Indian monsoon to change

Climate change will severely affect the Indian monsoon, with detrimental effects on the economy, say new findings.

Global warming could cause frequent and severe failures of the Indian summer monsoon in the next two centuries, new research suggests.

The effects of these unprecedented changes would be extremely detrimental to India's economy, which relies heavily on the monsoon season to bring fresh water to the farmlands.

The findings have been published on November 6 in the Institute of Physics' (IOP) Publishing's journal Environmental Research Letters, by researchers at the Potsdam Institute for Climate Impact Research and Potsdam University.

They found that as we move towards the end of the 21st, and into the 22nd, century, increasing temperatures and a change in strength of the Pacific Walker circulation in spring could cause more frequent and severe changes in monsoon rainfall.

The Walker circulation usually brings areas of high pressure to the western Indian Ocean but, in years when El Niño occurs, this pattern gets shifted eastward, bringing high pressure over India and suppressing the monsoon, especially in spring when the monsoon begins to develop.

The researchers' simulations showed that as temperatures increase in the future, the Walker circulation, on average, brings more high pressure over India, even though the occurrence of El Niño doesn't increase.

These failures of the monsoon system - defined in the study as a 40 to 70 percent reduction in rainfall below normal levels - were unprecedented in the researchers' observational record, which was taken from the India Meteorological Department and goes back to the 1870s.

Effects

The immediate effects of climate change on monsoon rainfall have already been observed by some researchers; however, the patterns of response in the coming decades are not uniform across different models and studies.

Jacob Schewe, lead author of the study, said: "Our study points to the possibility of even more severe changes to monsoon rainfall caused by climatic shifts that may take place later this century and beyond."

Earlier, scientists said: The vagaries of South Asian summer monsoon rainfall impact the lives of more than one billion people.

A review in Nature Climate Change of over 100 recent research articles concludes that with continuing rise in CO2 and global warming, the region can expect generally more rainfall, due to the expected increase in atmospheric moisture, as well as more variability in rainfall .

In spite of the rise in atmospheric CO2 concentration of about 70 parts per million by volume and in global temperatures of about 0.50°C over the last 6 decades, the All India Rainfall index does not yet show the expected increase in rainfall.

The reviewers Andrew Turner from the Department of Meteorology at the University of Reading and H. Annamalai from the International Pacific Research Center at the University of Hawaii at Manoa give several reasons for why the region's observed rainfall has not yet increased, among them are inconsistent rainfall observations, decadal variability of the monsoon, the effects of aerosols resulting from industrialisation, and land-use changes.

Regional projections for devastating droughts and floods - which are most meaningful for residents living in South Asia - are still beyond the reach of current climate models, according to the reviewers' detailed analyses of the present state of research.

The authors conclude that in order to make regional projections that can help in disaster mitigation and in adapting to climate change, the following is needed: establishing more consistent rainfall datasets by expanding observations to include, for example, agricultural yield; a better grasp of the complicated thermodynamics over the monsoon region and of the interactions among monsoon rainfall, land-use, aerosols, CO2, and other conditions; and an evaluation in coupled circulation models (which allow feedbacks among variables) of those processes that have been shown in simpler models to affect the monsoon and rainfall.Findings [3] related to the Arabian Sea is not also a good one. The tropical cyclones in the Arabian Sea during the pre-monsoon season (May - June) have intensified since 1997 compared to 1979 -- 1997.

This has been attributed to decreased vertical wind shear due to the dimming effects of increased anthropogenic black carbon and sulfate emissions in the region.

Decrease

The decrease in vertical wind shear, however, is not the result of these emissions, but due to a 15-day on average earlier occurrence of tropical cyclones, according to a study spearheaded by Bin Wang at the International Pacific Research Center, University of Hawaii at Manoa and published in "Brief Communications Arising" in the September 20, 2012, issue of Nature.

"About 90 percent of the pre-monsoon tropical cyclones occur during a small widow in late spring.

The mean date during which the cyclones with maximum intensity occur has advanced from June 8 in the earlier period to May 24 in the second period," explains Bin Wang.

"This advance has been accompanied by a significant decrease in vertical wind shear, which leads to tropical cyclone intensification, because large vertical wind shear is most destructive to intensification.""The ultimate reason for this earlier occurrence of storms and their intensification is the tendency we have noticed for the southwesterly monsoon to begin earlier in recent years," says Wang.

"This earlier monsoon onset is related to the greater warming of the Asian landmass than the ocean and thus an increased temperature ocean-land contrast over the last years. This greater temperature difference may strengthen the monsoon and create more favorable conditions for the formation of tropical cyclones."

"All the changes that we see in the pre-monsoon storms and the earlier monsoon onset since the late 90s, can be the result either of natural variability, namely the Interdecadal Pacific Oscillation, or of warming effects due to greater greenhouse gas emissions, but not the effect of increased aerosols. Only time and more research will tell."

- Third World Network Features.