A further study of the tropical Indian Ocean asymmetric mode in boreal spring

Abstract

The present study performs a mixed-layer heat budget analysis and a hybrid coupled model experiment to understand the roles of surface heat fluxes and oceanic processes in the tropical Indian Ocean asymmetric mode in boreal spring. The heat budget analysis shows that the surface heat fluxes play a dominant role in sea surface temperature (SST) changes in most of the tropical Indian Ocean. In the southwest tropical Indian Ocean, the SST warming during October-November that initiates the cross-equatorial SST gradient for the development of the asymmetric mode is primarily due to surface latent heat flux, with secondary contribution from meridional and vertical advection. The SST warming around April that sustain the cross-equatorial SST gradient is also mainly due to latent heat flux. The ocean downwelling is large during late winter and boreal spring, which prevents the weakening of the SST anomalies. The results of a numerical experiment with a hybrid coupled model in which an atmospheric general circulation model is coupled to a slab ocean model in the tropical Indian Ocean demonstrates that the asymmetric mode can develop without ocean dynamics. Thus, the thermodynamic air-sea interactions play an essential role for the development of the tropical Indian Ocean asymmetric mode.