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.
All Science Journal Classification (ASJC) codes
- Atmospheric Science