Weather Regime Transitions and the Interannual Variability of the North Atlantic Oscillation. Part II: Dynamical Processes

Abstract

n this study, attention is focused on identifying the dynamical processes that contribute to the negative North Atlantic Oscillation (NAO)? to positive NAO (NAO+) and NAO+ to NAO? transitions that occur during 1978?90 (P1) and 1991?2008 (P2). By constructing Atlantic ridge (AR) and Scandinavian blocking (SBL) indices, the composite analysis demonstrates that in a stronger AR (SBL) winter NAO? (NAO+) event can more easily transition into an NAO+ (NAO?) event. Composites of 300-hPa geopotential height anomalies for the NAO? to NAO+ and NAO+ to NAO? transition events during P1 and P2 are calculated. It is shown for P2 (P1) that the NAO+ to SBL to NAO? (NAO? to AR to NAO+) transition results from the retrograde drift of an enhanced high-latitude, large-scale, positive (negative) anomaly over northern Europe during the decay of the previous NAO+ (NAO?) event. This finding cannot be detected for NAO events without transition.Moreover, it is found that the amplification of retrograding wavenumber 1 is more important for the NAO? to NAO+ transition during P1, but the marked reintensification and retrograde movement of both wavenumbers 1 and 2 after the NAO+ event decays is crucial for the NAO+ to NAO? transition during P2. It is further shown that destructive (constructive) interference between wavenumbers 1 and 2 over the North Atlantic during P1 (P2) is responsible for the subsequent weak NAO+ (strong NAO?) anomaly associated with the NAO? to NAO+ (NAO+ to NAO?) transition. Also, the weakening (strengthening) of the vertically integrated zonal wind (upstream Atlantic storm track) is found to play an important role in the NAO regime transition.