.   doi: 10.1007/s13131-021-1829-8
[Abstract](166) [FullText HTML](50) [PDF 1856KB](14)
Antarctic Bottom Water (AABW) plays an important role in the meridional overturning circulation and contributes significantly to global heat transport and sea level rise (SLR). Based on the Global Ocean (1/12)° Physical Reanalysis (GLORYS12V1) products and conductivity-temperature-depth instrument data from the World Ocean Circulation Experiment hydrographic program, we analyzed the trends in the thickness, volume, temperature, salinity, and neutral density of the AABW in the Amundsen Sea from 1993 to 2017. Over the past 25 years, the volume has decreased by 3.45×1012 m3/a, thinning at a rate of 5 m/a. In the vertical direction, the contraction of the AABW is compensated by the volume expansion of the Circumpolar Deep Water. As the volume of AABW decreases, the temperature of the AABW increases by about 0.002°C/a. This warming is equivalent to a heat flux of 0.27 W/m2. A local SLR is produced due to thermal expansion of 0.35 mm/a. During the study period, the neutral density decreased by 0.000 3 kg/(m3∙a) due to warming. In the horizontal direction, the volume of AABW flowing from the Ross Sea into the Amundsen Sea gradually decreases and the temperature of the AABW increases continuously. The horizontal transport loss of the AABW volume is 4.07×1014 m3 and the horizontal heat transport results in a 0.03°C increase in the temperature of the AABW.
Article$Marine Chemistry
Geochemical speciation and spatial distributions of phosphorus in surface sediments from the basin of the Marcus-Wake seamounts in the western Pacific Ocean
Chao Yuan, Fansheng Meng, Xuying Yao, Jianyu Ni
2022, 41(4): 80-90.   doi: 10.1007/s13131-021-1942-8
[Abstract](143) [FullText HTML](42) [PDF 924KB](15)
The concentrations of five forms of phosphorus (P) including exchangeable or loosely adsorbed P (Ex-P), Fe-bound P (Fe-P), authigenic P (Auth-P), detrital P (Det-P), and organic P (Org-P) from the basin among the Marcus-Wake seamounts (19.4°–24°N, 156.5°–161.5°E) in the western Pacific Ocean were quantified using a sequential extraction method (SEDEX) to investigate the distribution and sources of different P species. Concentrations of total P (TP) varied from 14.0 μmol/g to 44.1 μmol/g, with an average of (32.4±7.7) μmol/g. Inorganic phosphorus, which was the major chemical form of sedimentary P, ranged from 12.6 μmol/g to 40.6 μmol/g, while the concentration of Org-P varied between 1.38 μmol/g and 5.18 μmol/g, accounting for 83.4%–93.4% and 6.6%–16.6% of the TP, respectively. The relative proportions of the five P species followed the order of Det-P>Auth-P>Org-P>Fe-P>Ex-P. On average, Det-P was the major P sink resulted from the atmospheric input and accounted for approximately 58.9%±12.4% of the TP. Auth-P and Org-P comprised 22.8%±11.4% and 11.5%±3.0% of the TP, respectively, while Fe-P accounted for 5.1%±2.6%. Lastly, Ex-P comprised 1.6%±0.3% of the TP. Org-P exhibited a negative correlation with Fe-P and Auth-P, while Fe-P showed a positive correlation with Auth-P. This indicated that the formation of Fe-P and Auth-P was at the expense of the regeneration or remineralization of Org-P during early diagenesis. High concentrations of Det-P and Auth-P as well as a low ratio of total organic C to reactive P (TOC/Rea-P) suggested that the aeolian input may play a significant role in sedimentary P budget in the study area. Additionally, well-oxygenated bottom water and low sedimentation rate could be responsible for the low TOC/Org-P ratio in the sediment.
Article$Marine Geology
Experiment and analysis of the formation, expansion and dissipation of gasbag in fine sediments based on pore water pressure survey
Shiyun Lei, Xiujun Guo, Haoru Tang
2022, 41(4): 91-100.   doi: 10.1007/s13131-021-1851-x
[Abstract](105) [FullText HTML](17) [PDF 1053KB](5)
Deep-seated gas in seabed sediments migrates upwards from effect of external factors, which easily accumulates to form gasbags at interface of shallow coarse-fine sediments. Real-time monitoring of this process is important to predict disaster. However, there is still a lack of effective monitoring methods, so we attempt to apply multi-points pore water pressure monitoring technology when simulating forming and dissipation of gasbags in sediments through laboratory experiment. This study focuses on discussion of sensitivity of pore water pressure monitoring data, as well as typical changing characteristics and mechanisms of excess pore water pressure corresponding to crack generation, gasbag formation and gas release. It was found that the value of excess pore water pressure in sediments is negatively correlated with vertical distance between sensors and gas source, and the evolution of gasbag forming and dissipation has a good corresponding relationship with the change of excess pore water pressure. Gasbag formation process is divided into three stages: transverse crack development, longitudinal cavity expansion, and oblique crack development. Formation of gasbag begins with the transverse crack at the interface of coarse-fine sediments while excess pore water pressure attenuates rapidly and then drops, pressure remains almost unchanged when cavity expanses longitudinally, oblique crack appeared in final stage of gasbag evolution while excess pore water pressure accumulated and dissipated again. The variation curve of excess pore water pressure in gas release stage has saw-tooth fluctuation characteristics, and the value and time of pressure accumulation are also fluctuating, indicating the uncertainty and non-uniqueness of gas migration channels in sediments.
Article$Ocean Engineering
Forecasting the western Pacific subtropical high index during typhoon activity using a hybrid deep learning model
Jianyin Zhou, Mingyang Sun, Jie Xiang, Jiping Guan, Huadong Du, Lei Zhou
2022, 41(4): 101-108.   doi: 10.1007/s13131-021-1965-1
[Abstract](132) [FullText HTML](41) [PDF 2419KB](9)
Seasonal location and intensity changes in the western Pacific subtropical high (WPSH) are important factors dominating the synoptic weather and the distribution and magnitude of precipitation in the rain belt over East Asia. Therefore, this article delves into the forecast of the western Pacific subtropical high index during typhoon activity by adopting a hybrid deep learning model. Firstly, the predictors, which are the inputs of the model, are analysed based on three characteristics: the first is the statistical discipline of the WPSH index anomalies corresponding to the three types of typhoon paths; the second is the correspondence of distributions between sea surface temperature, 850 hPa zonal wind (u), meridional wind (v), and 500 hPa potential height field; and the third is the numerical sensitivity experiment, which reflects the evident impact of variations in the physical field around the typhoon to the WPSH index. Secondly, the model is repeatedly trained through the backward propagation algorithm to predict the WPSH index using 2011–2018 atmospheric variables as the input of the training set. The model predicts the WPSH index after 6 h, 24 h, 48 h, and 72 h. The validation set using independent data in 2019 is utilized to illustrate the performance. Finally, the model is improved by changing the CNN2D module to the DeCNN module to enhance its ability to predict images. Taking the 2019 typhoon “Lekima” as an example, it shows the promising performance of this model to predict the 500 hPa potential height field.
The model of tracing drift targets and its application in the South China Sea
Yang Chen, Shouxian Zhu, Wenjing Zhang, Zirui Zhu, Muxi Bao
2022, 41(4): 109-118.   doi: 10.1007/s13131-021-1943-7
[Abstract](194) [FullText HTML](56) [PDF 2048KB](16)
A Leeway-Trace model was established for the traceability analysis of drifting objects at sea. The model was based on the Leeway model which is a Monte Carlo-based ensemble trajectory model, and a method of realistic traceability analysis was proposed in this study by using virtual spatiotemporal drift trajectory prediction. Here, measured data from a drifting buoy observation experiment in the northern South China Sea in April 2019, combined with surface current data obtained from the finite volume community ocean model (FVCOM), were used for the traceability analysis of humanoid buoys. The results were basically consistent with the observations, and the assimilation of measured current data can significantly improve the accuracy of the traceability analysis. Several sensitive experiments were designed to discuss the effects of wind and tide on the traceability analysis, and their results showed that the wind-driven current and the wind-induced leeway drift are both important to the traceability analysis. The effect of tidal currents on traceability could not be ignored even though they were much weaker than the residual currents in the experimental area of the northern South China Sea.
A case study on the soil classification of the Yellow River Delta based on piezocone penetration test
Jiarui Zhang, Qingsheng Meng, Lei Guo, Yan Zhang, Guanli Wei, Tao Liu
2022, 41(4): 119-128.   doi: 10.1007/s13131-021-1944-6
[Abstract](201) [FullText HTML](55) [PDF 1474KB](20)
Piezocone penetration test (CPTu), the preferred in-situ tool for submarine investigation, is significant for soil classification and soil depth profile prediction, which can be used to predict soil types and states. However, the accuracy of these methods needs to be validated for local conditions. To distinguish and evaluate the properties of the shallow surface sediments in Chengdao area of the Yellow River Delta, seabed CPTu tests were carried out at ten stations in this area. Nine soil classification methods based on CPTu data are applied for soil classification. The results of classification are compared with the in-situ sampling to determine whether the method can provide sufficient resolution. The methods presented by Robertson (based on soil behavior type index Ic), Olsen and Mitchell are the more consistent and compatible ones compared with other methods. Considering that silt soils have potential to liquefy under storm tide or other adverse conditions, this paper is able to screen soil classification methods suitable for the Chengdao area and help identify the areas where liquefaction or submarine landslide may occur through CPTu investigation.
Article$Marine Technology
Ocean wave parameters retrieved directly from compact polarimetric SAR data
Yu Liu, Yijun He, Biao Zhang
2022, 41(4): 129-137.   doi: 10.1007/s13131-021-1855-6
[Abstract](113) [FullText HTML](27) [PDF 1706KB](5)
We aim to directly invert wave parameters by using the data of a compact polarimetric synthetic aperture radar (CP SAR) and validate the effectiveness of ocean wave parameter retrieval from the circular transmit/linear receive mode and π/4 compact polarimetric mode. Relevant data from the RADARSAT-2 fully polarimetric SAR on the C-band were used to obtain the compact polarimetric SAR images, and a polarimetric SAR wave retrieval algorithm was used to verify the sea surface wave measurements. Using the data and algorithm, there is no need to estimate complex hydrodynamic modulation transfer functions, even at large radar incidence angles. First, the radar backscattering cross-sections and backscattering cross-section of the radar linearly polarized with any polarization orientation angle were calculated in the two compact polarimetric SAR modes. Then, the wave slopes along the azimuth direction and the range direction were calculated directly using CP SAR data. Finally, we obtained the slope spectrum of the wave from the estimated wave slopes along azimuth and range directions. The wave parameters extracted from the synthetic wave slope spectrum were compared with those obtained from buoy observations of the National Data Buoy Center, verifying a suitable agreement.
A novel synthetic aperture radar scattering model for sea surface with breaking waves
Xiaochen Wang, Yuxin Hu, Bing Han, Wei Tian, Chunhua Zhang
2022, 41(4): 138-145.   doi: 10.1007/s13131-021-1842-y
[Abstract](82) [FullText HTML](18) [PDF 1368KB](5)
In this study a novel synthetic aperture radar (SAR) scattering model for sea surface with breaking waves is proposed. Compared with existing models, the proposed model considers an empirical relationship between wind speed and wave breaking scattering to present the contribution of wave breaking. Moreover, the scattering weight factor p, and wave breaking rate q, are performed to present the contribution of the quasi-specular scattering term, Bragg scattering term, and wave breaking scattering term to the total scattering from the sea surface. To explore the modeling accuracy of sea-surface scattering, a simulated normalized radar cross-section (NRCS) and measured NRCS are compared. The proposed model generated the simulated NRCS and a matching GF-3 dataset was used for the measured NRCS. It was revealed that the performance of the VV polarization of our model was much better than that of HH polarization, with a correlation of 0.91, bias of −0.14 dB, root mean square error (RMSE) of 1.26 dB, and scattering index (SI) of −0.11. In addition, the novel model is explored and compared with the geophysical model of CMODs and satellite-measured NRCS from GF-3 SAR wave mode imagery. For an incidence angle 40°–41°, the relationship between the NRCS and wind speed, relative wind direction is proposed. As with the SAR-measured NRCS, the performance of VV polarization was much better than HH polarization, with a correlation of 0.99, bias of −0.25 dB, RMSE of 0.64 dB, and SI of −0.04.
Article$Marine Information Science
Spatio-temporal analysis of the melt onset dates over Arctic sea ice from 1979 to 2017
Shuang Liang, Jiangyuan Zeng, Zhen Li, Dejing Qiao
2022, 41(4): 146-156.   doi: 10.1007/s13131-021-1827-x
[Abstract](110) [FullText HTML](29) [PDF 3628KB](7)
The melt onset dates (MOD) over Arctic sea ice plays an important role in the seasonal cycle of sea ice surface properties, which impacts Arctic surface solar radiation absorbed by the ice-ocean system. Monitoring interannual variations in MOD is valuable for understanding climate change. In this study, we investigated the spatio-temporal variability of MOD over Arctic sea ice and 14 Arctic sub-regions in the period of 1979 to 2017 from passive microwave satellite data. A set of mathematical and statistical methods, including the Sen’s slope and Mann-Kendall mutation tests, were used to comprehensively assess the variation trend and abrupt points of MOD during the past 39 years for different Arctic sub-regions. Additionally, the correlation between Arctic Oscillation (AO) and MOD was analyzed. The results indicate that: (1) all Arctic sub-regions show a trend toward earlier MOD except the Bering Sea and St. Lawrence Gulf. The East Siberian Sea exhibits a significantly earlier trend, with the highest rate of −9.45 d/decade; (2) the temporal variability and statistical significance of MOD trend exhibit large interannual differences with different time windows for most regions in the Arctic; (3) during the past 39 years, the MOD changed abruptly in different years for different sub-regions; (4) the seasonal AO has more influence on MOD than monthly AO. The findings in this study can improve our knowledge of MOD changes and are beneficial for further Arctic climate change study.

Latest Issues