Weather Regime Transitions and the Interannual Variability of the North Atlantic Oscillation. Part I: A Likely Connection

In this study, the relationship between weather regime transitions and the interannual variability of the North Atlantic Oscillation (NAO) in winter during 1978-2008 is examined by using a statistical approach. Four classical weather regimes-the two phases of the NAO (NAO(+), NAO(-)) and the Scandinavian blocking and Atlantic ridge patterns-are obtained with k-means cluster analysis. Observations show that the transition between the NAO(+) and NAO(-) regimes is markedly different between 1978-90 (P1) and 1991-2008 (P2). Within P1 (P2), the frequency of the NAO(-) to NAO(+) (NAO(+) to NAO(-)) transition events is almost twice that of the NAO(+) to NAO(-) (NAO(-) to NAO(+)) transition events. On this basis, further cluster analysis performed for two cases with and without NAO transition events indicates that within P1 (P2) the NAO(+) (NAO(-)) anomaly is markedly enhanced as the NAO(-) to NAO(+) (NAO(+) to NAO(-)) transitions take place. Furthermore, the NAO regime transition is found to be more likely to enhance the eastward shift of the NAO (NAO(-)) anomaly. Thus, it is hypothesized that the interannual change in the winter-mean NAO index from P1 to P2 is related to the intraseasonal NAO(-) to NAO(+) (NAO(+) to NAO(-)) transition events during P1 (P2) because of the variation of the NAO pattern in intensity, location, and frequency (number of days). This finding is also seen from calculations of the winter monthly mean NAO index with and without NAO regime transitions.