Air-Mass and Frontal Extreme Precipitation Occurrence and the North Atlantic Oscillation (NAO)

Abstract

The North Atlantic Oscillation is a significant teleconnection pattern driving weather and climatic conditions in the Northern Hemisphere, including Europe. This chapter discusses relationships between the occurrence of extreme precipitation and daily NAO. The latter’s influence on extreme precipitation in Europe was assessed by comparing empirical distribution functions of daily NAO values on days with extreme precipitation with distribution functions on dry days, while also taking into account origin-based precipitation types. The statistical significance of the differences between empirical distribution functions was tested with the Mann–Whitney U test (U M–W) and with the Kolmogorov–Smirnov test (K–S). The influence of the NAO phase on the spatial variability of the frequency of extreme precipitation was analysed using conditional probability of the occurrence of extreme precipitation during positive NAO phase (NAO>0, or NAO+) and the negative NAO phase (NAO<0, or NAO−).

Both the strength and the spatial extent of these relationships between NAO and extreme precipitation display seasonality, with a winter peak, and deppend on origin-based precipitation type. In autumn, the influence of NAO is weaker than in winter, but clearly stronger than in summer and spring. In winter, the positive NAO phase accompanies a majority of extreme precipitation in an overwhelming majority of the Northern and Western Europe, as moist air masses move over this part of Europe. A majority of precipitation occurring at that time is associated with either a cold front (Fc) or with an occluded front (Fo), depending on the region. In southern Europe advection from the Atlantic Ocean and the occurrence of extreme precipitation is associated mostly with the negative NAO phase. Extreme precipitation occurring then is associated with the passage of different fronts (Ff) or with an occluded front (Fo).