Explained: How rocks in Meghalaya cave connect Northeast monsoon to El Niño

Ph.D. student Elli Ronay studies a stalagmite in Mawmluh Cave. Jessica Oster/Vanderbilt University

A new study led by researchers from a US university, on the rock formations in a cave near Cherrapunji in Meghalaya, has found new evidence to suggest that India's winter rainfall could be influenced by the state of the ocean waters in the faraway Pacific. What is that connection, and how do the rocks provide evidence of it?

El Ni o & monsoons

India's summer monsoon, in the months of June, July, August and September, which brings in about 70% of annual rainfall in the country, is already known to be heavily influenced by the variability in sea-surface temperatures of Pacific Ocean, a condition referred to as El Ni o Southern Oscillation (ENSO). A warmer than usual Pacific Ocean, off the coast of South America, is known to suppress the monsoon rainfall in India.

This relationship is not so strongly established with the winter monsoon, also called as the northeastern monsoon, which occurs during the months of October, November and December and is vital for several regions in the Northeast and India's eastern coast. More than 50% of the annual rains in coastal Andhra Pradesh, Rayalaseema, Tamil Nadu, south interior Karnataka, and Kerala comes during these winter months.

ENSO is known to have an impact on the winter monsoon as well but is weaker and opposite. The warming of sea-surface waters, for example, is seen to help winter rainfall rather than suppressing it. The impact varies in time and space. The influence is weaker in October and stronger in November and December. Similarly, the rainfall over southeastern peninsular India and Sri Lanka is strengthened with warming ocean, but is diminished over Thailand, Vietnam and the Philippines.

Unexpected connection

The latest study, led by researchers of Vanderbilt University in Nashville, US, claims to have found new evidence to suggest that the state of Pacific Ocean do indeed impact the winter rains. It says the "unexpected connection" between winter rainfall amounts in northeast India and climatic conditions in the Pacific Ocean, could help in predicting the rainfall during the winter months.

The researchers, Jessica Oster of the Vanderbilt University, and Sebastial Breitenbach of Ruhr University Bochum in Germany, have published their findings in the journal Scientific Reports. Their findings are based on more than three years of research on stalagmites (mineral deposits, mainly limestone, in caves) of the Mawmluh Cave, near Cherrapunji, in the East Khasi Hills district. These solid stalagmite structures, or mineral deposits, are the result of slow but steady water dripping in the caves, and contain several thin layers of different kinds of minerals that that get picked up while the water is flowing.

From a careful study of the composition of these stalagmites, scientists can deduce the amount of rainfall that could have happened over the caves in the past, or even whether the water was a result of local rainfall, or had flown in from a different place. Using such techniques, the researchers in this case were able to estimate local variations in rainfall in the past, and then correlate it with old ocean records of the Pacific Ocean.

"These new results… suggest that potentially powerful information about annual rainfall variability in northeast India has gone unnoticed in stalagmite records thus far," Vanderbilt University said on its website.

The stalagmites indicate the recurrence of intense, multi-year droughts in India over the last several thousand years, the university said. It added that stalagmite records from monsoon regions, including India, are vital to understanding past variability in the global climate system and the underlying reasons for this variability.