A Large Subsurface Anticyclonic Eddy in the Eastern Equatorial Indian Ocean

Abstract

Abstract 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 24.7σ 0 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.