Characteristics and mechanisms of the intraseasonal variability of sea surface salinity in the southeastern Arabian Sea during 2015–2020
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Abstract: Based on Soil Moisture Active Passive sea surface salinity (SSS) data from April 2015 to August 2020, combined with Objectively Analyzed Air-Sea Heat Flux and other observational data and Hybrid Coordinate Ocean Model (HYCOM) data, this work explores the characteristics and mechanisms of the intraseasonal variability of SSS in the southeastern Arabian Sea (SEAS). The results show that the intraseasonal variability of SSS in the SEAS is very significant, especially the strongest intraseasonal signal in SSS, which is located along the northeast monsoon current (NMC) path south of the Indian Peninsula. There are remarkable seasonal differences in intraseasonal SSS variability, which is very weak in spring and summer and much stronger in autumn and winter. This strong intraseasonal variability in autumn and winter is closely related to the Madden-Julian Oscillation (MJO) event during this period. The northeast wind anomaly in the Bay of Bengal (BOB) associated with the active MJO phase strengthens the East India Coastal Current and NMC and consequently induces more BOB low-salinity water to enter the SEAS, causing strong SSS fluctuations. In addition, MJO-related precipitation further amplifies the intraseasonal variability of SSS in SEAS. Based on budget analysis of the mixed layer salinity using HYCOM data, it is shown that horizontal salinity advection (especially zonal advection) dominates the intraseasonal variability of mixed layer salinity and that surface freshwater flux has a secondary role.
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Figure 1. Annual mean Soil Moisture Active Passive (SMAP) sea surface salinity (SSS) and Ocean Surface Current Analysis Real-time surface currents (arrows; unit: m/s) (a), root mean square (RMS) of SMAP SSS and RMS of evaporation minus precipitation (E−P, contours; unit: mm/d) (b), intraseasonal RMS of SMAP SSS (c), and percentage of intraseasonal RMS to total RMS for SSS (d). The purple line represents the pathway of the northeast monsoon current (NMC) around Sri Lanka, and the orange line represents the pathway of the East India Coastal Current (EICC), and the blue line represents the pathway of the West India Coastal Current (WICC). BOB: Bay of Bengal.
Figure 2. Seasonal distribution for the intraseasonal root mean square (RMS) of SMAP sea surface salinity (SSS): spring (March–May, MAM) (a), summer (June–August, JJA) (b), autumn (September–November, SON) (c), and winter (December–February, DJF) (d). The contours in c and d represent the ratio of intraseasonal RMS to total RMS for SSS.
Figure 3. Seasonal distribution of the intraseasonal root mean square (RMS) of OSCAR zonal currents and of E−P (contours; unit: mm/d): spring (a), summer (b), autumn (c), and winter (d). The areas bounded by thick green curves represent the regions with an intraseasonal SSS RMS larger than 0.35. Region A (4°–6°N, 75°–80°E) bounded by purple lines in d represents a region with strong intraseasonal variability in the southeastern Arabian Sea.
Figure 4. Time series averaged in region A (shown in Fig. 3d) for: sea surface salinity (SSS) anomaly (blue curves) and E−P anomaly (red curves) (a), SSS anomaly and zonal current anomaly (b). The blue line is 1.5 times the standard deviation of the SSS anomaly. The dark points represent the lowest anomalous salinity for the strong intraseasonal SSS events. cor: correlation coefficient.
Figure 5. Composite surface wind anomaly (arrows, unit: m/s) and outgoing longwave radiation anomaly from −40 d to +40 d with 10-d intervals for the four strong intraseasonal events listed in Fig. 4. The date with the minimum salinity of each event is selected as the reference time (0 d).
Figure 6. Composite sea surface salinity (SSS) anomalies and surface current anomalies (arrows, unit: m/s) from −40 d to +40 d with 10-d intervals for the four strong intraseasonal events listed in Fig. 4. The date with the minimum salinity of each event is selected as the reference time (0 d).
Figure 7. Composite E−P anomalies (contours, unit: mm/d) from −40 d to +40 d with 10-d intervals for the four strong intraseasonal events listed in Fig. 4. The date with the minimum salinity of each event is selected as the reference time (0 d). Composite sea surface salinity (SSS) anomalies are also shown for comparison.
Figure 8. Soil Moisture Active Passive (SMAP) SSS and Hybrid Coordinate Ocean Model (HYCOM) SSS (contours) for winter during 2015 and 2018 (a), RMS of HYCOM SSS (b), intraseasonal RMS of HYCOM SSS (c), and time series of the HYCOM SSS anomaly (blue curves) and SMAP SSS anomaly (red curves) of region A (d). The purple shading and yellow shading in d represents Stage 1 and Stage 2, respectively. The red (blue) dashed line represents 1.5 times the standard deviation of HYCOM (SMAP) SSS.
Figure 9. Evolution process of the HYCOM SSS anomaly and surface current anomaly (arrows; unit: m/s) from −40 d to +40 d with 10-d intervals for the strong intraseasonal event in 2016 winter. The date with the minimum salinity of this event is selected as the reference time (0 d).
Figure 10. Salinity budget components of the mixed layer salinity anomaly over region A for the strong intraseasonal event in 2016 winter calculated from the HYCOM output. In a, ST is the tendency of the mixed layer salinity anomaly; FW and ADV represent the tendencies of the mixed layer salinity anomaly caused by surface freshwater flux and horizontal advection, respectively; the sum of these two terms is denoted as SUM. ADVU and ADVV in b as shown in a but for zonal advection and meridional advection, respectively.
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