Intraseasonal variability of the surface zonal current in the equatorial Indian Ocean: Seasonal differences and causes

Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current (u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80 degrees-90 degrees E and near the western boundary at 45 degrees-55 degrees E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations (ISOs), which explains 91% of the standard deviation of the total u ISV. Further analysis suggests that it takes less than ten days for the intraseasonal zonal wind stress to generate the u ISV through the directly forced Kelvin and Rossby waves. Driven by the stronger zonal wind stress associated with the Indian summer monsoon ISO (MISO), the eastern u ISV in boreal summer (May to October) is about 1.5 times larger than that in boreal winter (November to April). In the western basin, both the atmospheric ISOs and the oceanic internal instabilities contribute substantially to the u ISV, and induce stronger u ISV in boreal summer. Energy budget analysis suggests that the mean flow converts energy to the intraseasonal current mainly through barotropic instabilities.

Automated summary

Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current over the equatorial Indian Ocean. The study finds that atmospheric intraseasonal oscillations are the main driver of u ISV in the eastern basin, while both atmospheric ISOs and oceanic internal instabilities contribute to u ISV in the western basin. The study also highlights the seasonal differences, with eastern u ISV being stronger in boreal summer compared to boreal winter.