IIT Bombay: Extreme rainfall not associated with local warming

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The researchers did not find evidence to support the notion that intensification in either the mean or extreme rainfall over India was due to warming.

A study carried out by researchers at IIT Bombay has found that extreme rainfall events over India do not have significant association with land surface air temperature over India and sea surface temperature over central Indian Ocean.

The results are contrary to the general notion that global warming or increased summer temperature is responsible for extreme summer monsoon rainfall events witnessed in some parts of India in the last few years. The results are applicable at both large and small scales — Indian subcontinent and at a grid level of 100 x 100 sq. km. Data for 50 years pre- and post-1975s were used for the study. The results are published today (August 3, 2016) in the journal Scientific Reports.

In the recent past, there has been considerable change in heavy summer monsoon rainfall pattern in India and there is considerable debate on whether such extreme events are caused by global warming or by urbanisation.

A December 2006 paper in Science by B.N. Goswami of the Indian Institute of Tropical Meteorology, Pune, attributed the increase in magnitude and frequency of heavy rainfall events over central India to global warming. But Goswami used a spatial average over central India, a convenient way to carry out the analysis but unfortunately missed some of the spatial details and overstated some of the conclusions. But the latest study found insignificant dependence of precipitation extreme on temperature over India.

At a local level, some of the grids of 100×100 sq. km. experienced a change in extreme rainfall with increasing temperature, while other grids witnessed a decrease or no significant change in rainfall with increased temperature. The model simulation of Indian rainfall extremes tends to “overestimate the changes in both mean and extreme precipitation”.

The study also found that dynamic moisture transport caused by thermal gradient between land and water had a significant role on mean and extreme rainfall compared with direct thermodynamic effect (where the atmosphere has a greater capacity to carry moisture when temperature rises).  “The dynamic transfer into the subcontinent is associated with thermodynamic processes in the nonlocal regions involved — it is the competition for rain over India versus over the ocean,” Prof. Raghu Murtugudde a co-author from the University of Maryland, Maryland, U.S. explains in an email to me.

The new generation climate model and historical simulations tend to over-estimate the association between extreme rainfall events and temperature.

On a global scale, increase in rainfall extremes in a warming world is generally explained by the Clausius-Clapeyron equation.  The C-C equation states that with an increase in temperature the precipitation too increases as the moisture-carrying capacity of air increases with temperature (at a rate of 7.5 per cent per degree increase in temperature). But the C-C scaling will not be applicable in many of the tropical regions.

A study carried out in Brazil revealed that there was a decrease in extreme rainfall with increasing temperature. In the latest study too, the researchers did not find evidence to support “intensification in either the mean or extreme rainfall over India in a warming world”.

“The claim is not really that climate change is not important. But the novel results indicate that the local warming is not the controlling factor. The clues are in the rapid warming of the Indian Ocean and the changing roles of the Arabian Sea, Bay of Bengal and the Indian Ocean in monsoon variability and extremes. It is the nonlocal controls that are of great importance,” says Prof. Murtugudde.

“The new generation climate model and historical simulations tend to over-estimate the association between extreme rainfall events and temperature. We must be careful while using climate change for estimating future changes in extreme rainfall with increasing temperature over India and Central Indian Ocean,” says Prof. Subhankar Karmakar a co-author of the paper from the Centre for Environmental Science and Engineering, IIT Bombay.

“The changing patterns of extremes over the Indian subcontinent need a scientific re-evaluation.  Our results highlight the need for further research to resolve the temperature dependence of rainfall,” says Prof. Subimal Ghosh, a co-author of the paper from the Department of Civil Engineering, IIT Bombay.

Published in The Hindu on August 3, 2016