Effects of climate modes on interannual variability of the equatorial currents in the Indian ocean

This study uses two reanalysis datasets and ocean model experiments to examine the dynamics of the interannual variability of the equatorial currents in the Indian Ocean, and to quantify the effects of the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) climate modes on the currents. Strong interannual variability of the equatorial currents mainly occurs in the upper-central basin (UCB) where the Wyrtki jets are located, and in the subsurface-eastern basin (SEB) where the equatorial undercurrent is located. Equatorial waves directly forced by equatorial winds dominate the interannual current anomalies in both the UCB and the SEB, and the reflected waves have a secondary role. The reflected waves tend to weaken the current anomalies in the UCB but intensify the currents in the SEB. In general, ENSO and the IOD have a comparable effect on the interannual current anomalies in the SEB, but the IOD has a larger role than ENSO in the UCB. In some years, either ENSO or the IOD may play a dominant role. Composite analysis suggests that the interannual current anomalies occur about two months prior to the peaks the climate modes. As ENSO and the IOD have apparent seasonality, the current anomalies mainly occur during August to October for pure IOD years, and during October to December for pure ENSO years. The co-occurrence of ENSO and the IOD enhances their respective impact, resulting in the surface current anomalies lasting from July to December and the subsurface current anomalies from July to March of the following year.