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 the main cause of monsoon initiation and evolution in southeast Asia. Several forcing factors such as tectonic, solar insolation, latent heat transport, albedo of the earth surface and deep water circulation changes drive the variability of southwest (SW) monsoon in the Indian subcontinent. Different forcing factors act on different time scales. Arabian 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 Arabian Sea sediments provide a record of past monsoon variability covering a time span of a few million years.

Detailed analysis of numerous monsoon indices and General Circulation Models (GeM) show that SW monsoon was strong during interglacials (warm periods) and weak during glacials (cold periods). Spectral analysis of detailed time series of monsoon indices reveal that the intensity of the SW monsoon fluctuates with periodicities of 100 ka and 23 ka. These periodicities of SW monsoon were attributed to the orbitally induced changes in solar radiation and surface boundary conditions (Milankovitch Theory). Recent high-resolution studies from the Oman Margin provide new evidences on the SW monsoon variability on century time scale and a Sub-Milankovitch Periodicity 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 ¹⁴C changes and recent onset of aridity in the tropics, which is also linked to the SW monsoon variability in one way or other.