Impact of salinity on the 1997 Indian Ocean dipole event in a numerical experiment

Abstract

The role of salinity during the onset and growth of the 1997 Indian Ocean Dipole Mode event is explored with an ocean general circulation model thoroughly validated to observations. In fall 1997, anomalous easterlies drive an Indian Ocean equatorial circulation similar to that regularly observed in the Pacific Ocean: a South Equatorial Current (SEC) near the equator, an Equatorial Under Current (EUC) subsurface, and intense upwelling in the eastern part of the basin. The SEC transports westward the eastern Indian Ocean fresh pool creating, along the equator, a shallow salinity stratification favoring the creation of a barrier layer. In our experiment, the shallow top of the thermocline limits the barrier layer thickness to about 10 m; nevertheless, salinity has a significant impact on sea surface temperature (SST). Indeed, the shallow salinity stratification along the equator traps the wind forcing in a thin surface mixed‐layer. The reduction of wind momentum penetration decreases the deceleration of the EUC, and the greater amplitude of wind momentum input in surface layers strengthens the SEC. This intensification of the equatorial zonal circulation increases the Sumatra upwelling and its associated meridional circulation. This strengthening of the whole equatorial circulation shifts upward the thermohaline structure and reinforces the cold SST anomaly off Sumatra by about 20%. Overall, the effect of salinity on the 1997 Indian Ocean Dipole is to reinforce the oceanic anomalies favoring a strengthening of the air‐sea interactions.