LINEAR AND NONLINEAR STUDIES OF THE SUMMER MONSOON ONSET VORTEX

We have carried out a combined barotropic-baroclinic instability analysis using a moist three-level primitive equation model with a view to investigate the role of moist convection and non-geostrophic effects on the growth of the monsoon onset vortex and upper tropospheric easterly waves. We have also conducted an idealized study of the growth and energetics of the monsoon onset vortex using a non-linear five-level global spectral model incorporating parameterized cumulus heating. In the linear model we found a lower tropospheric growing mode whose scale length, structure and the growth rate resemble the observed characteristics of the monsoon onset vortex. This mode had maximum amplitude centered around 10-degrees-N and had a wavelength of 3500 km, a doubling time of 7 days and a westward phase speed of 1 ms-1. Our analysis revealed two upper tropospheric growing modes. These upper tropospheric modes have scale length and westward phase speed similar to the observed tropical monsoon easterly waves in the upper troposphere. In an idealized numerical experiment, by imposing a south to north heating gradient in a multilevel non-linear global spectral model, we have simulated the monsoon onset type of vortex. Using wave-CISK type of cumulus heating, we investigated the growth of this generated vortex. The growth of the cyclonic vortex was found in the presence of cumulus heating clearly elucidating the role of moist processes in the growth of the monsoon onset type of vortex. The energetics calculations of the simulated vortex revealed that there was a steady increase of A(Z), A(E), K(Z) and K(E) after the cumulus heating was switched on in the model. Barotropic exchange of energy shows some positive exchange initially and later on baroclinic energy conversion in the presence of cumulus heating becomes very prominent and is the main source of energy for the growth of the vortex.