Seasonal variability and long-term trends of the surface and subsurface circulation features in the Equatorial Indian Ocean.

Herein, we have examined the seasonal variability and long-term trends in the recent 35 years (1980-2014) of the wind stresses and circulation features in the Equatorial Indian Ocean (EIO) using reanalysis and observed data. Annual mean flow of the EIO is eastward along the equator, with the strong current in the upper 80 m and weak below that. The prominent surface currents in the EIO are the eastward Wyrtki Jets (WJs) occurring during spring (April-May) and fall (October-November), with active flows within 2° of the equator between 60° E and 90° E. The fall WJs are stronger than the spring WJs. The Equatorial Undercurrent (EUC), the prominent subsurface eastward flow with core within 70-150 m, occurs twice annually-February-April and August-October-with the well-developed flow in March-April. Long-term trends reveal that during 1980-2014, the annual mean eastward surface current in the upper 75 m has increased, primarily due to the rise of WJs forced by the westerly wind stresses during November-December. But the subsurface eastward current (below 75 m) has decreased due to the weakening of winter North Equatorial Current (NEC) and EUC during February-April. Consolidation of the westerly wind stresses during November-April results in the strengthening of the wind-driven surface WJs and weakening of the pressure-driven EUCs during 1980-2014. Intensification of WJs are the direct effects of wind forcing. However, strengthening of westerly stresses also results in weakening of the surface westward NEC and thereby restricts the strong eastward gradient of SSH which is required to build up the essential eastward pressure gradient for the EUC. Since WJs and EUCs modulate the hydrographic structure of the eastern part of the EIO, any change in the strength of these flows, in turn, will influence mostly this oceanic region and thereby the local as well as global climate.