Winter weather set to get more erratic

Extreme variations in pressure over the North Atlantic are responsible for our increasingly erratic winter weather. Conditions in the UK range from mild, wet and stormy, to incredibly cold and snowy: which we get depends on the North Atlantic Oscillation (NAO), a north-south seesaw for barometric pressure variations over the North Atlantic. It indicates the strength of westerly winds imposing on the UK shore and the resulting weather patterns When the winds are strong – a positive NAO phase – we get milder weather, but weaker or reverse airflow, a negative NAO, brings the snowy conditions. Research from the University of Sheffield shows our weather patterns have become increasingly volatile, and are likely to become even more unsettled. “Our study highlights the changing nature of North Atlantic atmospheric circulation patterns that has given the UK more variable winter conditions in recent years,” said Professor Edward Hanna from the Department of Geography. “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 – either mild/stormy or cold/snowy – has significantly increased in the last few decades.” Researchers analysed year-to-year monthly and seasonal changes in the NAO since 1899. Although most seasons failed to show over-all long-term trends, winter – especially early winter months like December – showed a systematic rise in variation over the last century. 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 in the last decade, indicating British winters have become increasingly unsettled. Statistical analysis showed that there was a less than 0.04% probability of getting such clustering by chance. Researchers suggest that although this trend could be due to random fluctuations in the climate system, it could also have been influenced by driving factors including changing weather or pressure systems over the Arctic, and changes in the energy coming from the Sun. The team – which also included scientists from the University of East Anglia and the Met Office Hadley Centre – are now working to better understand these links. Recent seasonal asymmetric changes in the NAO (a marked summer decline and increased winter variability) and associated changes in the AO and Greenland Blocking Index