The team, which includes two Indian-origin researchers, reveal that the intensity of extremely wet spells and the number of extremely dry spells during the south Asian monsoon season have both been increasing in recent decades.
"We are looking at rainfall extremes that only occur at most a few times a year, but can have very large impact," said senior author Noah Diffenbaugh, an associate professor at the Stanford Woods Institute for the Environment.
For the new study, Diffenbaugh and graduate student Deepti Singh collaborated with Bala Rajaratnam, an assistant professor of statistics and environmental earth system science.
The team compared rainfall data gathered by the Indian Meteorological Department (IMD) and other sources over a 60-year period.
They used rigorous statistical methods to compare peak monsoon rainfall patterns during two time periods: from 1951 to 1980, and from 1981 to 2011.
The team looked specifically at rainfall during the months of July and August, which is the peak of the South Asian summer monsoon.
The analysis focused on central India, which is the core of the monsoon region and has extremely high population densities.
"We discovered that although the average total rainfall during the monsoon season has declined, the variability of rainfall during the peak monsoon months has increased," the researchers noted.
In particular, the researchers observed increases in the intensity of wet spells and in the frequency of dry spells.
"The statistical techniques show that the changes in these characteristics are robust and that these changes are unlikely to happen purely by chance," Singh said.
The team also found changes in the atmosphere, such as winds and moisture, that are likely responsible for the changes in wet and dry spells.
"There are many predictions that global warming should cause heavier downpours and more frequent dry spells," Diffenbaugh said.
"That is what we have found here, but India is a complex region, so we want to be sure before we point the finger at global warming or any other cause," he maintained.
The south Asian summer monsoon is an annual wind-driven weather pattern that is responsible for 85 percent of India's annual precipitation and is vital for the country's agricultural sector.
The monsoon season starts in June and lasts through September.
The monsoon typically starts in southern India and moves across the subcontinent. By mid-July, it is established over the entire subcontinent.
The study appeared in the journal Nature Climate Change.
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