Using a two-level quasi-geostrophic model, the effect of the inclusion of a long stationary monsoon wave (basic meridional wind) on the combined barotropicbaroclinic instability of the monsoon current has been investigated. Both barotropic and baroclinic interactions of the perturbation with the monsoon wave and the zonal flow are included. This analysis, using realistic velocity profiles, yields fast-growing westward-moving upper-tropospheric waves corresponding to easterly waves, and lower-tropospheric modes corresponding to monsoon disturbances.
The stability of the monsoon zonal flow has been studied by Shukla (1977) and Keshavamurty et al. (1977). They found that the monsoon atmosphere is not baroclinically unstable. Shukla found that the CISK mechanism can lead to a growth of disturbances of the same scale as the observed monsoon disturbances, but that there was no preferred scale with a fastest rate of growth. Keshavamurty et al. found that the monsoon zonal flow is barotropically unstable in the lower and midtroposphere and that this instability can yield disturbances of reasonable growth rate and scale. Mak (1975) studied the effect of the meridonal motion on baroclinic instability of a monsoon flow, but found that the meridonal components required for growth were very large compared to those obtained from observed winds. Lorenz (1972) studied the barotropic instability of Rossby waves and found that shorter waves can be barotropically unstable. The observed monsoon flow is seen to have an appreciable meridional component.