The spatial signature of the North Atlantic Oscillation in monthly rainfall patterns in Great Britain (1899/1900-2014/2015)

West, Harry; Quinn, Nevil; Horswell, Michael

Abstract

Although some studies have shown the influence of the North Atlantic Oscillation (NAO) on rainfall in Great Britain, these have focussed on wet extremes rather than the full wet-dry continuum. They have also been based on either analysis of a limited number of stations, undertaken only for particular seasons, or for a relatively short record. Improvements in long-range NAO prediction skill mean there is now renewed value in developing an understanding of how the NAO influences the temporal and spatial distribution of rainfall. Recent publication of nationally consistent gridded data sets such as the UK Centre for Ecology & Hydrology Gridded Estimates of Areal Rainfall (CEH-GEAR) and the Standardised Precipitation Index (SPI) time series provide new opportunities for analysis. The latter offers an additional advantage as these are conveniently scaled as a continuum of extreme wetness to extreme dryness.

We analyse regional correlations between two alternative monthly NAO indices (NAO-ST - Hurrell station based, and NAO-PC - Hurrell principal components based) and two measures of rainfall; firstly, monthly SPI (SPI-1) (5x5 km grid) which we averaged spatially by the 9 UK Met Office climate regions, and secondly, CEH-GEAR rainfall, treated in the same way. We also map at a national scale the mean monthly rainfall differences under NAO positive and NAO negative conditions, relative to NAO neutral conditions.

Contrasting monthly rainfall signatures are observed under each NAO phase and season. A winter NAO negative phase produces very dry conditions (up to 31% in drier) in the north, while the south and east experience marginally wetter conditions (around +13%). This pattern is inverted under NAO positive conditions, with the north being significantly wetter than under neutral conditions (up to 58% wetter), and the south and east being slightly drier (around -17%). During summer months however, a negative NAO produces significantly wetter summers for much of the country (+39%). Meanwhile a positive NAO produces extreme dry conditions (with the south experiencing a 36% reduction in rainfall compared to NAO neutral conditions).

• Presentation Conference Type: Poster
• Conference Name: British Hydrological Society National Symposium
• Start Date: Sep 12, 2018
• End Date: Sep 13, 2018
• Acceptance Date: Sep 1, 2018
• Publication Date: Sep 12, 2018
• Deposit Date: Sep 12, 2018
• Publicly Available Date: Oct 3, 2018
• Peer Reviewed: Peer Reviewed
• Public URL: https://uwe-repository.worktribe.com/output/860671
• Additional Information: Title of Conference or Conference Proceedings : British Hydrological Society National Symposium
• Contract Date: Oct 3, 2018