Origin of low frequency (intraseasonal) oscillations in the tropical atmosphere. Part III: monsoon dynamics

Abstract

We show that as a result of the interaction of the large scale monsoon flow with the near-equatorial intraseasonal oscillation, unstable baroclinic disturbances are generated over the monsoon region. These disturbances have spatial scales of ~3000-4000km and periods of 5-6 days with the vertical wave axis tilting eastward with height. The rapid development of these cyclonic disturbances along 15°-20°N and the concomitant weakening of the equatorial disturbances are accompanied by the rapid northward shift of the rising branch of the local Hadley circulation. From a linear stability analysis of quasi-geostrophic motion in a two-level model, it is shown that the westward propagating disturbances generated over the monsoon region are the manifestation of heat-induced unstable Rossby waves. -from Authors