MONSOON WETTING: Over the last 50 years, the number and size of strong monsoon downpours has increased, though this increase has been masked by a decline in weaker storms. Image: © BEN SPENCER; EYE UBIQUITOUS/CORBIS
The monsoon is the great life-giver and the great destroyer of the subcontinent. Without rain from these annual storms, crops wither, animals die and more than half the world's population suffers from potential famine. With too much rain, crops are inundated, animals drown and people suffer from floods and the diseases that follow in their wake. Observations of this critical climate system stretch back decades, and the overall level of rainfall has changed little over the years. But now researchers have discovered a trend within the annual measurements toward fewer, more extreme downpours--a trend that bodes ill for flooding and other natural disasters.
B. N. Goswami of the Indian Institute of Tropical Meteorology and his colleagues studied rain gauge data from 1,803 stations scattered throughout central India from 1951 to 2000. As expected there was a wide range of rainfall: from a maximum downpour of 58.2 centimeters in one day (nearly 23 inches) to none, with an average of just 5.7 millimeters (just under one quarter of an inch) a day during the season.
The researchers divided storms into several categories ranging from light (between five and 100 millimeters a day) to very heavy (more than 150 millimeters a day). Over the last few decades of the 20th century, such light events have declined significantly while their heavier counterparts have increased. In fact, the largest storms of the 1950s are smaller than their counterparts of the 1990s. "Heavy and very heavy rain events over central India have increased significantly since the 1950s," Goswami notes. "Also, the magnitude of the very heavy events in a given year has shown a clear increasing trend."
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 and very heavy events make an increasing contribution to the mean," Goswami explains. "These two opposing contributions roughly balance each other and keep the mean unchanged."
But even though the average has not changed, the potential for extreme downpours--and hence flooding and other ills--has, jumping 10 percent and still rising. This is an important and increasing risk going forward, according to the researchers. The number of strong tropical cyclones continues to increase as well, linked perhaps to the gradual increase in Indian Ocean sea surface temperatures. "The results are consistent with what may be expected under global warming," Goswami adds. "We are working on various other aspects of the impact of climate change on the Indian monsoon using this data set." In the meantime, his forecast calls for floods.