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By CHRIS HILL, Rural affairs correspondent
Monday, July 16, 2012
After months of thundery torrents, flash flooding and waterlogged outdoor events, rural affairs correspondent CHRIS HILL asks: What on earth has happened to our summer?
For the more superstitious weather-watchers among us, yesterday was a time to watch the skies in nervous anticipation.
It was St Swithin’s Day, when tradition dictates that the prevailing weather will continue for another 40 days.
Mercifully, a few showers across East Anglia represented a relief compared to much of the preceding few months, which have brought torrential thunderstorms, roads and rivers flooding, and a watery demise for some stalwarts of the outdoor events calendar.
But what is the reason for our soggy summer?
Well, according to meteorologists the culprit is the jet stream; a powerful high-altitude wind whose mysterious meanderings have an overbearing effect on our weather.
Normally, the west-to-east flow of air shifts to the north of our shores in the summer, directing areas of low pressure and bad weather further northwards, leaving us nestled in a comforting oasis of warm air.
But this year it has stayed to the south, so we have found ourselves on the jet stream’s colder, less amenable side – and at the mercy of prolonged rain and winds.
The precise reason for this is something of a puzzle to scientists, although some have claimed that melting polar ice, driven by global warming, could have reduced the temperature of the Atlantic and altered the route of the winds.
Climate researchers at the UEA say there is good evidence to support that theory – but not enough to say conclusively that this year’s wet summer is caused by anything other than natural variation.
Phil Garner, a forecaster with Norwich-based Weatherquest, said: “In a normal summer, we would start off with the jet stream to the south and it would move northwards as the earth warms up from the equator. But its position has been meandering, like a skipping rope in a playground. If you shake it up and down, you get it moving in great waves around the globe.”
So why is it in a different position this year?
“That is the £10m question,” said Mr Garner. “As the position of the jet stream is influenced by the Rocky Mountains in America it could be something that’s happening over the Pacific. The other factor that has come out recently is the research that says we are getting a lot more melting of the polar ice caps, making the water colder over the eastern seaboard of the States, and that is having the effect of buckling the jet stream.”
Mr Garner said although the short-term forecast doesn’t show much sign of change, there could be more encouraging signs on the horizon for the rest of the summer.
“Everyone is saying what a terrible summer it is, but we are not even half-way through yet,” he said. “The jet stream is stuck in the position we would expect it to be in spring, but that is not to say it won’t move, and give us better weather for the second half of the summer.
“There is low pressure for the end of the week which will bring more showers and rain - but that buckling effect of the jet stream is starting to develop such that we may be looking at something more warm developing after that. But it is early days.”
Mr Garner said the peculiar weather system could also be to blame for a recent glut of flooding incidents.
“With low pressure sitting over one place for a time, you get very heavy, pinpoint downpours,” he said. “The British rivers are designed to deal with normal frontal bands of rain from the west which give you heavy periods of rain, but not that heavy localised intensity which we have been seeing this year.”
The rainy weather has been with us since early April – ironically, when a short-lived hosepipe ban was introduced because of water shortages.
But the increase in rainfall has not been as severe in East Anglia as in other parts of the country. From the start of May until July 12, the Marham weather station recorded 174mm compared to its normal 148mm, a rise of about 20pc. Across the country, June was wettest month on record, but in East Anglia it was only seventh wettest.
“It reinforces the fact that the overall amounts of rain have not been the problem,” said Mr Garner. “But we have been having showers most days, and then localised torrential rain has broken the system.”
Although the jet stream is always moving, one great unknown is the extent of the influence of climate change.
Clare Goodess, a senior researcher at the climatic research unit at the UEA, said there was plausible evidence to suggest that melting polar ice could be a factor, but that it was difficult to attribute individual weather events or unusual seasons to human intervention.
“There is always going to be natural variability in the climate system and human influence imposed on that natural pattern of variability,” she said. “I think that at the moment we are at a period where the influence, certainly for rainfall, is still within that natural variability.
“Our projections for the middle of the century are for an increase in heavy winter rainfall in the East of England, and a decrease in the summer.
“There is strong evidence to say human influence can cause an increase in extreme rainfall, but the one slightly puzzling thing, which is contradictory to our models, is why this seems to be happening in summer rather than winter.”