Climate experts predict more rain in Tamil Nadu

Following a cyclonic circulation that caused rains in the city on November 6-7, the northeast monsoon, this month, has brought two back-to-back weather systems, with the third in the line. Weather systems brewing in succession, while not unusual, according to climate experts, could be due to the northeast monsoon not organising well. 

This could be the reason why systems may not intensify into cyclones, which typically bring more rain during the season. Nonetheless, the two storms on November 10 and 18 dumped 61% more rain in Tamil Nadu and 59% more in Chennai. 

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Prof Raghu Murtugudde, a climate expert from the University of Maryland and a visiting faculty member at IIT-B, told Times of India that the southwest monsoon, which brought excessive rain in September and October, did not dissipate completely, and the northeast monsoon, which arrived later, did not organise well.

When the monsoon is well-organized, conditions are favourable for systems to develop into cyclones. When cyclones shape up, strong winds cause ocean upwelling, causing cold subsurface water to move to the sea 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 another system. This year, the systems remained in a state of depression. “Conditions are still favourable for more systems to form till December… What we have to watch out for is whether they’ll bring rain or grow into cyclones,” he added. 

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Prof Murtugudde believes that one of the reasons for the poorly organised northeast monsoon could be heavy southwest monsoon spells in September and October, which have been linked to the melting of arctic ice, and the strong southwesterly winds not allowing the northeasterly winds to become strong and streamlined.

Former IMD deputy director general Y E A Raj predicted heavy rain due to a strong jet stream around 12km above sea level and an intense Siberian anticyclone on the surface.

“There are also other parameters like Indian Ocean Dipole (difference in sea surface temperature between two areas). At times, these relations could change with time or intra-seasonally. A monsoon will not normally obey a single parameter. It is unclear why NE monsoon becomes active or inactive,” he told Times Of India. 

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According to N Puviarasan, director of the IMD’s Area Cyclone Warning Centre, sea surface temperatures may have stayed warmer as the systems intensified into depressions near the coast. As a result, any winds it may have carried may not have impacted the sea surface, and the warmer sea may have continued to provide energy to systems in succession.