A muffling of the monsoons

New research conducted at the Potsdam Institute for Climate Impact research suggests that global warming could cause frequent and severe failures of the Indian summer monsoon in the next two centuries.

The study joins a growing body of work conducted by different research groups across the last five years that demonstrate a negative relationship between the two phenomena.

The researchers, Jacob Schewe and Anders Levermann, defined failure as a decrease in rainfall by 40 to 70 per cent below normal levels. Their findings, published on November 6 in the Environmental Research Letters, show that as we move into the 22nd century, increasing temperatures contribute to a strengthening Pacific Walker circulation that brings higher pressures over eastern India, which weaken the monsoon.

The Walker circulation was first proposed by Sir Gilbert Walker over 70 years go. It dictates that over regions such as the Indian peninsula, changes in temperature and changes in pressure and rainfall feedback into each other to bring a cyclic variation in rainfall levels.  The result of this is a seasonal high pressure over the western Indian Ocean.

Now, almost once every five years, the eastern Pacific Ocean undergoes a warm phase that leads to a high air pressure over it. This is called the El Nino Southern Oscillation.

In years when El Nino occurs, the high pressure over the western Indian Ocean shifts eastward and brings high pressure over land, suppressing the monsoon.

The researchers’ simulation showed that as temperatures increased in the future, the Walker circulation brings more high pressure over India on average, even though the strength of El Nino isn’t shown to increase.

The researchers described the changes they observed as unprecedented in the Indian Meteorological Department’s data, which dates back to the early-1900s. As Schewe, lead author of the study, commented to Phys Org, “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.”

A study published in 2007 by researchers at the Lawrence Livermore National Laboratory, California, and the International Pacific Research Centre, Hawaii, showed an increase in rainfall levels throughout much of the 21st century followed by a rapid decrease. This is consistent with the findings of Schewe and Levermann.

Similarly, a study published in April 2012 by the Chinese Academy of Sciences demonstrated the steadily weakening nature of the Indian summer monsoon since 1860 owing to rising global temperatures.

The Indian economy, being predominantly agrarian, depends greatly on the summer monsoon which lasts from June to September. The country last faced a widespread drought due to insufficient rainfall in the summer of 2009, when it had to import sugar and pushed world prices for the commodity to a 30-year high.