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The Cause

Chennai weather, Aside from the cyclonic circulation that delivered rain to the city on November 6-7, the northeast monsoon has brought two back-to-back weather systems to the city this month, with a third one on the way. Weather systems forming in rapid succession, according to climate experts, maybe owe to the northeast monsoon’s poor organisation.

This could also explain why systems aren’t strengthening into cyclones, which are known for bringing more rain during the season. Despite this, the two depressions that hit Tamil Nadu on November 10 and 18 had already dumped 61 percent more rain in the state and 59 percent more in Chennai.

An analysis of Chennai’s and the state’s average rainfall from October to December.

The Research Conducted

Prof Raghu Murtugudde, a visiting faculty member at IIT-B and a climate expert from the University of Maryland, said the southwest monsoon, which produced excessive rain in September and October, did not evaporate fully, and the northeast monsoon, which arrived late, did not organise properly. When the monsoon is well-organized, the conditions are often favourable for systems to develop into cyclones.

When cyclones occur, powerful winds create ocean upwelling. Which causes cold subsurface water to rise to the surface, while warm sea surface water would have provided energy to the system. As a result, it takes a few days for the ocean surface; to warm up and provide energy to a different system.

The Downward Spiral

 The systems remained in a downward spiral this year. “Until December, the conditions are favourable for more systems to emerge… “What we need to keep an eye on is whether they will provide rain or develop into cyclones,” he added.

As the systems intensified into depression along the coast, N Puviarasan, director of the IMD’s Area; Cyclone Warning Centre, said, sea surface temperatures may have remained warmer. As a result, the winds it carried may not have had an impact on the sea surface; and the warmer sea may have continued to provide energy to systems in a loop.

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