The intensity of slow pulses of rainfall in the country has decreased over the past sixty years and is replaced by an increase in short, high intensity rainfall events known as extreme events, a study by the Indian Institute of Science (IISc) has revealed.

The study separated the slow pulses in the monsoon which last 20-60 days from the faster pulses. “On investigating the data from 1961 to 2013, it was found that the intensity of these slow pulses has decreased over the past sixty years. This decrease has been compensated by an increase in short, high intensity rainfall events, known as extreme events,” Nirupam Karmarkar, lead author of the study and a PhD candidate at the IISc, said. The research also detected an increase in extreme events over coastal regions in May and June and over central India in July and August.

“Previous studies have documented this increase, but our main concern was to find how these extreme events are spread between the active, break and transition periods in a season,” Karmarkar, whose paper was published in Environment Research Letters journal, said.

“A step towards addressing the issue of subseasonal changes in the monsoon in a changing climate has been taken by the study. The impact of natural variations in climate and human induced climate change is, however, unclear,” Karmarkar added.

The decrease in the intensity of the slow pulses is associated with a decreasing trend in the percentage of extreme events during the active phase of the monsoon. The rhythm of the Indian Monsoon is set by weeks of continuous rain (active phase) punctuated by weeks with no rain at all (break phase).

“The ultimate challenge is to predict seasonal/subseasonal rainfall, which has direct implications for agriculture and disaster management.

“Adaptation to changes insubseasonal variability of rainfall can be harder” says Nirupam, which makes it important to understand subseasonal changes in the monsoon,” Karmarkar added.

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