Figure  1.  Schematic showing the zonal transection of surface and subsurface eddies (in the northern hemisphere). The blue lines indicate the sea-level anomaly. The black lines indicate the isopycnals. The color shading indicates the meridional current velocity generated by eddies. The white dots and crosses indicate the southward and northward current directions, respectively.

Figure  2.  The observation area and the sea-level anomaly (SLA) during observation. a. Map of the observation area in the southeastern tropical Indian Ocean. Black contours indicate the 200-m isobath. b. Color codes showing the SLA on December 3, 2017. Black solid and dashed contours show the outermost enclosed SLA isolines of the surface anticyclonic eddy (AE) and cyclonic eddy (CE), respectively. Black arrows represent geostrophic current vectors. Red arrows show the current vectors measured by vessel-mounted acoustic Doppler current profiler (VADCP) at the 33-m depth (first layer). The red triangles represent the Conductivity-Temperature-Depth stations. The red circle in a and b represents the location of the cruise-observed subthermocline eddy. The red straight lines in a and b represent the transect for VADCP observation. ITF: Indonesian Throughflow.

Figure  3.  Vertical distribution of the current velocity and the Rossby number. a. Meridional velocity measured along the vessel-mounted acoustic Doppler current profiler (VADCP) transect. Blue and red dashed boxes represent the western and eastern sides of the surface anticyclonic eddy (AE), respectively. The black dashed box endorses the cruise-observed subthermocline eddy (STE). Blue and red vertical lines show positions of maximum southward and northward velocities, respectively. Locations of Conductivity-Temperature-Depth stations are denoted at the top of the panel. b and c are the same as a, but for the zonal velocity and the Rossby number Ro, respectively. d. The blue and red lines represent the horizontally averaged meridional velocity profiles of the western and eastern sides of the AE, respectively. e. The blue and red lines represent the STE-induced meridional velocity anomaly (the VADCP-measured velocity minus the horizontally averaged meridional velocity profiles of the eastern side of the AE) at the position marked by the blue and red lines in a, respectively. f. Vertical distribution of the Ro averaged between the blue and red lines indicated in a and c. The two horizontal dashed lines indicate the e-folding vertical length scale of the STE.

Figure  4.  Vertical distribution of the temperature, salinity, and their anomalies. a. Depth-longitude plot of water temperature observed along the Conductivity-Temperature-Depth transect. The black contours represent the isothermals of 11℃, 13℃, 16℃, and 20℃. The white contours from up and down denote isopycnals of 25 σ₀, 26 σ₀, and 26.50 σ₀, respectively. The thick (thin) blue and red contours indicate the subthermocline eddy (STE)-induced meridional velocity anomaly isolines of −0.3 m/s (−0.2 m/s) and 0.3 m/s (0.2 m/s), respectively. The black dashed box shows the cruise-observed STE area. Observation station locations are denoted at the panel top. The image in b is the same as the image a, but for the salinity. Black contour represents the isohaline of 34.55. c. The temperature anomaly generated by the cruise-observed STE, that is, the temperature at s-1 minus that at s-4. The image in d is the same as image c, but for the salinity anomaly.

Figure  5.  The background salinity and potential temperature and salinity (T-S) diagrams of observation and background. a. Annual mean salinity distribution of the BOA_ARGO on the 26.0 σ₀ surface in the south Indian Ocean. The red circle represents the location of the cruise-observed subthermocline eddy (STE). b. Depth-longitude plot of annual mean salinity of the BOA_ARGO along 10°S. The black bar represents the location of the cruise-observed STE. The white contours from top to bottom represent the isopycnals of 25.0 σ₀, 26.0 σ₀, and 26.5 σ₀, respectively. The red contour in a and b represent isohaline of 34.55 (isohaline of 34.55 was discontinuous in a because of the annual mean Argo data that was unavailable near the coast). c. T-S diagrams of water mass within the areas of red and green dashed boxes in a are shown as red and green shadings, respectively. The red dotted line shows the T-S distribution in the cruise-observed STE center (station s-1), and the blue dotted line represents the results measured out of the STE (station s-4). The contours show the potential density. The vertical dashed line indicates the isohaline of 34.55.

Figure  6.  Spatial distribution and potential temperature and salinity (T-S) diagrams of the identified low-salinity Argo profiles. a. Spatial distribution of the identified low-salinity Argo profiles (black dots). The red and green dots show the location of the cruise- and Argo-observed subthermocline eddy (STE), respectively. The color codes show the topography. b. T-S plot. The gray dots represent the T-S distribution of the identified low-salinity Argo profiles corresponding to the black dots in a. Red and green dotted lines show the observed T-S distributions within the cruise-observed (at station s-1) and Argo-observed (on March 26, 2006) STE, respectively.

Figure  7.  The Argo float trajectory and the sea-level anomaly (SLA). The yellow line indicates the trajectory of the Argo float with platform number 5900860. The red part indicates the position where the Argo-observed subthermocline eddy (STE) was detected. Color codes show SLA on March 26, 2006, when the Argo float was at the location indicated by the red star (also in the inset). Black arrows represent geostrophic current vectors. Black solid and dashed contours show the outermost enclosed SLA isolines of the surface anticyclonic eddy and cyclonic eddy, respectively. The red circle indicates the cruise-observed STE. The observation area is shown in an inset on the bottom right corner. ITF: Indonesian Throughflow.

Figure  8.  Depth-time plots of temperature (a) and salinity (b) of the Argo-observed STE. Black contours denote isotherms of 16℃, 13℃, and 11℃ in a, and isohalines of 34.55 in b, respectively. White contours denote isopycnals of 25 σ₀, 26 σ₀, and 26.50 σ₀. c and d are the temperature and salinity anomalies generated by the Argo-observed STE (the temperature and salinity observed in the STE, minus those observed out of the STE).

Figure  9.  The relative locations between low-salinity profiles (LSPs) and surface eddies and the distribution of different LSPs. a–d. Examples showing the relative locations between LSPs and surface eddies. The white stars indicate the positions of LSPs. The green and red circles indicate the adjacent surface cyclonic eddy (CE) and anticyclonic eddy (AE), respectively. Color codes show sea-level anomaly (SLA) on the day indicated in the upper left corner. a. The two stars indicate the stations s-1 and s-2, which are located inside the cruise-observed subthermocline eddy (STE) and outside of the adjacent surface CE. The white points indicate stations s-3, s-4, and s-5 from left to right. b. The star indicates the LSP located inside the Argo-observed STE and outside of adjacent surface eddies. c. An LSP, captured by Argo float with platform number 5900462, is located inside a surface CE. d. An LSP, captured by Argo float with platform number 5905018, is located inside a surface AE. e. The distribution of LSPs located inside surface CEs, AEs, and outside the surface eddies. The color codes show the eddy kinetic energy (EKE).

Figure  10.  The zonal distribution of low-salinity profiles (LSPs). a. The blue, red, and yellow bars show the zonal distribution of the number of LSPs located inside cyclonic eddys (CEs), anticyclonic eddys (AEs), and outside of surface eddies, respectively. The green line indicates the zonal distribution of the number of the total Argo profiles. The zonal bin is 2°. b. The blue, red, and yellow bars show the percentage of LSPs located inside CEs, AEs, and outside of surface eddies in the total Argo profiles. The green line indicates the ratio of the LSPs located outside surface eddies (OP) to that located inside surface eddies (IP). The green line becomes a dotted line when the Argo float number is relatively small. The black line indicates the eddy kinetic energy (EKE) along 10°S.

Figure  11.  Temperature (a) and salinity (b) profiles observed at the five stations. The local observation time (DD-HH) is denoted.