Chennai floods due to climate change?

Sea surface temperatures were above normal.  

The recent heavy record deluge of rainfall over Chennai was due to above normal sea surface temperatures in the Bay of Bengal, presence of monsoon trough close to Chennai, prevailing easterly waves and strong easterly winds. The 2015 El Nino phenomenon weakened South west monsoon winds which in turn led to less churning and upwelling in the Bay of Bengal and Arabian Sea and resulted in higher sea surface temperatures due to less mixing of cold waters with warm surface waters. This had a warming effect in Bay of Bengal. During the post-NW monsoon withdrawal period (later part of October and November) a monsoon trough prevailed around 13N latitude (latitude of Chennai) over peninsular India and Bay of Bengal.

The monsoon trough is a large area of low pressure that can move in any direction and carry convective winds with it. The high SSTs moistened the boundary layer (the lowest layer of the atmosphere in touch with the ocean surface) by evaporation. This moist air was lifted high up in the atmosphere by easterly waves emanating some distance from the coast at Chennai. Easterly waves are an atmospheric phenomenon wherein the wind moves in a wave like motion with the distance from crest to crest being as much as 2000 km (with a forward and rear sections of 1000 km each) and the period about two to three days. “Therefore it can be believed that the phenomenon might have persisted for 3 -4 days with spatial scales of approximately 500-1000km,”said Dr. J.R. Kulkarni, Team Expert, World Meteorological Organization, Ex-Adviser, IITM, Pune, to this Correspondent with whom he was in telephone and email contact.

The upward motion of the winds in the wave is called divergence and the downward, convergence. The divergence lifted moisture bearing air with it and carried it high up into the atmosphere as rain bearing clouds, and the easterly winds drove these clouds towards Chennai where they precipitated as rain. The continuous formation of clouds over Bay of Bengal, their transformation into deep convective clouds, their movement towards land and to Chennai city provided continuous heavy rainfall over land and the Chennai area. These clouds provided very heavy rainfall of the order of 150 mm/hour. If the rainfall is more than 100 mm/hour, then it is termed as a cloud burst.

For converting water into vapour, energy is required to provide to the water molecules. This energy is called latent heat of vaporization. As the water vapor goes up it cools due to ambient cooler atmosphere. It becomes saturated. The water vapor turns into water droplets. In this process, the energy stored in the water vapor is released to the atmosphere. This heats the cloud air. The cloud air becomes warm and light. The cloud air gets acceleration in upward direction because of receiving of latent heat. Temperature in the atmosphere decreases with height. At about 5 km above the ground, temperature becomes 0°C. This is called freezing level. The cloud drops freeze and become ice above freezing level. In the freezing process heat is released which is called latent heat of freezing. The cloud droplets and ice particles get additional upward motion by getting this energy. This way the cloud grows to a very high altitude.

Studies have shown that south west and north east monsoons have negative correlation. North east monsoon during the 2009 El Nino year was stronger than normal. Rainfall in 2009 north east monsoon was 12 per cent more than long term normal (IMD Departmental website Chennai). This year’s high north east monsoon rainfall performance is consistent with earlier findings of negative correlation between the two monsoons in the El Nino years. The higher than normal sea surface temperatures are the real fuel of such deep convection. Climatologically these anomalies weaken in the December. Therefore it may be conjectured that heavy rainfall activity may weaken in December. “Radar data are required to check movement of clouds, their transformation into deep convective clouds, and their spatial and temporal scales. Modeling studies are required for examining various linkages in the proposed mechanism,” said Dr. Kulkarni.

“Extreme weather events are indeed increasing due to global warming. This is because a warmer climate can hold more moisture in the atmosphere, leading to heavier rainfall when it does occur. However, it’s difficult to say for certain that a particular extreme event – like the Chennai floods – is attributable to anthropogenic climate change,” says Dr. Roxy Mathew Koll of The Indian Institute of Tropical Maeteorology, Pune.

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Printable version | Oct 15, 2021 9:00:44 PM | https://www.thehindu.com/sci-tech/energy-and-environment/chennai-floods-due-to-climate-change/article7980332.ece

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