Warmer Atlantic Ocean blamed for UK's gloomy summers

8 October 2012 by Tamera Jones

The UK's recent run of dismal summers were strongly influenced by a major warming of water in the North Atlantic Ocean which started back in the 1990s and continues today, say scientists.

Wind and rain

Wind and rain

They add that while the North Atlantic remains as warm as it is, the country is unlikely to see an end to wet summers.

"You're not always going to get one, but as long as the Atlantic is warm, the chances of a wet summer are increased," says Professor Rowan Sutton director of Climate Research NERC's National Centre for Atmospheric Science (NCAS), who led the study.

Last time the Atlantic was as warm as it is now, it persisted for nearly 30 years, from 1931 until 1960. This led to a run of wet summers over the UK. Lynmouth in Devon experienced disastrous flooding in August 1952, and severe flooding during August 1948 closed the east coast mainline for three months.

The state of the oceans tells you about weather patterns that are likely to evolve several years ahead.

- Professor Rowan Sutton, NERC's National Centre for Atmospheric Science (NCAS)

The warm period the North Atlantic is experiencing right now only started around 1996, which suggests it might be some time before the ocean cools down again and we see a return to more agreeable summers. But, as with all things weather related, how long the current warm period will last isn't easy to predict.

"We can't assume that the current warming will be the same length as the previous one. We just don't know how long it'll go on for," says Sutton.

He and his colleague, Dr Buwen Dong - also from NCAS - describe in a study published in Nature Geoscience how they analysed long-term records of air temperature, rainfall and pressure at sea level for these two warm periods. They compared these records with those from a cool period in between.

By comparing the observed changes with computer simulations of the climate system, they found compelling evidence that the temperature of the Atlantic Ocean influences the whole of Europe's climate.

"The state of the oceans tells you about weather patterns that are likely to evolve several years ahead," says Sutton.

In particular, they found that a warming of the North Atlantic in the 1990s coincided with a shift to wetter summers in the UK and northern Europe. At the same time, the Mediterranean experienced a shift to hotter and drier summers than usual.

The patterns the researchers unearthed match those experienced in the UK this year, when the country recorded its wettest summer in 100 years. In contrast, countries in the Mediterranean suffered under temperatures of 40 degrees centigrade or higher.

"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," says Sutton.

Scientists know that the temperature of the North Atlantic swings slowly between warm and cool conditions, which can each last for decades at a time. Not just that, but previous studies have linked sea surface temperatures in the Atlantic with changes in Europe's climate. But exactly how the two are linked was, until now, unclear.

Now, the pair's findings suggest that changes in ocean temperature affect the atmosphere directly above, ultimately causing a trough of low pressure over western Europe during the summer, which, "steers rain-bearing weather systems slap-bang into the UK".

"We know the jet stream is involved in this: there's a close relation between it and the path weather systems take," Sutton says. 'When the North Atlantic is cool, rain-bearing weather tends to meander northwards over Iceland. But when it's warm, they tend to slam into the UK.'

The next obvious area of research suggests Sutton is to figure out how long the current warm state could last.

'Atlantic Ocean influence on a shift in European climate in the 1990s' - Rowan T Sutton and Buwen Dong,Nature Geoscience vol 5 no. 10, published 7 October 2012, doi