The University of Hyderabad and the Indian Institute of Technology Bombay together conducted a study to find out the reasons behind the 2015 Chennai floods.
As per the study, extreme El Nino conditions and warming of the Bay of Bengal had resulted in unprecedented heavy rainfall in Chennai between 30 November 2015 and 2 December 2015.
The study was published in the journal Climate Dynamics.
• Generally, El Nino causes less than normal rainfall in the case of the southwest monsoon. However, it brings about above-normal rainfall during the northeast monsoon. This happens due to the difference in seasonal wind patterns between the two monsoons.
• The scientists also suggest that the consistent warming of the Bay of Bengal off the coast of Tamil Nadu and Andhra Pradesh have played an important role in causing the floods in the area.
• A simple linear correlation analysis carried out by the researchers indicates that the Bay of Bengal sea surface temperature positively and significantly correlated with northeast monsoon rainfall.
• The magnitude of correlations of northeast monsoon rainfall with El Nino conditions and the Bay of Bengal warming are nearly similar.
• Based on sensitivity experiments, the researchers conclude that about 21 per cent of the intensity of the extreme Chennai rainfall can be attributed to the extreme El Nino conditions.
• However, the researchers also caution that these results have been arrived using just one dynamical model.
• It remains to be seen if this kind of contribution from the tropical Pacific to extreme rainfall during the northeast monsoon happens only during extreme El Nino or whether normal El Ninos too can cause it.
What is El Nino?
• El Nino is the warm phase of the El Nino Southern Oscillation (ENSO).
• The phase is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific, including off the Pacific coast of South America.
• ENSO refers to the cycle of warm and cold temperatures, as measured by Sea Surface Temperature (SST), of the tropical central and eastern Pacific Ocean.
• It is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific.
• The cool phase of ENSO is called La Nina with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific.
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