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

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  • 1 RCE-TEA, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • | 2 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
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Abstract

In 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.

Corresponding author address: Dr. Dehai Luo, RCE-TEA, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. E-mail: ldh@mail.iap.ac.cn

Abstract

In 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.

Corresponding author address: Dr. Dehai Luo, RCE-TEA, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. E-mail: ldh@mail.iap.ac.cn
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