Seasonal and interannual variations of mixed layer salinity in the southeast tropical Indian Ocean

In this study, seasonal and interannual variations of the mixed layer salinity (MLS) in the southeast tropical Indian Ocean (SETIO) are analyzed using satellite observations, historical data sets, and data-assimilating ocean model outputs. On the seasonal cycle, the MLS in the SETIO becomes fresher in austral winter and saltier in austral summer: between the Java-Lesser Sunda coast and the South Equatorial Current (SEC, 12 degrees S), where positive entrainment and fresh advections counterbalance each other, the annual cycle of the MLS closely follows the variation of the air-sea freshwater forcing; off the northwest and west Australian coasts, the MLS variations are influenced by the annual cycles of the Indonesian Throughflow (ITF) and Leeuwin Current (LC) transports as well as the air-sea freshwater forcing, with eddy fluxes acting to freshen the MLS along the SEC, the Eastern Gyral Current, and the LC. On the interannual-scale, El Nino (La Nina) events are typically associated with saltier (fresher) MLS in the SETIO. Composite and budget analyses reveal that interannual variations in precipitations drive the MLS anomalies off the Java-Lesser Sunda coast; between 12 degrees S and the northwest Australian coast, the MLS variations are influenced by both advection anomalies and local precipitation anomalies; whereas anomalous meridional currents contribute to the MLS variations off the west Australian coast. Both enhanced local precipitations and the ITF transport anomalies have substantial contributions to the drastic freshening of the Indonesian-Australian Basin between the Java-Lesser Sunda coast and the northwest Australian coast during the extended La Nina events in 1999-2001 and 2010-2012.