Warming of Indian Ocean may

 weaken monsoon

The study by scientists from Pune's Indian Institute of Tropical Meteorology (IITM), Sorbonne University in Paris and Pune's Fergusson College found the warming of the Indian Ocean has been a major contributor to the total global sea surface warming, which may have long-term effects on the climate such as weakening the south-west monsoon and being detrimental to marine biodiversity. 

Global ocean surface warming has long-term effects on the climate since it persists for a longer time compared with land temperatures. 

Mathew Koll Roxy, lead author of the research and senior scientist at the Centre for Climate Change Research at IITM, told TOI this is the first time scientists have discovered warming of this rate and magnitude over the Indian Ocean. "Earlier analysis were for shorter periods (e.g. past 50 years), while our current analysis is with multiple datasets for the past 112 years," he said. 

The study was recently published online by the Journal of Climate of the American Meteorological Society. It is part of an Indo-French collaboration carried out under the National Monsoon Mission set up by the ministry of earth sciences. Ritika Kapoor, Pascal Terray and Sebastien Masson are the study's co-authors. 

Roxy said among the major oceans, Indian Ocean is the smallest but also the warmest. Hence it plays a critical role in regulating the climate and variability of the Asian monsoon, as well as the global climate. This research found warming over the Indian Ocean to be happening at a rate faster than any other region of the tropical oceans, and with a 'potential' to alter the strength and course of the monsoon. 

Generally, the western Indian Ocean has cool surface waters, while the central-east Indian Ocean is a warm pool with sea surface temperature (SST) greater than 28 degrees C. "Earlier studies have shown that the warm pool has considerably warmed over the past half-century, though the reasons have remained elusive until now," said Roxy. "However, our research, which used extended data of sea surface temperature during 1901-2012, revealed that the relatively cool western Indian Ocean has been warming for more than a century, and has attained the warm pool SST values of 28 degrees C." 

Scientists attributed the warming over western Indian Ocean to increasing El Nino events, among other causes. "The periodical El Nino events over the east Pacific change the global atmospheric circulation, and weaken the winds over the western Indian Ocean," said Roxy. "This induces abnormal warm surface waters over the western Indian Ocean. Meanwhile the La Nina events fail to make any significant changes over the Indian Ocean. Along with this, the frequency of El Nino events has increased during recent decades. This piles up the heat over the western Indian Ocean, resulting in excessive warming." 

He said the results of the study may help in understanding the long-term decrease in monsoon rainfall, but not for individual years.