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.

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.

Reason for flood

• 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.
• 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.

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.

El Nino Effect

• El Nino events are associated with a warming of the central and eastern tropical Pacific.
• El Nino occurs when the normal trade winds weaken (or even reverse), which lets the warm water that is usually found in the western Pacific flow instead towards the east.
• This warm water displaces the cooler water that is normally found near the surface of the eastern Pacific, setting off atmospheric changes that affect weather patterns in many parts of the world.
• These consequences include increased rainfall across the southern tier of the US and in Peru, and drought in the West Pacific.

El Nino effects on Indian Monsoon

• El Nino signal shows up in atmospheric circulation, the local sea surface temperature can also change.
• It 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.

Source : The Hindu

GS II : Important Geophysical phenomena