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Clouds originating over Indian Ocean cool Bay of Bengal waters

The formation of a pool of cooler water around India and Sri Lanka during summer monsoon, called the Bay of Bengal cold pool, has intrigued many a scientist in the past. Now, a team of researchers at Indian Institute of Science, using computer simulations and satellite data, have discovered the processes that lead to the formation of this extraordinary pool of cold water. As it turns out, clouds originating over the Indian Ocean are to blame for the cold pool in the Bay of Bengal.

“We have discovered the processes that lead to the formation of the cold pool whereas previous studies noted its existence”, says P. N. Vinayachandran, Professor, Centre for Atmospheric and Oceanic Sciences , IISc.

Enclosed by Bangladesh on the North, India and Sri Lanka on the west and Myanmar and Andaman and Nicobar Islands on the east, the Bay of Bengal is the largest bay in the world. Every year, the appearance of a pool of relatively colder water in Southern Bay of Bengal, has perplexed scientists ever since it was reported for the first time back in 2005. The transition from warmer waters in pre-monsoon months to the cooler waters has never been fully understood.

Based on  satellite derived Sea Surface Temperature (SST), precipitation and surface wind data, the researchers from the IISc have attempted to explain why the water cools down. They simulated the interaction between the oceans and the atmosphere using mathematical models which take into consideration the influence of the Indian Ocean, the ocean currents, salinity and temperature.

The cooling starts from mid April and continues till September. Analysing  the data obtained from the Indian Meteorological Department and satellites, the researchers identified three major cooling events corresponding with the onset of Monsoon.  In April, prior to the onset of Monsoon, in what is called 'pre-Monsoon cooling', the sea surface temperature of the cold pool dropped by around 0.8°C.  Between May and June,  sea surface temperatures dip furher making the waters cooler by about 1.8o C. Since this event coincides with the onset of the monsoon over Kerala, it is called 'onset cooling'. Between June and September, after the onset of monsoon, the cold pool experiences a number of weaker cooling events. This phase is called 'intraseasonal cooling'.  

From simulations, the researchers observed that the formation of  the cold pool entailed  the formation of cloud bands over the Equatorial Indian Ocean (EIO). If a cloud band that originated in western EIO caused the   pre-monsoon cooling,  , a cloud band with origins in the eastern EIO led to onset cooling. These cloud bands, which  travel northward, make a pool of water Bay of Bengal to become unusually cooler.   The researchers observed the same sequence of events every year, with a small change in the time at which it started .  This study  confirmed that  atmospheric variations can significantly influence sea surface temperatures.

“When the cloud band arrives over the cold pool regions, the clouds blocks  a major part of the solar radiation which would otherwise reach the sea surface. In addition, the cloud bands are usually accompanied by stronger winds. These winds increase evaporation from the sea surface and cause stronger mixing in the ocean. Thus reduced radiation, increased  latent heat loss and stronger mixing in the ocean are the processes that decreases sea surface temperature after the arrival of the cloud band”, explains Prof. P.N. Vinayachandran.

The implication of the cold pool to the summer monsoon is still not fully  understood and therefore further investigations are  required to throw light on the various affects the cold pool has on the region. By providing an explanation for the formation of the cold pool, the current research provides a platform to further probe the mysteries of the curious phenomenon of the Bay of Bengal cold pool.

About the authors:

P.N Vinayachandranis a professor at the Centre for Atmospheric and Oceanic Sciences at IISc. Umashankar and Ambica Behara are PhD students at the Centre for Atmospheric and Oceanic Sciences at IISc.

Contact details:

Umashankar Das: umasankardas0705@gmail.com

P.N Vinayachandran: vinay@caos.iisc.ernet.in

Ambica Behara: ambika@caos.iisc.ernet.in

About the publication:

The paper 'Formation of the southern Bay of Bengal cold pool'appeared in the journal Climate Dynamics.