A typical case for observation of the relationship between the Arctic Oscillation and the North Atlantic Oscillation
Whether or not the Arctic Oscillation(AO) and the North Atlantic Oscillation(NAO) belong to the same mode of climate variability is a topic of long-lasting disputation in the field of the AO dynamics.From the case study on the abnormal behaviors of the AO and the NAO during the period of freezing disaster around mid-Jan-initial Feb 2008 over the South China and their possible causes,this paper reviews the relationship between the AO and the NAO.The main analytical tools utilized in this study are cross wavelet transform and wavelet coherence,ft is found that the phase of the AO leads or lags that of the NAO about 90°at the time scale of 30 - 60 days,while they are roughly in phase at the time scale of 10 - 20 days.Our analysis of the dynamical characters of both the AO and the NAO under the background of the 2007/08 La Nina event indicates that this variation of phase difference between the AO and the NAO with time scales origins from the regionality of wave-flow interaction. As we all know,the formation of three centers of action of the AO has something to do with the intensive centers of wave-flow interaction over the North Atlantic,the Northern Pacific and in the stratospheric polar vortex,respectively,while the NAO is mainly associated with wave-flow interaction centers over the North Atlantic.By affecting the Pacific jet stream and planetary waves,the La Nina event intensifies this difference between the AO and the NAO at time scale of 30 - 60 days.The reasons are as below,the Pacific jet stream or planetary waves in troposphere can directly affect the Pacific center of action of the AO.Meanwhile,planetary waves propagating upwards also affect the polar vortex in the stratosphere,and then affect the Arctic centers of action of the AO.On the other hand,at the time scale of 10 - 20 days,the in-phase relation of the AO and the NAO suggests that they may reflect the same physical phenomenon of wave-flow interaction,i.e.the synoptic wave breaking process over the Atlantic,which is another center of action of the AO.Based on the analysis above,the results of this paper make us incline to regarding the AO and the NAO as different.