Equatorial Indian Ocean Westward Jets: Intraseasonal Variability, and Impact on Salinity

While westerly winds dominate the equatorial Indian Ocean and generate the well-known eastward flowing Wyrtki Jets during boreal spring and fall, there is evidence of a strong westward surface jet during winter which is swifter than eastward currents during that season. A weaker westward jet is found in summer. Previous studies on zonal currents in the equatorial Indian Ocean have focused mainly on the seasonal and interannual variabilities with limited studies on subseasonal variability. In this study, we examine the occurrence, characteristics and intraseasonal variability of westward jet and its impact on mixed layer salinity in the equatorial Indian Ocean using the HYbrid Coordinate Ocean Model (HYCOM) reanalysis with the Navy Coupled Ocean Data Assimilation (NCODA). The westward jet builds up gradually, decays rapidly, and is primarily forced by local intraseasonal wind stress anomalies generated by atmospheric intraseasonal oscillations (ISOs). The intraseasonal westward jet generates strong horizontal advection and is the leading cause of mixed layer freshening in the western equatorial Indian Ocean. Without it, a saltier mixed layer would persist and weaken any barrier layers. A rapid reversal of ISO surface winds terminates the westward jet.