"The results reveal a widespread significant increase in the frequency of hot days over all-India, except over the Indo-Gangetic plains," said Manish Kumar Joshi, the study lead author from IITM.
The grounds below the Koyna Dam Reservoir in Satara District, Maharashtra runs dry. Photo: Ashwin Arun Yadav / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
A study, published in the journal Scientific Reports have identified factors leading to an extension of hot extreme weather events which are becoming more common in India.
According to the researchers, between 1976 and 2018, large parts of India, except the indo-Gangetic Plains, experience on average nearly 10 extremely hot days in the April-June season, which they said is approximately 25 per cent more than in the 1951-1975 period.
Before the 1976 climate shift, they said eastern and southern parts of India experienced a significant increase in the frequency of hot days. However, after this climate shift, the study noted that hot days have significantly increased over the northwestern parts of the interior peninsula, and along the western coast.
Joshi and his team believe this indicates a spatial shift of temperature increasing trends in India, particularly in the frequency of hot days. The researchers also point out that the increase in frequency of hot extremes is associated with an atypical high pressure atmospheric system over northern parts of India.
This high-pressure system, the study reported, is linked to the flow of winds that make up anticyclones -- large-scale circulation of winds around a central region of high atmospheric pressure which spins clockwise in the Earth''s northern hemisphere, and counterclockwise in the southern hemisphere.
Joshi explained that parts of this wind system descend on northern parts of India, and mounts pressure on the land. He said this mounting pressure leads to increased atmospheric temperature, in a process called adiabatic compression.
"The high-pressure system (linked with the anti-cyclonic flow) over the northern parts of India causes sinking motion that leads to surface warming due to adiabatic compression, which in turn causes more hot extremes," Joshi said.
"Furthermore, this sinking motion is associated with reduced cloud cover, which has allowed more solar radiation reaching the Earth’s surface that in turn leads to warmer temperatures over the Indian landmass," he added.
The scientists also noted in the study that there is a significant reduction in soil moisture levels throughout India, which are also contributing to the hot extremes.
As the soil moisture evaporates, it results in less warming of the lowest part of the atmosphere that is closest to the ground, which the scientists said, limits the maximum temperatures during hot extremes.
With the soil already dried up in several other places across the country in the recent period, the study said, this has led to the larger proportion of heat being conducted from the Earth to the atmosphere, than from liquid moisture into the air.
The scientists said this induces a cyclic process between the atmospheric heating and further drying of the soil.
One of the major reasons why the cities are trapping heat are the urban building materials said the author. "These materials are water-resistant surfaces, which don’t allow water to flow through surfaces. So, without the cycle of flowing and evaporating water, these surfaces can’t be cooled automatically," Joshi added.
He also added that the hot extremes can be predicted in advance based on Indian Ocean surface temperatures as this is dependent on the El Nino/Southern Oscillation (ENSO) weather phenomenon -- an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean.