'Anomaly in Indian Ocean warming could affect future monsoon pattern'

Researchers have found evidence of a major anomaly in the warming of the Indian Ocean, which could alter the strength of the southwest monsoon.

It could also adversely impact the marine food-webs over one of the most biologically productive regions, says Roxy Mathew Koll, lead researcher and co-author of a paper on the subject.

WARMEST OCEAN

“The results of our study challenge our current understanding of Indian Ocean warming and climate change,” Koll told BusinessLine .

Koll is associated with the Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune. The co-authors are Ritika Kapoor, Terray Pascal, and Masson Sebastien.

Titled ‘The curious case of Indian Ocean warming,’ the paper is scheduled to appear in the Journal of Climate , published by the American Meteorological Society.

Being the warmest among the major oceans, the Indian Ocean plays a critical role in regulating the monsoon as well as the dynamics over the tropics.

During summer, the central-east Indian Ocean hosts a ‘warm pool’ of waters with sea surface temperatures (SSTs) greater than 28 deg Celsius.

ENHANCED CONVECTION

This makes it highly conducive for enhanced convection, which involves the process of building rain-bearing clouds.

Studies on trends during the past half century have pointed out substantial warming over this warm pool, though the reasons have remained ambiguous.

But an analysis of the warming during 1901-2012 by Koll and team has revealed a different picture of Indian Ocean warming.

“The focus now shifts to the relatively cool western Indian Ocean,” Koll says.

“We find that the western tropical Indian Ocean has been warming for more than a century at a rate faster than any other region of the tropical oceans, and has attained the warm pool SST values of 28 deg Celsius.”

While the warm pool (in the east) went through a warming of 0.7 deg Celsius, the western basin experienced an anomalous increase of 1.2 deg Celsius in summer.

GRADIENTS WEAKEN

Thus, the warming of the generally cool western Indian Ocean against the warm pool region (in the east) weakens the SST gradients.

This has the potential to change the monsoon circulation and rainfall, as well as alter the marine food webs in this biologically productive region.

This is because warming causes the air over the ocean to expand and lower the atmospheric pressure. It unsettles the wind pattern which, in turn, decides the monsoon flows.

Analysing the causes, the research said the warming trend over the Indian Ocean is highly dependent on what happens in the equatorial Pacific.

The El Nino induces anomalous warming over the western Indian Ocean; but its alter ego La Nina fails to do the inverse.

A second, prominent reason is that the frequency of El Nino events has increased in recent decades.

EL NINO IMPACT

In an El Nino, the Pacific Ocean appears to throws out its heat, which partially gets accumulated in the Indian Ocean.

Post-1950, a few warm events over the Indian Ocean have attained the threshold value for El Nino (anomalies greater than 0.77 deg Celsius).

This places these warm events almost on a par with the El Nino in magnitude, Koll says.

In this manner, the Indian Ocean warming turns out to be the largest contributor in phase, with the overall trend in the global SST.

Over the long term, the warming scenario and related climate dynamics are factors to be vigilant of, while assessing climate change and variability.

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