Origin of Low Frequency (Intraseasonal) Oscillations in the Tropical Atmosphere. Part III: Monsoon Dynamics

Abstract

This is the third in a series of papers to study the origin of intraseasonal oscillations. In this paper, we address the issue of the interaction of the monsoon large-scale circulation and intraseasonal oscillations. 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 approximately 3000–4000 km 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. They may also be identified with the observed sudden jump of the Mei-yu rainband over East Asia and the inverse relationship between the monsoon ITCZ and the equatorial ITCZ over India and East Asia. 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. The instability is favored in the region with large vertical wind shear and reduced effective static stability. The monsoon large scale circulation over India and southeast Asia and the plentiful supply of moisture in the region appear to be favorable for the development of these unstable waves. It is argued that the prevailing easterly waves found over the subtropical western Pacific during northern summer may also be due to the above unstable Rossby wave mechanism.