Interannual Variability in the Large-Scale Dynamics of the South Asian Summer Monsoon

In this study, reanalysis and other observational data are used to identify coherent and robust large-scale atmospheric patterns of interannual variability associated with the strength of the South Asian summer monsoon (SASM). A decomposition of the water vapor budget into dynamic and thermodynamic components shows that interannual variability in the SASM net precipitation (P-E) is primarily due to variations in winds rather than those in moisture. Using linear regression analysis, we find that strong monsoons relative to weak monsoons are associated with changes in the strength and structure of the cross-equatorial monsoonal circulation, which include a northward shift of its maximum and its ascending branch. Interestingly, and in disagreement with the view of monsoons as large-scale sea-breeze circulations, strong monsoon years are found to be associated with a decreased meridional gradient in the near surface atmospheric temperature in the SASM region. Extending our analysis to the southern hemisphere, we show how on interannual time scales a positive correlation exists between the strength of the monsoon and poleward eddy energy fluxes in the southern hemisphere mid-latitudes. Possible implications of these teleconnection patterns for increased understanding of the SASM interannual variability are discussed. Patterns of large-scale atmospheric variability associated with the SASM in CMIP5 simulations are explored and compared with observational results.