Interannual Variability of the South Equatorial Current in the Southeast Indian Ocean Associated with El Niño–Southern Oscillation

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Abstract Interannual variability of the South Equatorial Current (SEC) in the southeast Indian Ocean is investigated using Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO) and Global Ocean Reanalysis and Simulations (GLORYS) data from January 1993 to December 2022 and a 1.5-layer reduced-gravity model. Interannual variability of the SEC averaged over the upper 200 m in the southeast Indian Ocean is influenced by El Niño–Southern Oscillation (ENSO). The SEC has a significant positive correlation with the Niño-3.4 index and lags the Niño-3.4 index by 12–15 months, with the SEC weakening significantly during the decaying years of El Niño events and vice versa for La Niña events. Mechanistic studies suggest that during the developing years of El Niño events, the northward sea level anomaly (SLA) pressure gradient between the negative SLA off Sumatra–Java coast and the positive SLA of SEC region (90°–115°E, 9°–13°S) strengthens the SEC under geostrophic equilibrium. During the decaying years of El Niño events, the negative SLA in the tropical western Pacific Ocean propagates through the Maritime Continent to the SEC region. The SLA pressure gradient between the Sumatra–Java coast and the SEC region reverses to southward. This induces an eastward geostrophic current anomaly under geostrophic equilibrium, significantly weakening the SEC. The converse holds for La Niña events. Local wind in the southeast Indian Ocean has a suppressive effect on the SLA propagated to this area. The results of the 1.5-layer reduced-gravity model verify these phenomena, and the westward-propagating SLA from the Maritime Continent induced by the Pacific wind plays a dominant role in the SLA variation in the SEC region from model experiment. This study can improve the understanding of the effects of ENSO processes on ocean circulation in the southeast Indian Ocean.