New Delhi, Sept. 3: The bouts of rainfall that battered Houston, Mumbai and Calcutta last week, albeit in vastly different amounts, may earn tags of extreme rain events that weather scientists say are becoming more common under the influence of global warming.

The tropical storm Harvey set a record for continental US with 132cm rainfall about 45km southeast of Houston on August 29. Mumbai's Santa Cruz weather station documented 31cm rain over a 12-hour period the same day. And Calcutta's weather office recorded 9cm through the torrential downpour on Friday evening.

"Any rain 6.5cm or more is heavy rainfall," said Asish Sen, a senior weather scientist at the India Meteorological Department, Calcutta. Episodes of heavy or intense rain when concentrated over small pockets of space and spans of time are extreme events, he said.

Meteorologists at the Calcutta weather office said the 9cm rain on Friday was concentrated between 11.30am and 8.30pm, with most of the rain during the late evening hours. "On the outskirts, beyond about 30km, the rain didn't have the intensity it had in the city," Sen said.

Climate scientists in India and elsewhere have documented a rise in the frequency of such extreme weather events - not just intense rainfall, but heat waves, droughts and floods too.

Indian researchers Madhavan Rajeevan and Dushmanta Pattnaik were among the first to document what they said was a "significant increasing trend" of extreme rainfall events during the June-to-September monsoon season through an analysis of daily rainfall from 1951 to 2005.

Their study conducted eight years ago had also shown that concentrations of extreme rainfall events, marked by rainfall 12.5cm or higher, were likely to occur along the west coast, in central India and eastern India.

US-based researchers have cautioned that global warming - driven by emissions of greenhouse gases - is also likely to lead to an increase in the occurrence of "very intense cyclones", although the actual global numbers of cyclones are likely to decrease or change little.

The warming in the decades to come is likely to "cause tropical cyclones to have substantially higher rainfall rates than present-day ones", the US National Oceans and Atmospheric Administration's geophysical fluid dynamics laboratory has said in a document reviewing current research.

Six days ago, as sections of scientists and members of the public turned to social media to discuss Harvey, Michael Mann, a climate scientist at the Pennsylvania State University tweeted that Hurricane Sandy that struck northeastern US in October 2012 was a 500-year event. "It's now a 30-year event because of climate change. Will be a 3-5 year event if we don't act," he tweeted.

The evidence for a rise in the frequency of extreme weather events has been accumulating. But it was only in March this year that Mann collaborating with scientists in Germany proposed for the first time a likely mechanism that might explain seemingly diverse and distant extreme weather events.

The scientists examined historical atmospheric observations to document conditions under which extreme weather events have occurred and found that they are likely to occur when jet streams - atmospheric waves of air that span the globe - become stationary for short periods.

Jet streams are high-speed winds at altitudes of 9km to 16km that span the planet. When the waves turn stationary, although for short periods, hot summer days could translate into heat waves and accumulated moisture might lead to concentrated spells of extreme rain events. "We're able to connect the dots when it comes to human-caused global warming and an array of extreme weather events," Mann had said in a news release issued by the university.

Global warming-linked temperature changes may indirectly contribute to the "standing" jet stream waves which could cause unusual weather events, the scientists suggested in a paper in the journal Scientific Reports. Their study points to a link between temperatures and stationary jet stream waves associated with extreme events - droughts, floods and heat waves.

The US-German study is among the first that could in principle link diverse extreme events such as the rainfall in Houston, Mumbai and Calcutta last week.

But some scientists say the jet stream theory still needs to be backed by more evidence and observations.

"Jet streams are critical components of the upper atmosphere that can influence lower-level atmospheric conditions," said Pattnaik, a senior scientist at IMD, New Delhi. "But the precise mechanisms to explain how jet streams influence local weather at different regions need to be better understood."