Researchers find ‘intriguing’ trends in monsoon rainfall

Researchers from IIT Madras and IIT Bombay tracking monsoon rainfall pattern in India over the past century have come out results which they described as “intriguing and contradictory to common belief.”

These insights are critical not only for understanding geographic variations in seasonal rainfall, but also for framing long-term water management policies of the country, they said. The work has been published in the peer-reviewed journal PLOS.

The research team comprised first author Subimal Ghosh and Subhankar Karmakar from IIT Bombay, and KS Kasiviswanathan, KP Sudhir and 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, and Union Ministry of Earth Sciences, it analysed countrywide rainfall data over the past century to find trends and variations.

The team warned that recent observations that monsoon has grown more unpredictable than before bodes ill for a country whose societal and economic well-being is critically linked to seasonal rains.

Extreme events such as the floods in Kerala and the ongoing zero-water situation in the adjoining Tamil Nadu stand testimony to the recent vagaries of the Indian summer monsoon.

Convection as driver

The research team sought to understand the nature of these variations in the monsoon rainfall and impacts of climate change on the temporal and spatial rainfall patterns through analysis of historic data and simulation studies.

Pointing out a major finding, Sachin Gunthe said the research showed that the amount of rains has decreased in river basins with surplus water and has increased in basins with deficit water. It is common knowledge, he said, 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.

More studies needed

This is important because it contradicts the traditional notion of dry areas becoming drier and wet areas becoming wetter in response to climate change, said Gunthe. Concerted and logical approaches based on these observations would greatly benefit national-scale climate-water adaptation and regional preparedness and framing long-term water management policies for the country.

The regional aspects of the Indian summer monsoon rain pattern are difficult to understand because it is affected by many factors. Reasons for the unusual rain pattern remain unclear and rigorous hypothesis-driven models and process studies are required.

The summer monsoon contributes approximately 80 per cent of the annual total rainfall of the country and plays a decisive role on its agricultural output. Sixty per cent of agriculture depends upon the monsoon rain for irrigation.

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