Weather regimes and their connection to the winter rainfall in Portugal

Abstract Wintertime rainfall over Portugal is strongly coupled with the large‐scale atmospheric flow in the Euro‐Atlantic sector. A K ‐means cluster analysis, on the space spanned by a subset of the empirical orthogonal functions of the daily mean sea‐level pressure fields, is performed aiming to isolate the weather regimes responsible for the interannual variability of the winter precipitation. Each daily circulation pattern is keyed to a set of five weather regimes (C, W, NAO−, NAO+ and E). The dynamical structure of each regime substantiates the statistical properties of the respective rainfall distribution and validates the clustering technique. The C regime is related to low‐pressure systems over the North Atlantic that induce southwesterly and westerly moist winds over the country. The W regime is characterized by westerly disturbed weather associated with low‐pressure systems mainly located over northern Europe. The NAO− regime is manifested by weak low‐pressure systems near Portugal. The NAO+ regime corresponds to a well‐developed Azores high with generally settled and dry weather conditions. Finally, the E regime is related to anomalous strong easterly winds and rather dry conditions. Although the variability in the frequencies of occurrence of the C and NAO− regimes is largely dominant in the interannual variability of the winter rainfall throughout Portugal, the C regime is particularly meaningful over northern Portugal and the NAO− regime acquires higher relevance over southern Portugal. The inclusion of the W regime improves the description of the variability over northern and central Portugal. Dry weather conditions prevail in both the NAO+ and E regimes, with hardly any exceptions. The occurrence of the NAO+ and the NAO− regimes is also strongly coupled with the North Atlantic oscillation. Copyright © 2005 Royal Meteorological Society