Climate: UK weather trends toward extremes

An extratropical cyclone
An extratropical cyclone

North Atlantic pressure variations driving variable pattern

Staff Report

FRISCO — Weather patterns affecting the UK are  becoming more volatile, climate researchers concluded in a new study, concluding that the trend is being driven by extreme variations in pressure over the North Atlantic.

The month of December is showing the biggest variation, but contrasting conditions, from very mild, wet and stormy to extremely cold and snowy are a clear sign of less stable weather, University of Sheffield scientists reported in a study published last month in the Journal of Climatology.

Winter weather conditions are commonly defined using the North Atlantic Oscillation, a south-north seesaw of barometric pressure variations over the North Atlantic which determine the strength of the westerly winds that shape North Atlantic weather systems.

When westerly winds are strong during a positive NAO phase, Britain experiences mild, wet and often stormy weather – like last winter. Weaker or reverse airflow, known as negative NAO phase, typically brings cold, snowy weather, which featured prominently in the winters of 2009/10 and 2010/11.

Climatologists Professor Edward Hanna and Tom Cropper from the University of Sheffield’s Department of Geography and the rest of the team of scientists from University of East Anglia’s Climate Research Unit and the Met Office Hadley Centre, analyzed year-to-year monthly and seasonal changes in the NAO since 1899.

Although the experts found that most seasons didn’t show overall long-term NAO trends, winter – especially early winter including the month of December – showed a significant systematic rise in variation over the last century.

The researchers found that over the last 115 years, three out of five all time record high NAO values and two out of five record lows for the month of December occurred over the last decade (2004-2013).

Researchers found from their statistical analysis that there was a less than 0.04 per cent probability of getting such a clustering of extreme NAO years by chance.

Though this trend could be due to random fluctuations in the climate system, it could equally have been influenced by various driving factors including changing pressure/weather systems over the Arctic, especially Greenland, and changes in energy coming from the Sun.

“We cannot use these results directly to predict this winter’s weather but according to the long-term NAO trend we can say that the probability of getting extreme winter weather has significantly increased in the last few decades,” said Professor Edward Hanna, from the University of Sheffield’s Department of Geography. “Further research is needed to show whether or not this increased volatility is linked to global warming.”



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