Indian Monsoons Becoming More Extreme

By combining meteorological records spanning 60 years with rigorous statistical methods, scientists have shown that Indian monsoons have become more variable.

AsianScientist (May 9, 2014) – Scientists have identified significant changes in the patterns of extreme wet and dry events that are increasing the risk of drought and flood in central India, one of the most densely populated regions on Earth.

The discoveries, detailed in the journal Nature Climate Change, are the result of a new collaboration between climate scientists and statisticians that focused on utilizing statistical methods for analyzing rare geophysical events.

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 typically starts in southern India and moves across the subcontinent. By mid-July, it’s established over the entire subcontinent,” said Deepti Singh, lead author of the study and a PhD student at Stanford University.

Singh said that rainfall extremes during the months of the monsoon season can be as important as how much total water is received. For example, during critical crop growth stages, too many days without rain can reduce yields or lead to crop failure, which can reverberate through India’s agriculture-dependent economy. At the same time, short periods of very heavy rainfall can create humanitarian disasters, such as in 2005, when massive flooding killed thousands of people in Mumbai.

Because such extreme events are rare, it can be difficult to study them objectively. For the new study, the team wanted to test whether the pattern of extreme wet and dry “spells” during the monsoon season had changed in recent decades.

The team compared rainfall data gathered by the Indian Meteorological Department 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.

They looked specifically at spatial and temporal relationships, which are typically ignored in “classical” or “off-the-shelf” statistical tools that were originally designed for use in the fields of biology, medicine and agriculture. They found that although the average total rainfall during the monsoon season has declined, the variability of rainfall during the peak monsoon months has increased. 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 says that the next step is to investigate what might be causing the changes in the atmosphere.

The article can be found at: Singh et al. (2014) Observed Changes in Extreme Wet and Dry Spells During the South Asian Summer Monsoon Season.

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Source: Standford University; Photo: Karunaka Rayker/Flickr/CC.

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