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Changes in Monsoon rhythms, extreme rainfall detected

As the farmers of India are acutely aware, the success of a growing season depends not only on how much rain is received during the monsoon, but also when it is received. The intensity of the slow rhythms of the monsoon has decreased over the past sixty years, according to a recent study by scientists at the Indian Institute of Science. A compensating increase in the number of extreme rainfall events in the monsoon months has also been detected.

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” says Nirupam Karmakar, the lead author of the study which has been published in Environmental Research Letters. “[Adaptation to changes in] subseasonal variability of rainfall can be harder” says Nirupam, which makes it important to understand subseasonal changes in the monsoon.

The study separated the slow pulses in the monsoon which last 20 to 60 days from the faster pulses. They 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. They detected an increase in extreme events over the coastal regions in the months of May and June and over central India in the months of 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]”, says Nirupam. 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.

A step towards addressing the issue of subseasonal changes in the monsoon in a changing climate has been taken by this study. The impact of natural variations in climate and human induced climate change is, however, unclear. “The role of natural variability can be hard to detect in 63 years of data. To address [the issue of] climate change, modelling experiments need to be done”, says Nirupam.

About the study

The paper appeared in the journal Environmental Research Letters on 22nd May.

About the authors

Arindam Chakraborty is an Associate Professor ( and Ravi S. Nanjundiah a Professor ( at the Centre for Atmospheric and Oceanic Sciences and Divecha Centre for Climate Change, Indian Institute of Science, Bangalore. Nirupam Karmakar is a PhD student of Prof A. Chakraborty and Prof Nanjundiah.


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