The Hindu Business Line : `Heavy rainfall events may drive Indian monsoon'

India will have to live with fewer and more extreme downpours in a key emerging trend in monsoon rainfall pattern, throwing up increased risk of flooding and other natural disasters.

PARTIALLY OFFSET

According to Dr B.N. Goswami, who lead-authored a recent paper in the journal, "Science," the extreme downpours will largely even out perceived losses from a decreasing trend in moderate rainfall events, keeping overall precipitation to a balance.

"Although the relative contributions to the mean from these two classes do not balance in a given year, the contribution from the decreasing trend of moderate events is partially offset by that from increasing heavy rain events," the paper said.

Consequently, the seasonal total does not show any statistically significant change over longer time scales. But the observed trends suggest enhanced risks associated with extreme rainfall over India in the coming decades.

FOCUS AREA

Dr Goswami, Director, Indian Institute of Tropical Meteorology, and colleagues studied rain gauge data from 1,803 stations scattered throughout central India, the best available sample monsoon geography, from 1951 to 2000.

The researchers divided storms into several categories ranging from light (between 5 and 10 cm a day) to very heavy (more than 15 cm a day). They found that over the last few decades, the `light' rainfall events have declined significantly while their heavier counterparts have increased.

UNCHANGED MEAN

This increase in large storms has been masked in the overall rainfall data by the decline in more moderate downpours. As the weak and moderate events decrease, their contribution to the mean decreased while the increasing number of heavy to very heavy events makes an increasing contribution to the mean. These two opposing contributions roughly balance each other and keep the mean unchanged, the researchers found.

HIGH RISK

But even though the average has not changed, the potential for extreme downpours has changed - jumping 10 per cent and still rising. The number of very heavy events has more than doubled, indicating a large increase in disaster potential. This is an important and increasing risk going forward, according to the researchers.

GLOBAL WARMING

The number of strong tropical cyclones continues to increase as well, linked perhaps to the gradual increase in Indian Ocean sea surface temperatures. These findings are in tune with model projections and some observations that indicate an increase in heavy rain events and a decrease in weak events under global warming scenarios.

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