Large South Equatorial Current Meander in the Southeastern Tropical Indian Ocean Captured by Surface Drifters Deployed in 2019.

To investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander. Plain Language Summary: The South Equatorial Current (SEC) is westward in the tropical Indian Ocean. During boreal spring 2019, 18 drifters were deployed off Sumatra‐Java, and their trajectories show a large SEC meander, with a typical drifter moved southward over 1,100 km during 40 days along ∼100°E. The geostrophic component, associated with the Seychelles‐Chagos thermocline ridge and an anticyclonic eddy/Rossby wave, and ageostrophic component, induced by southeasterly wind, jointly drive drifters to move southward. Further diagnoses of historical drifters show that the ageostrophic component could take up to 45% contribution in the southward movement in the SEC meander. Key Points: South Equatorial Current in the Indian Ocean advected drifters southward more than 1,100 km within 40 days (∼32 cm s−1) in 2019Combined effects of the zonal pressure gradient and the southeasterly wind contribute to the southward movementSouthward movement usually occurs in spring and summer, contributing from ageostrophic current up to ∼45% [ABSTRACT FROM AUTHOR]