Extreme rainfall tied to local temperatures?

Extreme rainfall tied to local temperatures?

Extreme rainfall in India is more influenced by changes in local conditions than by changes in global conditions, a study has found. This result helps us better understand extreme rainfall events that have caused havoc in some of the Indian cities. The study was carried by the researchers from the Indian Institute of Science, Bengaluru.

By analysing climate related data collected from hundreds of weather stations spread across the country for over 35 years, the researchers 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 (ENSO) and global climate change do not influence as much as the local temperature.

El Nino is the warming up of the Pacific waters. ENSO is a periodic variation in winds and sea surface temperature over the Pacific. It is a naturally occurring phenomenon which looks at fluctuating oceanic temperatures in the equatorial Pacific. A coupled climate phenomenon, ENSO consists of three states: El Nino, La Nina and neutral. 

Some studies have shown that a strong El Nino in the far away 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’s rainfall, global climate change can influence the very seasonality of the rainfall. Interestingly, as the IISc study has shown, when it comes to intensity and frequency of the extreme rainfall, local conditions have a stronger influence than the other two global factors. 

Some calculationsUsing 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 between 1969-2005, they looked for clues to understand what actually influenced three extreme rainfall parameters — intensity, frequency, and duration.

Calculations showed that if the intensity and frequency danced to the tune of changes in local temperature, the duration of the extreme rainfall remained mostly uninfluenced.“We analysed rainfall data for summer monsoon months collected by the Indian Meteorological Organisation from nearly 2,000 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. Unlike earlier studies, we looked simultaneously at the intensity, duration and frequency of extreme rainfall at fine spatial scales,” said Dr Arpita Mondal, now an assistant professor in IIT Mumbai. She carried out the study when she was a research scholar at the Divecha Centre for Climate Change, Indian Institute of Science, working under the guidance of Pradeep Mujumdar, professor, department of civil engineering, 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 and 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. One example of such localised processes is urbanisation. Further research needs to be carried out to categorically link changes in urbanisation and other local processes to those in the extreme rainfall characteristics.

(The author is with Gubbi Labs, a Bengaluru-based research collective)

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