'Extreme rainfall influenced by changes in local temperature'

'Extreme rainfall influenced by changes in local temperature'
Extreme rainfall in India is more influenced by changes in local temperature than global conditions, a study by researchers from the Indian Institute of Science has claimed.

By analysing 35-year climate-related data collected from hundreds of weather stations spread across the country, the researchers said they found that the intensity and frequency of extreme rainfall are greatly influenced by changes in local temperature.

Other global factors like the El-Nino Southern Oscillation and the global climate change do not influence as much as the local temperature do, the study said.

El-Nino Southern Oscillation, or simply ENSO, is the warming up of the Pacific waters.

Some studies show that a strong El-Nino in the far Pacific is accompanied by a weak Indian summer monsoon.

India received less than normal rainfall in 2015, a year which also witnessed the strongest El-Nino on record. If El-Nino can affect one year rainfall, global climate change can influence the very seasonality of the rainfall.

Using closely related branches of statistics called the extreme value theory and generalised linear models, the researchers analysed how different climatic factors, both local and global, affected extreme rainfall in India.

By going through the maze of historically observed rainfall, and land and sea surface temperatures for the period 1969–2005, they looked for clues to understand what actually influenced three extreme rainfall parameters – intensity, frequency, and duration.

"We analysed rainfall data for summer monsoon months collected by the Indian Meteorological Department from nearly two thousand locations. We modelled the extreme rainfall in the country under the influence of changes in local temperature at the regional scale, while ENSO, and changes in global temperature are considered as global factors," said Arpita Mondal, an Assistant Professor in IIT Mumbai.

"Unlike earlier studies, we looked simultaneously at the intensity, duration and frequency of extreme rainfall at fine spatial scales," said Mondal.

She carried out the study when she was a research scholar at the Divecha Centre for Climate Change, IISc.

A rainfall event is considered extreme if it crosses a high threshold. The frequency is about the number of extreme rainfall spells in June, July, August, and September, the summer monsoon months. The duration of the extreme rainfall is the number of consecutive days on which the rainfall is above the threshold.

The strong influence of changes in local temperature on extreme rainfall indicates that localised effects play a more significant role.

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