Sci-Tech

Scientists find out why Chennai was deluged in 2015

The extreme El Nino conditions in 2015 and the warming trend in the Bay of Bengal contributed equally to the unprecedented heavy rainfall witnessed in Chennai for three days from November 30 to December 2, 2015, researchers have found.

El Nino generally causes less than normal rainfall in the case of the southwest monsoon. In contrast, it brings about above-normal rainfall during the northeast monsoon. This is because of the difference in seasonal wind patterns between the two monsoons.

A study by the University of Hyderabad and the Indian Institute of Technology – Bombay suggests that the extreme El Nino that occurred in 2015 played an important role in Chennai’s heavy rainfall. After 1982 and 1997, the 2015 event also turned out to be an extreme El Nino event.

“Another factor that seems to have played an important role is the consistent warming of the Bay of Bengal off the coast of Tamil Nadu and Andhra Pradesh,” says Dr. Karumuri Ashok from the Centre for Earth and Space Sciences, University of Hyderabad, and the corresponding author of the paper published in the journal Climate Dynamics.

Linear correlation

A simple linear correlation analysis carried out by the authors 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 we could conjuncture that about 21% of the intensity of the extreme Chennai rainfall can be attributed to the extreme El Nino conditions,” Dr. Ashok says. “Interestingly, the warming trend in the Bay of Bengal [sea surface temperature] contributed equally to the Chennai event.”

El Nino can affect the Indian monsoon through atmospheric circulation. Whenever the El Nino signal shows up in atmospheric circulation, the local sea surface temperature can also change.

“Experiments suggest that changes in local sea surface temperature seem to be stronger than local atmospheric changes,” Dr. Ashok says. He however cautions that these results have been arrived using just one dynamical model. “We need to confirm if this result is correct. So we need to repeat using more cases with the same model and similar type of studies using other numerical models,” he says.

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.

“This is important because the El Nino itself is changing,” Dr. Ashok says.

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Printable version | Aug 8, 2020 1:17:57 AM | https://www.thehindu.com/sci-tech/scientists-find-out-why-chennai-was-deluged-in-2015/article19217487.ece

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