Warm Indian Ocean, Weak Asian Monsoon

The South Asian summer monsoon contributes to more than ūth of the annual rainfall, and dictates the socio-economic livelihood in the subcontinent. However, there are large uncertainties looming over the status and fate of the monsoon, with several studies debating on whether the summer monsoon is weakening or strengthening in a changing climate. The observations and climate models have suggested that anthropogenic warming in the past century have increased the moisture availability, as well as the land-sea thermal contrast in the tropics, favoring an increase in tropical rainfall. However in the current study, we observe that the summer monsoon rainfall during 1901-2012 shows a significant weakening trend over the South Asian subcontinent, extending from Pakistan through central India to Bangladesh. We notice that the subcontinent experienced a relatively subdued warming during this period. In contrast, the tropical Indian Ocean experienced a nearly monotonic warming, at a rate faster than the other tropical oceans. Using multiple observed datasets and model simulations, we demonstrate that the subdued warming of the subcontinent along with the enhanced Indian Ocean warming results in a reduced land-sea thermal contrast, which extends from the surface to the upper troposphere, weakening the summer monsoon circulation and rainfall.