A Physical Basis For Changes in the Phases of the Summer Monsoon over India

Abstract

The summer monsoon over India is described as ?active? when large parts of India, particularly the central parts and the west coast receive normal or above normal rainfall. It is described as having a ?break? when the rainfall is below normal over most parts of India, except in the hills in the north and in the southeastern parts of the country. It is shown in this paper that when a break sets in, there is an in-situ formation of a zonally oriented trough at 10?15°N at 700 mb over south India and the adjoining seas. Simultaneously, there is a sharp fall in the 500 mb temperature over the Tibetan Plateau to much below normal values. As the plateau warms and attains near normal temperature, the trough in the south shifts northward and occupies the normal active monsoon trough position (around 20°N) over north India. Considering the importance of meridional circulations in the tropics, it is reasonable to expect that the changes in the 700 mb flow with the phase change are associated with corresponding changes in the meridional circulation. Computations of vertical motion along 75°E with the help of a quasi-geostrophic model show that under break monsoon conditions, the Hadley cell with its upward limb in the trough at 10?15°N over south India and descent further north, predominates. Whereas, in the active monsoon phase, the monsoon cell, with a rising limb at ?20°N and descent in the south, is prominent. The physical process through which the sudden cooling over the Tibetan Plateau brings about a change in the meridional circulation, and therefore a change in the phase of the monsoon from an active to a break situation, is explored through a quasi-geostrophic streamfunction for the meridional circulation. It is seen that eddy fluxes, particularly of heat, are one to two orders of magnitude larger at the northern end of the monsoon cell at the time of disruption of the monsoon. The vertical motions induced by these eddy fluxes are seen to be in the right direction to bring about a switch over from the monsoon cell to the Hadley cell. The re-establishment of the monsoon cell and with it the active phase appears to take place slowly through radiational heating over the Tibetan Plateau.