By Rajanikanth Poolla

Hyderabad: Its summer 2020 and the heat is on, albeit rather late in the season. The Telugu states have been reeling under a sweltering heatwave, the first of the season, for the last couple of days. The heatwave comes in the wake of super cyclone Amphan crossing the coast of WestBengal. The cyclone, however, did not have a cooling effect here and only left the Telugu states hot and dry.

This is not the first time severe heatwaves were observed in the aftermath of a cyclone drifting away from the east coast. Last year, cyclone Fani that struck Odisha also raised Mercury levels with the entire Andhra coast experiencing severe heatwave and temperatures above 45 degrees Celcius.

So, how are summer cyclones and heatwaves related? It is not unusual for the Bay of Bengal to produce cyclones in mid-summer. Due to high ocean heat and favourable moisture flow, they grow quickly into severe cyclonic storms. Also, due to the seasonal upper tropospheric wind patterns, they drift away from the east coast, usually making landfall over either east India, Bangladesh, or Myanmar. Both their intensity and their movement away from the southern peninsula contribute to dry atmospheric conditions and resultant heatwaves over south India.

To know this correlation, we must understand the mechanism of cyclone formation and sustenance. All cyclones begin as a low pressure that forms when warm moist air on sea surface expands and rises upwards. Warm sea surface water evaporates, supplying moisture. Thus, heat and moisture are the primary fuels for the cyclonic storm. Wind flows from the surrounding atmosphere at high pressure into this area and starts circulating around the low-pressure centre forming a cyclonic circulation. Not all moisture is supplied from sea surface. The storm draws moist winds from the surrounding atmosphere for its sustenance and further strengthening. According to a research conducted by physicists at Petersburg Nuclear Physics Institute, “Rather than being driven by heat extracted from the ocean, hurricanes gobble up pre-existing moisture stocks from the atmosphere as they move.” This results in the surrounding atmosphere getting suddenly dry and deprived of water vapour. Research corroborates this theory, “Such hurricanes leave a dry footprint, whereby rainfall is suppressed by up to 40 per cent.”

As the storm grows strong, more and more moisture is pulled, which aids in cloud formation and intense rains in its path. Researchers at the department of atmospheric sciences in the University of Illinois note, “Contribution of vertically integrated inward moisture flux increases from the tropical wave stage to the tropical cyclone stage.”

The movement away from east coast aligns the surface wind pattern to blow from NW direction across core heat zones Rajasthan, Madhya Pradesh, and Maharashtra towards Telangana and Andhra Pradesh. The convergence of this dry, hot air at certain spots dramatically increases Mercury levels.

The current dry, hot phase is most likely to continue for another week over Telugu states. Thereafter, the inflow of moisture due to monsoon winds can ease the situation. IMD expects monsoon to make onset over Kerala by 5 June.

(Rajanikanth Poolla is a weather analyst)

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