On the Onset Vortex of the Summer Monsoon

Abstract

In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-level flow which is an update on the well-known Findlater analysis. A number of calculations show that the horizontal shear of the monsoon current provides substantial energy during the evolution of the onset vortex. The flow satisfies the necessary condition for the existence of instability. Corresponding stability diagrams exhibit substantial growth rates around the period of the formation of the onset vortex. The scale of maximum growth rate is closely in correspondence with the scale of the onset vortex. Finally, the conversion from zonal to eddy kinetic energy is demonstrated via a simple prediction experiment with the conservation of absolute vorticity as a constraint. A reasonable simulation of the onset vortex is shown in this experiment.