Abstract/Summary

Understanding decadal changes in monsoon rainfall is essential for mitigation policies in the next few decades, but comparatively less is known about the Southern Hemisphere monsoon. We find that decadal variations of the Southern Hemisphere land monsoon (SHLM) have a zonal dipole pattern with a significant periodicity of 10-15 years. This spatial pattern is characterized by wet Southern African (SAF) and Australian (AUM) monsoons but a dry southeastern South American monsoon (SESAM). Moisture budget analysis shows that changes in monsoon circulation explain over 70% of the decadal variation. Tropical dynamics dominate the monsoon circulation change, and extratropical Rossby waves also have a contribution. The wet SAF and AUM but dry SESAM pattern is mainly driven by a tropical sea surface temperature (SST) gradient, specifically warm SST over the western Pacific and cold SST over the tropical Indian, central Pacific, and eastern Pacific Oceans. This gradient enhances the Walker circulation and subtropical highs over the South Indian and South Pacific Oceans, increasing moisture convergence over the SAF and AUM. The resultant Walker circulation change decreases ascending motion over the SESAM. The enhancement of AUM rainfall triggers the Pacific-South American pattern of the Rossby wave train, which propagates eastward along the SH subpolar jet, generating descending motion over the SESAM. The tropical and extratropical mechanisms are verified in a coupled climate model with nudged SST and a simplified model with artificial atmospheric heating imposed over eastern Australia, respectively. Our findings help unify understanding of decadal variations of the regional monsoons and may add confidence for decadal prediction of SHLM changes in the following decades.