10/08/2012

The jet stream and the rain stream

We all know about the jet stream, but there is apparently also a rain stream which brings water from the Amazon, perhaps to cause floods.


The study, which appeared in the New Scientist, suggests that these aerial rivers of moisture could have been the cause of the ten worst winter floods in the UK since the 1970s. They also can exist in other regions of the world, with California being one of these regions that can also be affected (by a rainstream from Hawaii to California known as the "pineapple express").

According to this report

Atmospheric rivers (ARs) are giant ribbons of moist air, at least 2000 kilometres long and several hundred kilometres across, which move water across the mid-latitudes. They flow in the lower troposphere, where winds with speeds in excess of 12.5 metres per second can carry as much water as the Amazon river. At any given time, four or five ARs carry nearly 90 per cent of the moisture that is moving towards the poles.

An article in the Sunday Times notes that these rivers of moisture can carry 1,000 times more water than the Thames river. I have no details of how that could relate to the great rivers of continental Europe, nor whether, as with the jet stream, the river could sometimes move to the North or the South. Equally, does being outside the stream imply drought conditions?

So how does this relate to global warming (oops: climate change!). It is tempting to think that this aerial river phenomenon shows that climate change is not the cause of the extremes of weather that we are experiencing, such as the long drought that has decimated grain yields in the USA. However, the New Scientist article suggested that climate change will actually increase the influence of these rivers. To quote from the ST article

Will climate change affect ARs? Michael Dettinger of the Scripps Institution of Oceanography in La Jolla, California, studied the "Pineapple Express", an AR that brings water from the tropics near Hawaii to California. He used seven global circulation climate models to look at the changes in AR-induced storms if greenhouse gas emissions accelerate during the 21st century.

Dettinger found that more evaporation due to higher temperatures could cause AR-induced storms to become more frequent in California (Journal of the American Water Resources Association, DOI: 10.1111/j.1752-1688.2011.00546.x). There is no reason to expect any different in the UK, he says.

Maybe our TV weather forecasters could show us these rain streams when they tell us whether (pun intended!) to take an umbrella and raincoat with us when we go out!

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