Role of equatorially asymmetric sea surface temperature anomalies in the Indian Ocean in the Asian summer monsoon and El Nino-Southern Oscillation coupling

Using an ocean general circulation model forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis, we examined possible roles of the tropical Indian Ocean in the Asian summer monsoon and El Nino-Southern Oscillation (ENSO) coupling. A dominant precursory signal of anomalous monsoon circulation relevant to ENSO is the presence of latitudinally asymmetric anomalies of tropical convection and sea surface temperature (SST) near the equator in the preceding spring. The model results show that the equatorial asymmetry of surface latent heat flux plays a vital role in generating the equatorially asymmetric SST in the preceding spring, which implies that a positive wind-evaporation-SST (WES) feedback proposed by Xie and Philander [1994] is crucially responsible for generation and maintenance of those equatorial asymmetries. The westward extension of anomalous convection from the vicinity of the Philippines into the northern Indian Ocean from the preceding winter to spring, which has a signal of ENSO, can be viewed as a developing process of WES. Persistence of the WES feedback regime affects land surface hydrologic processes in the Asian continent through a Rossby wave response to anomalous convective heating. It is anticipated that the WES mode in the tropical Indian Ocean serves as a bridge between the ENSO prevailing in the preceding winter and the anomalous summer monsoon.