Subsurface eddies are pervasive in the ocean, but those in the eastern equatorial Indian Ocean are not well understood due to the lack of in‐situ observations. Using multiple‐platform observations and reanalysis product, we investigate the characteristics, evolutionary process, origin, and underlying generation mechanism of a subsurface anticyclonic eddy (SAE) in the eastern equatorial Indian Ocean during January‒February 2020. Results show that the horizontal radius of the SAE was slightly greater than 250 km, and the SAE core was located at ∼100‐m depth near the 24.7σ0 isopycnal. This warm, saline and oxygen‐rich SAE was weakly stratified and characterized by tilted lens structures. Further water‐mass tracing analysis indicates that the SAE shifted westward along 4°N with a mean propagation speed of 0.18 m/s from the northwest coast of Sumatra Island. The hydrographic characteristics of the SAE are similar to those of the strong Equatorial Undercurrent, it is speculated that the SAE was probably generated by the north branch flow as the eastward undercurrent reached Sumatra Island. Both lateral shear of undercurrent and conservation of potential vorticity were attributed to the formation of the observed SAE.
The North Pacific Subtropical Countercurrent (STCC) is a zonal current consisting of a weak eastward current near the surface and a westward current (north equatorial current, NEC) below. Previous studies have proved that this region is an extremely active region of the North Pacific eddy. The activity of mesoscale eddy in this region can directly or indirectly affect the periodic variations of the Kuroshio and the Chinese coastal waters. In order to study the structure of mesoscale eddies in this region and their impact on the upper ocean, five Petrel-II underwater gliders were conducted as a network to collect hydrographic properties from December 09 2020 to February 05, 2021. It is found that a cyclonic eddy passed through the glider network from east to west during the observation period, and the glider grid almost covers the whole eddy. The radius, amplitude and propagation velocity of the eddy were analyzed. The underwater gliders were equipped with CTD sensors, collected about 1200 full thermohaline profiles. The observation results indicated that the cyclonic eddy can influence over 400 m in the vertical, which caused the isotherm rising up to 40 m. Analysis based on satellite sea surface temperature (SST) and sea level anomaly (SLA) data suggest that the cyclonic eddy is probably locally generated.
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