Zonal Momentum Balance in South Asian Summer Monsoon: Forces and Changing Winds

The balance of forces for the South Asian Summer Monsoon (SASM) gives valuable insights for the understanding of the mean circulation and the changes it has undergone in the past. In this study, we have analyzed the zonal momentum balance for SASM for the last few decades (1950-2010) using reanalysis data to understand the changes in different forces and relate them with the changes in the associated circulation. In the lower level (925 hPa), the Pressure Gradient Force (PGF), Coriolis Force (CF), and Residual Force (RF, which includes the unresolved sub-grid scale process and frictional terms) are found to be the dominant terms of the zonal momentum balance for SASM with a magnitude of order 10^-4m sec^-2 whereas, horizontal advection and eddy force terms are negligible with one or more order lesser in magnitude. The residual force can be estimated by Rayleigh friction induced by turbulence, particularly over ocean points, which, however, is not a good measure of the same over the land points because of high irregularity. The momentum balance at the upper level (200 hPa) is between the PGF, CF, and the advection term, unlike the lower level, where the residual force does not seem to be dominant. In the free troposphere, the Convective Momentum Transfer or in other words convective friction is a good estimator of the RF, which represents the vertical transport of momentum. The changes in SASM circulation in the past can be apprehended by looking into the changes in these vital forces that drive the motion.