Role of Land-Ocean Contrast in the Indian Summer Monsoon Rainfall

For well over 300 hundred years, the monsoon has been considered to be a gigantic land-sea breeze driven by the land-ocean contrast in surface temperature. This popular perception of the monsoon as a gigantic land-sea breeze is not only mentioned in text books and reviews but also used in some studies of the impact of global warming to deduce the expected monsoon variability from the expected land-ocean contrast. If the primary cause of the monsoon is the differential heating between land and ocean, we expect enhanced land-sea contrast to be associated with enhanced monsoon rainfall. It is important to assess whether observations of monsoon variability are consistent with this expectation. Since models are used for deriving the nature of the impact of a change in land-ocean contrast on the monsoon, it is also important to investigate if such a role of land-ocean contrast is simulated by models. Here, we address this problem with an analysis of the relationship between the Indian summer monsoon rainfall (ISMR) and the surface temperature of the land, the surrounding ocean, and the land-ocean contrast in observations and simulations using one high-resolution atmospheric model and one coupled model. We find that in all three cases, the ISMR is negatively correlated with the land surface temperature as well as land-ocean contrast. Thus, for observations and models, the relationship of the variation of land-ocean contrast with that of the monsoon rainfall is opposite to what is expected from the land-sea breeze hypothesis. This clearly suggests that the time has come to abandon the perception of the monsoon system as a gigantic land-sea breeze. © 2021 by World Scientific Publishing Co. Pte. Ltd.