A Numerical And Observational Study On The Phase Transition Of The North Atlantic Oscillation

First, using a nonlinear quasi-geostrophic barotropic model, the paper investigatesnumerically how the spatial pattern of synoptic scale waves and the westerly windstrength affect the phase transition of the North Atlantic Oscillation (NAO), and therole played by the large-scale topography. It is found that the weak basic westerly windis favorable for the westward shift of the downstream large-scale system, favoring thetransition of the NAO from positive (negative) phase into negative (positive) phase. Itis noted that when the synoptic scale waves are stronger and concentrated in a widerregion, the NAO event can transform more easily from the negative phase into positivephase; when the synoptic scale waves are stronger and shift northward, the NAO cantransform more easily from the positive into negative phase. In the presence of a largescale topography, the number of the cases in which the NAO transforms from positive(negative) phase to negative (positive) phase is reduced. The conlusions are easilyexplained in terns of the Rossby phase speed formula. It is clear that when the basicwesterly wind is smaller, the phase speed is more likely take on a negative value. In thiscase, the large amplitude blocking over understream will undergo a distinct westwardmovenent, then the patern will be changed and the phase of the NAO will be transited.The more small the synoptic-scale waves are, the more large the amplitude of thefluctuation is and the more likely the phase speed takes on a negative value. The largescale topography results the basic westerly wind in the high latitude enhanced, thosealso lead that the phase speed takes on a positive value, which is not favorable forwestward movenent of the understream systern.In addition to local factors such as the basic westerly wind, synoptic-scale waves, andlarge scale topography will affect the phase transition of the NAO, external forcing such assea surface temperature will also affect the phase transition of the NAO. Thus, we usethe bivariate Madden-Julian oscillation (MJO) index and NAO index, usingK-means clustering algorithms, four weather regimes over the North Atlantic -European region are obtained. Moreover, the relationship between the Madden-JulianOscillation(MJO) in the tropics and the four weather regimes over the North Atlantic-European region is investigated. It is found that the winter mean NAO index can becharacterized by the number of days of NAO envents calculated by K-means clusteringalgorithms. During the two periods:1978-1990(P1) and1991-2010(P2), phase3(6) ofthe MJO leads positive (negative) regime of the NAO. Especially, MJO is found to bevery important for the NAO transition during P1, but unimportant during P2. When theMJO is within the first (six) phase during the P1, the NAO event can transit from theweak negative (positive) phase to positive (negative) phase. But only the transitionbetween dipole and wave train patterns is evident during P2when the MJO in the firstphase.