TY - JOUR
T1 - Weather regime transitions and the interannual variability of the north atlantic oscillation. Part II
T2 - Dynamical processes
AU - Luo, Dehai
AU - Cha, Jing
AU - Feldstein, Steven B.
PY - 2012/8
Y1 - 2012/8
N2 - 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.
AB - 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.
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U2 - 10.1175/JAS-D-11-0290.1
DO - 10.1175/JAS-D-11-0290.1
M3 - Article
AN - SCOPUS:84867968442
SN - 0022-4928
VL - 69
SP - 2347
EP - 2363
JO - Journal of the Atmospheric Sciences
JF - Journal of the Atmospheric Sciences
IS - 8
ER -