It has been some summer in England: there has been glorious sunshine, long days of sweltering heat.
But the good times won’t last: According to a new study, winter flooding in England will get more severe and more frequent due to global warming as a result of a change in the characteristics of atmospheric rivers.
It’s a new term for a phenomenon only identified about 20 years ago.
Atmospheric rivers, or ARs, are described as “narrow regions of intense moisture flows in the lower troposphere of the atmosphere that deliver sustained and heavy rainfall to mid-latitude regions.”
They can be up to 300 kilometres wide and up to 2,000 kilometres long. They flow, invisibly, about one to 2.5 kilometres above the surface of the ocean. There can be multiple atmospheric rivers at any time covering the hemisphere.
Known to bring heavy rainfall, they have been blamed for severe flooding in California and in England. (Some have blamed atmospheric rivers for the recent catastrophic flooding in Calgary.) For instance, the AR that was responsible for flooding in northwest England in 2009 transported 4,500 times more water than the average flow in the River Thames in London.
There was staggering rainfall and extraordinary flooding: bridges collapsed, roads were washed away, hundreds were rescued.
In the paper, researchers examined five modelling scenarios to simulate possible conditions and found that a warming climate, which lets the atmosphere hold more moisture, made the rivers more likely.
David Lavers, lead author of the study, said the link between atmospheric rivers and flooding is already established.
“An increase in AR frequency is likely to lead to an increased number of heavy winter rainfall events and floods. More intense ARs are likely to lead to higher rainfall totals, and thus larger flood events.”
The study was conducted by researchers from the University of Reading, U.K., and the University of Iowa and was published by the Institute of Physics in Environmental Research Letters.
Atmospheric river isn’t the only new term in the weather lexicon.
There is also derecho (də-REH-choh), which means straight in Spanish. A derecho is a gigantic cluster of thunderstorms that affect a large area for a long time, bringing strong winds, thunder, lightning and torrential rain with it. Severe derechos are given the adjective “super.”
Derechos have been around for a long time, says Brett Anderson, a senior meteorologist with Accuweather in Stormstown, Pa.
Heat is critical for this kind of a system and “with global warming, we are expecting more heat waves,” said Anderson. “So it’s fair to say that we will see more intense derechos.”
But he says derechos are not caused by global warming.
Unlike tornadoes, which tend to be isolated and spread over a couple of kilometres, a derecho can engulf a broad area, as much as 300 or even 400 kilometres. They come with almost no warning, unlike hurricanes.
“They (derechos) are rare and don’t happen often but when they do, they cause a lot of damage,” he said.
The one that roared through the U.S. mid-Atlantic last June was a rare one in terms of its size, duration and impact, said Anderson.
It caused widespread damage from Chicago to Virginia. More than five million people lost power for days and 22 people were killed.
That derecho was triggered by a ripple in the jet stream and fuelled by the intense heat that caused Washington and Columbia, S.C., to smash temperature records last June.
“That kind happens maybe once in about 10 years,” Anderson said.
In Ontario, derechos are rare, says Peter Kimbell of Environment Canada. “Off the top of my head, I would say there is probably one every five years or so.” The last significant one was on July 17, 2006, in northeastern Ontario.
The term was first used by an American professor in the 1880s and then, says Kimbell, it died out. It was resurrected in 1983 by a forecaster. “It is a neat word,” he said.
And it’s got people talking.
“Everyone wants to know what exactly they are . . . and how to pronounce derecho,” said Anderson. “I’m not sure I do it right.”