Understanding the interannual variations of the zonal mean Indian summer monsoon seasonal rainfall

Abstract

In this study, we seek to understand the variations of the zonal mean Indian summer monsoon (ISM). It is observed that the zonal mean precipitation anomalies explain about 20–30% of the total seasonal precipitation of the ISM. Additionally, we also show that the interannual anomalies of the ISM, at least for some of the most extreme seasons display significant zonal symmetry. Therefore, understanding the interannual variations of the zonal mean precipitation is quite relevant. Furthermore, the reduced and simplified dimensionality of the zonally symmetric framework is an additional attraction. Our study shows that the zonal mean precipitation anomalies of the ISM are significantly correlated with the corresponding anomalies of the vertically integrated moist static energy (H) at interannual scales. The forcing terms of the tendency of the zonal mean H of the ISM is dominated by the meridional flux of H by the Hadley cell followed by comparably smaller, yet, significant forcing terms of transverse boundary and vertical fluxes of H. We find that the individual correlations of the zonal mean precipitation anomalies of the ISM with each of these dominant terms of H are comparably weak in the core latitudes of the ISM. But some of the relatively weaker forcing terms of H like the radiative heating and the diffusion of the enthalpy fluxes display a very strong relationship with the corresponding zonal mean precipitation anomalies. These results suggest that although the description of the regional Hadley cell offers a heuristic model to describe the ISM variation, it belies the intricacies of the forcing terms that maintain it. Our study suggests that the challenge of the ISM seasonal rainfall anomaly prediction even in a reduced dimensional space of a zonally symmetric framework requires all the forcing terms of H with reasonable fidelity, irrespective of their contribution to the variations of H.