A Diagnostic Study Of The NAO Phase Transition

With daily NCEP/NCAR reanalysis data and daily NAO Index data from CPC, this study examines NAO phase transitions during 1978-1990(P1) and 1991-2009(P2). The focus of this study is on the structure and evolution characteristics of different transitions, which is performed on some basic variables such as geopotential height and zonal wind.The results of a composite analysis with NAO Index suggest that the frequency of the NAO- to NAO+ transition is dominant during P1, while the NAO+ to NAO- transition prevails during P2. As the later phase of the transition(NAO-) is much stronger during P2 than that of P1, it is likely to partly account for the linear downtrending within P2. The composite of geopotential height shows that, the main characters of different transitions are enhancement and westward withdrawing of the positive or negative anomaly. However, these characters cannot be detected for NAO events without transition. The westward withdrawing also lead to NAO anomaly being located close to the downstream, which just reflects the "eastward shift" of the NAO pattern. Moreover, it is found that there exists the change between the double-jet state and single-jet state during the transition events in the Atlantic area,which would not happen during the non-transition events. The type of this jet change depend on the type of phase transition.It is much more significant with the intensity and the westward - withdrawing during P2 than that during P1. The reason for this difference may probably be the interference between wavenumber 1 and 2. During P1, as for the different intensities and phase velocities, it is hardly to achieve the "resonance" between wavenumber 1 and 2. Nevertheless, for the most period during P2, the two waves stay with most the same phase velocity and could be coupled well as a result. Thus it would be indicated that the the intensity and the westward - withdrawing of the NAO anomaly is extraordinarily significant. However, no matter of what phase transitions, the re-intensification of both wavenumbers 1 and 2 after the decay of the former NAO events is crucial for the transitions.