Belfast Telegraph

Tuesday 23 September 2014

Wet summers due to warmer Atlantic

If the jet stream ends in a southerly position, it can bring wet summers

Recent wet summers in the UK may have been caused by a major warming of the North Atlantic Ocean, leading scientists and meteorologists have said.

Experts gathered at the Met Office to discuss the UK's unusual weather patterns in recent years, looking at what might have caused the cold winter of 2010/11, the wet summer of 2012, and this year's cold spring.

Professor Stephen Belcher, head of the Met Office Hadley Centre and chair of the meeting, said the delegates heard about "exciting" research from the University of Reading into circulation of currents in the Atlantic. "These areas of warm and cold water can affect the atmosphere, and load the dice as to where the jet stream is," he said. If the jet stream ends in a southerly position, it can bring wet summers.

Professor Rowan Sutton, of the University of Reading, said: "There will always be a lot of variability in British weather, but recent persistent patterns - such as the series of wet summers since 2007 - are unusual. This spring was the coldest for over 50 years, 2012 was the wettest in a century and December 2010 was the coldest on record, with national records dating back to 1910.

"We discussed a range of research exploring how recent UK weather could arise from purely natural processes, such as cycles in the oceans, but we also looked at the influence of higher levels of greenhouse gases in the atmosphere. We have agreed to take forward new collaborative research to disentangle different influences and work out how to improve predictions.

"Research at the University of Reading suggests that recent wet summers could be caused by a major warming of the North Atlantic Ocean that occurred back in the 1990s. The North Atlantic ocean has alternated slowly between warmer and cooler conditions over the last 100 years.

"We saw a rapid switch to a warmer North Atlantic in the 1990s and we think this is increasing the chances of wet summers over the UK and hot, dry summers around the Mediterranean - a situation that is likely to persist for as long as the North Atlantic remains in a warm phase. A transition back to a cooler North Atlantic, favouring drier summers in the UK and northern Europe, is likely and could occur rapidly. Exactly when this will happen is difficult to predict, but we're working on it.

"Other research at Reading suggests recent cold winters may be linked to a dip in the energy coming from the Sun and more frequent blocking events in the Eastern Atlantic. Blocking occurs when the warm jet stream from the west on its way to Northern Europe is blocked allowing north-easterly winds to arrive from the Arctic. Blocking episodes can persist for several weeks, leading to extended cold periods in winter.

"As well as such natural processes, we know that weather across the UK and Europe is being affected by higher levels of greenhouse gases in the atmosphere. For example, rainfall events have become more intense and this is quite likely linked to a warmer climate. There is also some evidence linking the record low amounts of Arctic sea ice to UK weather, but this evidence is not yet conclusive either way."

The meeting was designed to assess the research done so far and discuss what needs to be studied in the future to get a better idea of what could be causing the weather extremes. Earlier this month the Met Office said below-average temperatures through March, April and May made it the fifth coldest spring in national records dating back to 1910 and the coldest spring since 1962. Provisional findings show the UK's mean temperature for the season was just 6C (42.8F), while March was "exceptionally" cold, averaging 2.2C (36F).

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