some studies on the mean Meriodional circulation droughts

Abstract

It has been observed that during a break in the Indian southwest monsoon, the middle latitude circulation enters low index regime or Rossby regime. These Rossby waves are found to propagate southwards even up to Indian region in such situations. Also, it is well known that the strength of the Rossby waves has significant influence on the Mean-Meridional Circulation (M.M.C). Therefore, one of the aims of this study is to examine how far the monsoonal circulation is influenced by the Rossby waves via the M.M.C. In this connection it is felt that one has to study two aspects:

The sensitivity of the global Mean Meridional Circulation to various controlling factors such as diabatic heating, large-scale eddy fluxes, small-scale eddy stresses, etc. The pattern of the M.M.C., particularly over the Indian and Asian regions, during the southwest monsoon season, for two contrasting years—one, a normal monsoon year and the other, a year of severe monsoon drought.

The following are the findings of this investigation within the framework of a mathematical model (Rao 1956):

The mechanical forcing due to the effect of small-scale turbulent stresses and large-scale momentum fluxes cannot by itself explain quantitatively the observed M.M.C. in the tropics especially. The thermodynamic forcing, due to the effect of diabatic heating and large-scale heat fluxes, is of overwhelming importance for the M.M.C. in tropics. This is not to suggest that the indirect heating effects due to the mechanical frictional forcing via the CISK process are unimportant. The M.M.C. is sensitive to the vertical distribution of both the small-scale frictional stress as well as the diabatic heating. For example, only when the diabatic heating is uniformly distributed throughout the column of the atmosphere, the computed Hadley cell resembles the observed one. If the heating decreases rapidly with height, the return current in the Hadley cell is weaker when compared with observation. The computed M.M.C. is sensitive even to the boundary conditions at the equator. For example, the removal of rigid wall at the equator by inclusion of the southern hemisphere introduced significant quantitative changes in the strength of the Hadley cell. The inclusion of the mountains does not affect the pattern of the M.M.C. in general, except near the mountains. Over the Indian region, during the monsoon season months of June, July and August, the atmosphere experiences heating around latitude 10°N. Hence there is rising motion over India while the air descends over 10°S in general throughout the season. Thus this reverse Hadley cell is controlled to a great extent by the heating pattern prevailing. However, perhaps because this heat source is situated north of tropics, the mechanical forcing due to Rossby waves can affect this reverse Hadley cell strength. For example, it is found that during the severe drought month of August 1965, the already weak reverse Hadley cell is considerably weakened further by the inclusion of the mechanical forcings arising due to large-scale eddy fluxes of momentum. The changes in the heating pattern are responsible for the changes in the strength of the M.M.C., especially in tropics, between a drought period and a good monsoon period. These changes in the diabatic heating pattern for the drought year may be the indirect effects of large-scale transient waves.