Interhemispheric moisture transport in the Indian Ocean summer monsoon: Data‐model and model‐model comparisons

Abstract

Exchange of energy and mass between the ocean and the atmosphere is an integral component of Indian Ocean summer‐monsoon dynamics. Most previous general circulation model simulations of the monsoon have prescribed sea surface temperature (SST), thus severely constraining atmosphere‐ocean energy exchanges. Here we compare model physics, based on both prescribed and computed SST simulations, to observed climate links between northern hemisphere monsoon strength and the cross‐equatorial transport of moisture (latent heat) from the southern subtropical Indian Ocean. For the computed SST case, stronger southern hemisphere southeast trades and colder southern subtropical SSTs are associated with increased cross‐equatorial moisture flux. Increased moisture flux induces strengthened monsoon circulation via increased condensational heating and decreased sea level pressure over Asia. For the prescribed SST case, stronger southeast trades and colder southern subtropical SSTs are also associated with increased cross‐equatorial moisture flux and, hence, strengthened monsoon circulation. However, in this case, the increased moisture flux is induced by strengthened cross‐equatorial winds associated with subsidence over the cold southern hemisphere SSTs (i.e., increased sea level pressure in the southern subtropics). Thus the model physics associated with changes in monsoon strength are very different for the prescribed and computed SST simulations. Only the computed SST simulation replicates observed meteorological relationships.