Scientists blame warmer Atlantic for wet summers

By David Shukman
Science editor, BBC News

image captionBritain experienced one of its rainiest summers in a century

Recent warming in the Atlantic Ocean is the main cause of wet summers in northern Europe, according to a new study.

A cyclical pattern of rising and falling ocean temperatures is seen as a major influence on our weather.

Scientists say the current pattern will last as long as the Atlantic warming persists.

The research was carried out at the University of Reading and is published in the journal Nature Geoscience.

The study investigated a phenomenon known as the Atlantic Multidecadal Oscillation - a cycle of change in which the waters either warm or cool over a period of several decades.

The researchers compared three periods in this cycle: a warm state from 1931-60, a cool period from 1961-90 and the most recent warm period starting in 1990 and continuing now.

The paper notes that conditions in the last warm period in the Atlantic are broadly similar to those observed now.

So the study compared weather conditions in Europe during the two warm Atlantic phases with those experienced in the cool phase.

One conclusion is that a warmer-than-usual Atlantic "favours a mild spring (especially April), summer and autumn, in England and across Europe."

Another finding - of greatest relevance to the search for a cause of rainy summers - is that the warmth of the ocean also tends to make northern and central Europe wetter than usual. By contrast southern Europe, from Portugal to Turkey, gets far less rain than normal.

That was the pattern observed last summer.

Jet stream

The precise mechanism by which the temperature of the surface of the ocean drives weather systems is not well understood.

But it is thought the changes in pressure influence the path of the jet stream, and how much it "meanders". Regions to the north of the stream, as Britain was for much of the summer, tend to be on the receiving end of a succession of rain storms.

The study was led by Professor Rowan Sutton, director of climate research at the National Centre for Atmospheric Science at the University of Reading.

"We know that a higher sea surface temperature in the ocean warms the air above which affects the weather systems and their path, shifting the Jetstream, but we don't yet know the full details of how this works.

"We also don't know the length of these periods of warmer or cooler conditions in the Atlantic Ocean - in the past they have varied a lot, from 20-50 years."

The oscillation is itself tied to another natural cycle known as the Altantic Meridional Overturning Circulation (AMOC) - a pattern of oceanic currents believed to be governed by an interplay of salt and freshwater, winds and tides, and possibly now also influenced by manmade greenhouse gases.

The research did not attempt to explore the causes of the warm and cool phases in the Atlantic, focusing instead on what it means for our weather.

Ice connection?

Last month we reported on the possible impact of the reduction in Arctic sea-ice on weather patterns.

Professor Sutton said that "clearly there is a link between Atlantic warming and Arctic sea ice though the details are not well understood."

"This does not mean that Arctic sea ice is not contributing to the effect on Europe's weather but it's an open question as to how much.

"The warming of the Atlantic Ocean may be amplified by the reductions in Arctic sea ice - these things are not independent."

The study is described as 'important' by Professor Alan Thorpe, head of the European Centre for Medium-range Weather Forecasting, showing how warmer seas can "set the scene" for wetter summer weather.

"These variations are on the decadal time-scale and remind us that the atmosphere and oceans can drive long-term fluctuations in our seasonal weather.

"Earlier research at the European Weather Centre had indicated that both the sea surface temperatures and the decreasing summer Arctic sea-ice may in combination have played a role in poor UK summer weather in 2007 and 2008, and one can speculate also in 2012.

"Overall we can see that there are a number of potential factors that can affect how good or poor the summers are in our part of the world. These factors typically involve far remote influences."

So what about the next few summers? Professor Sutton says there is no sign of the Atlantic Ocean cooling for the moment so the current pattern is likely to continue.

The only good news, he says, is that the transition between warm and cool phases in the ocean can happen quickly and cold winters may be a precursor to that change.

So, ironically, if the coming months bring freezing conditions, that may have an impact on the temperature of the Atlantic, and that in turn might mean the ocean switches to a cooler phase and so lead to drier summers for Britain.

But no one is betting on it - the research is still in its earliest days.

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