IIT Madras & IIT Bombay Researchers Study variations in Indian Summer Monsoon Rainfall for past Century

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Chennai: Researchers from Indian Institute of Technology Madras and Indian Institute of Technology Bombay have studied the rainfall pattern during Indian summer monsoon period for the past century.

Using recorded rainfall data and simulation studies, the Research team has gained surprising insights that are critical not only for understanding geographic variations in seasonal rainfall in India, but also for framing long-term water management policies of the country. Their work has been published in the reputed peer-reviewed journal PLOS.

The Research team comprised Dr. Subimal Ghosh and Dr. Subhankar Karmakar from IIT Bombay, Dr. K.S. Kasiviswanathan, Dr. K.P. Sudhir and Dr. Sachin Gunthe from IIT Madras along with their research students.

Supported by the Max Planck Partner Group at IIT Madras, Department of Science and Technology, Government of India, and Ministry of Earth Sciences, Government of India, this multi-institute team used rainfall data across the entire country over the past century to show the trends and variations in monsoon rains.

The Indian summer monsoon that falls between June and September contributes approximately 80 per cent of the annual total rainfall of the country and plays a decisive role on the country’s agricultural output. Sixty percent of Indian agriculture depends upon monsoon rain for irrigation, which in turn decides the economy because agriculture accounts for eighteen per cent of India’s gross domestic product.

Recent observations, both at the meteorological level and from local perceptions, that monsoon has grown more unpredictable than before, bodes ill for a country whose societal and economic wellbeing is critically linked to seasonal rains. Extreme events such as the floods in Kerala and the ongoing zero-water situation in the adjoining state of Tamil Nadu stand testimony to the recent vagaries of the Indian summer monsoon.

The collaborative team from IIT Madras and IIT Bombay seeks to understand the nature of these variations in Indian summer monsoon rainfall and the impacts of climate change on the temporal and spatial rainfall patterns through analysis of historic data and simulation studies.

Speaking about this Research, Dr. Sachin S Gunthe, Associate Professor (Environmental and Water Resources Engineering Division), Department of Civil Engineering, IIT Madras, said, “The regional aspects of the Indian summer monsoon rain pattern are difficult to understand because it is affected by many factors. In order to discern rainfall patterns reliably, the we (researchers) used IMDB’s daily rainfall data for the years 1901–2004 and performed simulations taking into consideration, phenomena such as maximum temperature, minimum temperature, rainfall and wind velocity. The model was validated with satellite-based observation from the European Space Agency- Climate Change Initiative (ESA-CCI) soil moisture data.”

The first author of the PLOS paper Dr. Subimal Ghosh of IIT Bombay added, “The results of the investigations were intriguing and contradictory to common belief.”

It is common knowledge that geographic variation of extremes in rainfall occurs due to convection – the movement of moisture-laden hot air upwards, followed by cooling at higher altitudes and shedding of the moisture as rain. Convection-based rains would mean that regions where there is excess moisture in the air should experience more rainfall. This, however, was not seen in the rainfall pattern analysed by the research team.

“Our study shows that the amount of rains has decreased in river basins with surplus water and has increased in basins with deficit water,” explained Dr. Gunthe.

This observation is important because it contradicts the traditional notion of dry areas becoming drier and wet areas becoming wetter in response to climate change.

The researchers caution that the reasons for the unusual pattern of rainfall remain unclear and rigorous hypothesis-driven models and process studies are required. The observations by the IIT Bombay-IIT Madras team would undoubtedly serve as a launchpad for future scientific investigations on the root causes of regional vagaries of rainfall. Concerted and logical approaches based on these observations would greatly benefit national-scale climate-water adaptation and regional preparedness.

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