A primitive equation barotropic instability study of the monsoon onset vortex, 1979

Abstract

A primitive equation barotropic stability analysis of the 700 mb zonal wind profile over the Arabian Sea was performed for each day between 10 and 14 June 1979, during the onset phase of the monsoon. the flow was found to become progressively more unstable with the passage of time. the most unstable inviscid primitive equation mode has an e-folding time of 3.1 days, a wavelength of 3500 km and a westward phase speed of 1.9ms^-1. The non-geostrophic effect reduces the growth rate and changes the structure of the unstable mode significantly. It is noticed that when the normalized unstable mode is superposed on the basic state, the total field is closer to the observations. Momentum transports by the primitive equation mode and by the quasi-geostrophic approximation are in opposite directions in equatorial regions. the energetics of the primitive equation unstable mode are computed and the physical processes responsible for the westward propagation of the wave are identified.