Seasonal structure and interannual variation of the South Equatorial Current in the Indian Ocean

The South Equatorial Current (SEC) in the south Indian Ocean (SIO) contributes to mass and heat exchanges among the Pacific, Indian, and Atlantic Oceans. By analyzing satellite and in situ observations, this study examines the seasonal structure and the interannual variability of the SEC. The SEC is mainly part of the Seychelles-Chagos thermocline ridge (SCTR) circulation during December to April, and is composed of the internal SCTR circulation and the intrusive Indonesian Throughflow during May to November. The SEC thus presents straight and meandering routes in these two periods, respectively. Furthermore, observations and model experiments show that the SEC's meandering route has obvious interannual variability, with exceptional northwestward extension in 1994, 1997, and 2019 during the past three decades. These exceptional routes are caused primarily by extreme positive Indian Ocean Dipole (IOD) events, which induce a strengthened westward transport of the fresh water. These exceptional routes thus reflect the abnormal freshening in the SIO. Mooring observations reveal that a significant long-term freshening event occurred in the SIO from August 2019 to at least July 2020. The persistent low salinity confirms that there is substantial water exchange in the SIO during the extreme positive IOD years.