The summer monsoon has been showing a weakening trend over the past century with decreasing rainfall over large regions of the Indian subcontinent. The monsoon occurs because the land heats up much more than the ocean and the warm air over the land rises and results in low pressure. This causes the rain-bearing winds from the relatively cooler ocean to blow on to the land and cause rainfall. That is, it is the strong thermal contrast between land and ocean that results in a strong monsoon.

However, a recent study by Dr. Roxy Mathew Koll of the Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune and others, and published recently in the journal Nature Communications contends that this thermal contrast has been decreasing in the past decades, i.e., the land has been cooling and the ocean warming and the monsoon has shown a decreasing trend during the past century.

Ideally, under a global warming scenario the land temperature should increase greatly in the hot summers and serve as a strong monsoon driver. But, in the case of the Indian subcontinent, over the past century, that has not been the case. Observed data dating from the 1870s are available for the summer monsoon rainfall, from the Indian Meteorological Department and other sources. Using the data from 1901-2012, it was found that the rainfall has been decreasing over central South Asia — from south of Pakistan through central India to Bangladesh.

The decrease is highly significant over central India where agriculture is still mostly rain-fed, with a reduction of up to 10-20 per cent in the mean rainfall.

Quite a few other studies indicate that the monsoon rainfall is weakening over the South Asian region during the past half century (since 1950s). Some of these studies suggest that though the extreme rainfall events have increased over some regions, the frequency of moderate-to-heavy rainfall events has decreased over the subcontinent.

The reduction in land-sea temperature contrast is attributed mostly to a strong warming in the Indian Ocean on a multi-decadal scale with the latest reason being climate change under a global warming scenario.

The surface warming in the Indian Ocean, especially in the western regions has reached values of up to 1.2 degrees C during the past century, much larger than the warming trends in other tropical oceans. The decrease in the land-sea thermal contrast surface temperature trends (1901-2012) is also visible in the upper atmosphere, as the warming trends in the ocean surface are transferred to the atmosphere above through convective processes.

Apart from the ocean warming, a part of the decrease in land-sea temperature difference is also due to suppressed warming over the Indian land mass, possibly due to increased aerosol levels. Aerosols in the atmosphere reflect the sun’s heat back into space and cause a cooling effect.

The warming Indian Ocean also plays a role in weakening the monsoon circulation. Increased warming in the ocean enhances the large-scale upward motion of warm moist air over the equatorial ocean. This enhanced upward motion over the ocean is compensated by subsidence of dry air over the subcontinent, inhibiting convection and rainfall over the Indian landmass. This means that a warming Indian Ocean has resulted in surplus rains over the ocean at the cost of the monsoon rains over land, simultaneously drying the Indian subcontinent.