Interannual variability in the large scale dynamics of the South Asian summer monsoon

In this study, the interannual variability of precipitation and large scale dynamics of the South Asian summer monsoon is investigated using reanalysis and precipitation data from various sources. The water vapor budget in the South Asian monsoon region is decomposed into dynamic and thermodynamic components, showing that the year-to-year precipitation variability is primarily due to variations in winds rather than those in moisture. The vertically integrated moisture flux convergence in the South Asian monsoon region is taken as an index for monsoon strength and is used for linear correlation analyses, to identify large scale circulation patterns associated with monsoon variations.

We find that in strong monsoon years relative to weak monsoon years, there is a northward expansion of the cross-equatorial meridional overturning circulation, with an increase in precipitation in the ascending branch. Our analysis shows that strong monsoons are associated with an increased meridional gradient in sub-cloud equivalent potential temperature, consistent with recent studies of the South Asian monsoon in a convective quasi-equilibrium framework and consistent with a northward displacement of the ascending branch of the meridional overturning. However, in contrast with the traditional view of monsoons as land-sea breeze circulations, strong monsoons are correlated with negative surface temperature anomalies over India and a decreased meridional gradient in surface temperature.

Motivated by theoretical studies suggesting a possible influence of extratropical eddy fluxes on monsoonal circulations, we extend our regression analyses into the southern hemisphere. Strong monsoon years are found to be correlated with increased meridional surface temperature gradients in the southern hemisphere subtropics and extratropics, and, by thermal wind balance, with a strengthened westerly jet. While there is no significant correlation with eddy momentum, we find that strong monsoons years are associated with enhanced moist static energy eddy fluxes in the southern hemisphere subtropics and extratropics. Possible mechanisms by which large-scale eddy fluxes might influence the monsoonal circulation and the location of its ascending branch are discussed within a zonally averaged energetic framework.