Possible Mechanisms of Interannual Variations in Surface Mixed Layer Temperatures off Somalia in Boreal Summer

Abstract

Abstract The key processes responsible for the interannual variation of the surface mixed layer temperature off Somalia in the western Arabian Sea in boreal summer are investigated by the use of a regional ocean model. Our focus is on influences of remotely forced annual Rossby waves as well as local southwesterly monsoonal winds in the years with anomalously warm or cold mixed layer temperature conditions. Composite and heat budget analyses of the simulated results indicate that the interannual mixed layer temperature variations in the region off the coast of Somalia are generated by the combined effects of local upwelling, Rossby wave intrusion, and horizontal advection. In particular, interannual modulation of the annual Rossby wave before the onset of the southwest monsoon causes a subsurface temperature anomaly below the mixed layer along the coast of Somalia. It is also shown that this subsurface temperature anomaly is upwelled into the mixed layer by the upward flow associated with the seasonally evolving coastal upwelling and that the mixed layer temperature anomaly in the coastal region is advected into the offshore region by seasonal developments of the Somali Current. Results from sensitivity experiments with different wind forcing scenarios demonstrate that the contribution of this remotely forced Rossby wave to the interannual variations of the mixed layer temperature anomaly off Somalia is comparable to that of the local wind stress anomalies alone. Significance Statement The purpose of this study is to investigate the key processes responsible for the interannual variation of the temperature within the ocean surface layer off Somalia in boreal summer. Because this temperature variation is shown to be related to the Indian summer monsoon rainfall, this study can lead to improvement in the predictability of the Indian summer monsoon. Previous studies have demonstrated that interannual modulation of the local monsoonal winds along the coast of Somalia in spring and summer generate the temperature variation. We find for the first time that the large-scale ocean waves excited remotely in previous winter play an important role, and its contribution is comparable to that of the effect of the wind influences.