HYDERABAD: In a cause for concern, experts in ocean sciences and meteorology have cautioned against "significant changes" in the quantum of rainfall , particularly during the south-west monsoon , in view of global warming and climate change.

A team of researchers drawn from prestigious institutions including the city-based Indian National Centre for Ocean Information Services ( INCOIS ), has pointed out that the monsoons over India are controlled by factors not only in the Indian Ocean and the Arabian Sea , but also in the Pacific and the Atlantic oceans.

The researchers said global factors as far away as the Atlantic too, play a key role in deciding the quantum of rainfall in India during the monsoon seasons. Based on the data (1979-2017), the team found a decreasing trend in rainfall (0.43 mm) during southwest monsoon and increasing trend in rainfall (0.11 mm) in northeast monsoon.

The results of the study were published in the latest issue of the scientific journal, Scientific Reports. Apart from the INCOIS in Hyderabad, researchers were drawn from the Centre for Oceans, Rivers, Atmosphere, and Land Science (CORAL), IIT, Kharagpur, and the Indian Institute of Tropical Meteorology, Pune.

The team, comprising scientists PJ Nair, A Chakraborty, H. Varikoden, PA Francis and J Kuttippurath, also introduced a new method for estimating variability and trends in rainfall over different climate regions in the country.

The new method is based on multiple linear regression and helps in assessing contributions of different remote and local climate forcings to seasonal and regional unevenness (inhomogeneity) in rainfall. Based on the new method, the team showed that the variability in rainfall during the southwest monsoon (June-September) is controlled by factors like Eastern and Central Pacific El Niño Southern Oscillation, equatorial zonal winds, Atlantic zonal mode and surface temperatures of the Arabian Sea and Bay of Bengal.

In case of northeast monsoon (October-December) the variability in rainfall is governed by sea surface temperature of the North Atlantic and extratropical oceans. “This study cautions against the significant changes in Indian rainfall in a perspective of global climate change,” researchers said. They assessed combined relationship between rainfall in India and different local and global climate processes.

“The results unravel the role of a number of climate modes including the local forcings, in addition to the commonly used global climate forcings, in determining the variability of rainfall distribution over India. A negative trend is estimated for southwest monsoon and positive trend for northeast monsoon rainfall in peninsular, north central and northeast regions. The trends are opposite in other regions.

The study also revealed some interesting facts on the variability of Indian rainfall, both in terms of contributing factors and long-term trends. The observed changes in rainfall also have signatures of the impact of regional and global climate change. It called for continued monitoring and long-term analyses of rainfall for climate impact assessments and government level decisions.

