Monsoon cyclogenesis and large scale flow patterns over South Asia

Abstract

An important problem of monsoon meteorology is understanding the mechanism of formation of monsoon disturbances over north Bay of Bengal. Using the linear stability analysis approach we have investigated in three different ways the stability of July mean monsoon zonal flow for the growth of disturbances. We have used a two-layer quasigeostrophic model and conducted combined barotropic­ baroclinic stability analysis. In the first approach, we included cumulus heating into our model through CISK mechanism. We obtained growing modes in the lower troposphere. One of thew modes has scale, structure and growth rate which agree well with those of the observed monsoon disturbances. In another study, we superposed a long stationary wave (the planetary monsoon wave) on the basic zonal flow and repeated the stability analysis. No heating was included in this case. This analysis also yielded growing disturbances in the basic flow. It turns out that one of these modes resembles well with the observed disturbances in respect of horizontal scale, growth rate and structure. Lastly, we examined the basic flow for instabili­ties when both cumulus heating and long wave are present. As expected there was enhancement in the growth rate and amplitude of perturbation as compared to their values in the earlier two cases. We have also studied the variability of monsoon cyclogenesis by considering the large-scale flow field over the monsoon region during active and break monsoon phases. We have conducted the above stability analysis of monsoon flow during break monsoon and just prior to distur­bance formation. It is found that the flow during break monsoon is stable whereas the flow just prior to depression formation is found to be unstable and yields disturbances with realistic scales and growth rates.