DRIVING FORCES OF INDIAN-SUMMER MONSOON ON MILANKOVITCH AND SUB-MILANKOVITCH TIME SCALES - A REVIEW

A scientific consensus exists that tectonic evolution of Himalaya is t he main cause of monsoon initiation and evolution in southeast Asia. S everal forcing factors such as tectonic, solar insolation, latent heat transport, albedo of the earth surface and deep water circulation cha nges drive the variability of southwest (SW) monsoon in the Indian sub continent. Different forcing factors act on different time scales. Ara bian Sea sediments consist of distinct fauna that are endemic to areas of upwelling. The SW monsoon wind strength, upwelling in the Arabian Sea and precipitation in the subcontinent are inter linked. Thus Arabi an Sea sediments provide a record of past monsoon variability covering a time span of a few million years. Detailed analysis of numerous mon soon indices and General Circulation Models (GCM) show that SW monsoon was strong during interglacials (warm periods) and weak during glacia ls (cold periods). Spectral analysis of detailed time series of monsoo n indices reveal that the intensity of the SW monsoon fluctuates with periodicities of 100 ka and 23 ka. These periodicities of SW monsoon w ere attributed to the orbitally induced changes in solar radiation and surface boundary conditions (Milankovitch Theory). Recent high-resolu tion studies from the Oman Margin provide new evidences on the SW mons oon variability on century time scale and a Sub-Milankovitch Periodici ty in the range of 2,200 to 2,400 years. The SW monsoon variability on century scale and high frequency periodicities might be forced by the deep water circulation changes in high latitudes. The present review discusses the atmospheric C-14 changes and recent onset of aridity in the tropics, which is also linked to the SW monsoon variability in one way or other.