The Monsoonal Mid-Tropospheric Cyclogenesis

Abstract

The initiation of the monsoonal mid-tropospheric cyclone over the west coast of India in summer is regarded to be an instability phenomenon of a rather unique class of baroclinic basic state. The latter is believed to be associated with the monsoon depression over the Bay of Bengal, the heat low over Pakistan, as well as with the strong heating on the Himalayan-Tibetan Massif. The essential elements of a representative flow over the northeastern Arabia Sea and central western India are incorporated into a modified Eady model in which the basic flow is assumed to have a zonal component with a uniform easterly shear, and a southerly component vanishing at the surface and in the upper troposphere but having a maximum value at a low tropospheric level. The instability analysis indicates that with plausible values for the parameters, the horizontal length scale, the growth rate and the phase speed of the most unstable mode are quite comparable to the corresponding observed values. More importantly, such an unstable mode has a strong cyclonic circulation in the 700–500 mb layer, but is very weak both near the surface and in the upper troposphere. There is a cold core below the center of the disturbance and a warm core above. It has a horizontal convergence below the 600 mb level and a weaker and thicker layer of divergence above. The relative phase between the horizontal convergence and the geopotential is such that the upward motion occurs slightly to the west of the disturbance with a downward motion to the east. These properties are strikingly similar to the observed counterparts, thereby lending support to the basic hypothesis on the origin of the mid-tropospheric cyclone. The relative roles of the zonal and meridional components of the basic flow are also examined.