An instability study of the onset-vortex of the southwest monsoon, 1979

A dry two-layer linear-balanced model is used to investigate the relative effects of the horizontal and vertical shears of the zonal flow over the Arabian Sea in the early phase of the onset of the southwest monsoon in 1979. The basic state is obtained from the MONEX data. Two aspects of the cyclogenesis which immediately preceded the onset of the monsoon can be adequately accounted for by the process of dynamic instability mainly associated with the horizontal shear of the basic flow. These two aspects are the growth rate and the scale selection of the disturbance. To this extent, the results of this study agree with Krishnamurti et al. (1981). But the mainly barotropic instability mechanism incorrectly predicts two other aspects of the cyclogenesis. The maximum intensity of the disturbance is predicted on the southern flank of the jet and the vertical extent of the disturbance is essentially confined to the lower troposphere. These discrepancies reflect the limitations of a dry model.

The vertical shear and stratification significantly influence the structure of the disturbance although their effect on the growth rate is small. The comparison of the linear-balanced model results with the quasi-geostrophic model results suggests that the full variation of the Coriolis parameter with latitude should be retained in future modeling of the onset-vortex.