The Indian Ocean has been warming at a rate faster than thought before (1.2 deg C during the past century). It is also the largest consistent contributor to the global ocean warming trends. Recent studies show that a warm Indian Ocean can in turn modulate the Pacific conditions including the El Nino events. So basically, such large warming over the Indian Ocean has implications on the global climate.

The western Indian Ocean, traditionally thought to have cooler sea surface temperatures (SSTs) than the central and eastern Indian Ocean, is surprisingly showing an even stronger summer warming trend over the whole of the 20th century than the central and eastern Indian Ocean. The warming is significantly so large that it may alter the monsoon circulation, monsoon rainfall over the ocean and land, marine food webs and fisheries (western Indian Ocean is one of the most productive oceans) and global climate including the El Nino.

A recent study focused on the causes for this warming and found that it was mainly due to El Nino events, which are getting stronger and more frequent during recent decades, possibly due to a changing climate. These El Nino events weaken the summer westerly (blowing from west to east) winds over the Indian Ocean. Winds have the effect of cooling the sea surface. Strong winds cause evaporation and loss of latent heat from the ocean leading to cooling. When the winds are weakened the opposite happens — the ocean warms.

The study published recently in the Journal of Climate was undertaken by Dr. Roxy Mathew Koll of the Indian Institute of Tropical Meteorology, Pune and co-authored by Ritika Kapoor, Pascal Terray and Sebastien Masson. This work is part of an Indo-French collaboration, carried out under the National Monsoon Mission set up by the Ministry of Earth Sciences.

The study found that the ocean atmospheric phenomenon — the El Nino, and its influence on the Walker Circulation were responsible for periodic weakening of monsoon westerlies and led to abnormally high summer sea surface temperatures in the western Indian Ocean. Walker Circulation represents the zonal (east-west) circulation over the tropics. In the Pacific, the trade winds blow from east to west. This is because of the low pressure in the west and high pressure in the east, which drives these winds. This is linked to the warm waters over the west Pacific and cool waters over the east Pacific. In the upper atmosphere, this trade wind circulation is completed by winds blowing from west to east.

During summer, this Pacific cell is linked to the monsoon westerlies in the Indian Ocean. During an El Nino, due to warm waters in the east and cool waters in the west, the pressure systems also change, reversing (shifting) the Walker circulation. The wind direction reverses and the whole circulatory system shifts eastward leaving only weak surface westerlies in the western Indian Ocean. This results in ocean warming in the Western Indian Ocean.

A simultaneous correlation analysis between the eastern Pacific and

global summer mean SST anomalies, after removing the global warming trends, depicts significant positive correlation over the western Indian Ocean. This indicates that ENSO dominates the western tropical Indian Ocean variability during summer.

“The frequency and magnitude of El Ninos have also increased in the recent decades, possibly due to global warming. This means a piling up of heat on the Indian Ocean,” notes Dr. Roxy Koll in an email.

Unlike the Pacific or the Atlantic Oceans, the Indian Ocean is land-locked on the north. This means that the ocean circulation which carries the tropical heat towards the poles is restricted in the Indian Ocean, withholding the piled up heat.

When queried how Indian Ocean warming affects the El Nino, Dr. Koll noted: “Earlier studies have shown that enhanced tropical Indian Ocean warming in recent decades favours stronger trade winds in the western Pacific via the Walker circulation and hence is likely to suppress the El Nino events.”

“The western Indian Ocean, due to its cool, nutrient rich waters, is

abundant with marine phytoplankton and the fisheries thrive on it. Excessive warm waters can be detrimental to phytoplankton production which in turn affects the fisheries,” he notes.