Weather regime transitions and the interannual variability of the north atlantic oscillation. Part II: Dynamical processes

In this study, attention is focused on identifying the dynamical processes that contribute to the negativeNorth 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 strongerAR(SBL) winterNAO ^- (NAO ⁺) event canmore easily transition into an NAO ⁺ (NAO ^-) event. Composites of 300-hPa geopotential height anomalies for the NAO ^- toNAO ⁺ andNAO ⁺ toNAO ^- 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 retrogradingwavenumber 1 ismore important for theNAO ^- to NAO ⁺ transition during P1, but themarked reintensification and retrogrademovement 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.