Heavy rainstorms have become a more common feature of the Indian monsoon over the past 50 years, according to new research. At the same time, moderate rainfall has become rarer.
This internal switch from moderate to extreme storms may explain why the rain delivered by monsoons has remained stable, a fact that has puzzled meteorologists who expect them to grow more ferocious as the Indian Ocean warms.
It is well established that the Indian Ocean is warming, an effect thought to be caused by climate change. And according to our understanding of rain production, this should have a clear outcome.
Warmer seas should heat up the monsoon winds that carry moisture from the ocean to the land. In turn, warmer winds should carry more moisture, so warmer oceans should lead to more rain. Yet observations show total rainfall during Indian monsoons to be remarkably stable.
Most previous research on this puzzle has been confined to individual rain gauges, which would have missed heavy rainfall just a short distance away. To get a wider perspective, B N Goswami and colleagues at the Indian Institute of Tropical Meteorology looked at readings from a grid of 1800 gauges over a large chunk of central and eastern India (see map, right).
Their results show that storms producing very heavy rainfall (more than 150 mm in a day) were twice as common in 2000 as they were in 1951. Heavy storms, delivering more than 100 millimetres a day, also increased in frequency. But those giving moderate rainfall became slightly rarer (see graph, bottom right).
Julia Slingo, of the University of Reading in the UK says the trends are significant but adds a number of caveats. For starters, 50 years is too short a time to reach definitive conclusions because rainfall patterns from Indian monsoons seem to vary with a period of several decades. Slingo points out that Goswami's research shows a downturn in the frequency of heavy rainfall in the late 1990s, and wonders if this signals the start of a reverse in the trend.
Goswami and his colleagues also relate the increase in heavy rainfall to warming of the Indian Ocean, but Slingo argues that the area the Pune group looked at includes all of central India where monsoon rains are influenced by other factors, such as the proximity of the Himalayas.
But Slingo adds that Goswami's trend does correspond to the small amount of modelling work her team has done on seasonal frequency of extreme rainfall in the region. "No doubt there is something here and we should take it seriously," she says. "But exactly how you interpret that in terms of danger, of human lives at risk, is difficult."
The research does not shine any new light on the frequency of flash floods, such as that on 26 July 2005, which dumped 944 mm on Mumbai in 24 hours and killed about 1000 people across the state of Maharashtra (see Mumbai monsoon's record deluge kills 1000). Goswami's data is averaged for rainfall across an area 100 kilometres square, making it difficult to say anything about flash floods in particular areas.
Journal reference: Science (vol 314, p 1442)
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