An examination of extreme storms across the British Isles over the last 85 years during the boreal autumn [October, November, December (OND)] and winter [January, February, March (JFM)] shows that large-scale natural climate variability plays an important role in modulating the intensity and frequency of these events.
Severe storms across the British Isles were most prominent in the 1920s and 1990s in OND, and in the 1920s, 1980s and 1990s in JFM. There is a significant correlation between JFM severe storminess across the British Isles and both the Gibraltar–South-West (SW) Iceland and Azores–Iceland indices of the North Atlantic Oscillation (NAO), but this relationship fluctuates over the 85 years of data. Strongest NAO relationships occur during 1970–1990 and 1940–1960, with a weaker correlation in the 1920s–1940s, and effectively no correlation in 1950–1970. There is no significant relationship between the Gibraltar–SW Iceland NAO and severe storms in OND, but a significant correlation exists with the Azores–Iceland NAO and there is a clear link to a pattern in mean sea level pressure (MSLP) extending from the tropical Atlantic to higher latitudes of the North Atlantic. El Niño Southern Oscillation (ENSO) influences from the Pacific Ocean also appear to play a role in modulating OND severe storms over the British Isles. Importantly, severe storms in OND and JFM seasons respond to different physical mechanisms.
Future work is needed to extend this study back into the late 19th century in order to evaluate fully any changes in severe storms across the British Isles using a longer instrumental record. This may be best achieved through long historical surface-observations-only global reanalyses, which can reconstruct tropospheric weather variables using longer instrumental records of daily to sub-daily MSLP. Copyright © 2008 Royal Meteorological Society