The Deep-Penetrating South Equatorial Undercurrent in the Tropical South Indian Ocean

A narrow eastward jet is detected beneath the westward-flowing South Equatorial Current in the tropical South Indian Ocean (SIO) using in-situ observation and reanalysis data. This jet is dubbed the South Equatorial Undercurrent (SEUC). The SEUC exists across the basin from about 60 degrees to 105 degrees E, with a core centered around 8 degrees S in the west and shifted to approximately 10 degrees S in the east and vertically extending from 200 m to more than 2,000 m. The SEUC is a seasonal feature that mainly occurs from January to June, and gradually weakens in July and August. The SEUC exhibits similar features as Tsuchiya jets in the Pacific and Atlantic Oceans, but also has its own uniqueness. This is the first time the SEUC is detected in the Indian Ocean, contributing to our understanding of the current system in the SIO, and basin-scale and interbasin mass and property exchanges. Plain Language Summary The current system in the South Indian Ocean is critical for the Indian Ocean basin and interbasin mass and property exchanges. However, our knowledge on the subsurface currents remains limited because of the especially sparse observations. A typical feature of the South Indian Ocean is a westward flowing South Equatorial Current (SEC). In this study, using in-situ observation and reanalysis data, we report an eastward current beneath the SEC, named the South Equatorial Undercurrent (SEUC). We further examine the SEUC's spatial structures and temporal evolution.