Weather regime transitions and the interannual variability of the North Atlantic Oscillation. Part II: Dynamical processes
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6 Conclusion and discussion In this paper, we have examined the dynamical processes that contribute to the NAO  to NAO (NAO to NAO) transition within P1 (P2). It was shown that the NAO to NAO (NAO to NAO) transition is closely related to changes in the Atlantic ridge (AR) and Scandinavian blocking (SBL) patterns. During stronger AR (SBL) winters, the NAO to NAO (NAO to NAO) transition is more likely to take place. This transition follows the route NAO to AR to NAO (NAO to SBL to NAO) within P1 (P2). By further calculating composites of the 300-hPa geopotential height anomalies, it was revealed that the NAO to SBL to NAO (NAO to AR to NAO) transition events within P2 (P1) can be attributed to the re-intensification and retrograde movement of a large-scale positive (negative) anomaly over northern Europe. Michelangeli and Vautard (1998) have discussed the contribution of the retrograding wavenumber 1 to Atlantic blocking. Michel and Rivière (2011) have recently analyzed the NAO to SBL to NAO transition and have shown that the SBL to NAO transition is mainly due to cyclonic wave breaking. In that sense, the retrograde displacement of the positive height anomaly can be viewed as triggered by nonlinearities. Cassou (2008) also noted that the NAO to SBL to NAO transition tends to be excited by the MJO in tropics. However, Luo et al. (2011) found that an excessively strong storm track can lead to the NAO to SBL to NAO transition in that the European blocking is more easily enhanced and undergoes a marked westward drift. This result does not contradict the results of Ulbrich and Christoph (1999), who found that the positive trend of the NAO from 1970s is due to the presence of a stronger storm-track. This is because the NAO anomaly is enhanced as the Atlantic storm track is strengthened, as long as the storm track is not too strong. In contrast, it can play the reversed role and contribute to the NAO to NAO transition if the Atlantic storm track is sufficiently strong. This result has been shown by Luo et al. (2011) from both observational and theoretical perspectives. In the present study, we present a new finding that enhanced retrograding high latitude low-frequency negative (positive) anomalies over the European continent play an important role in the NAO to NAO (NAO to NAO) transition.
Within P2, the NAO Although our present study provides observational evidence for the role played by the weakening of the prior barotropic zonal wind and the strengthening of the prior upstream Atlantic storm track in the NAO regime transition, a theoretical study of this process was not made. This is an unsolved problem that we plan to address in a future study.
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Figure 2. Time series of normalized winter (DJF) mean AR and SBL indices, in which the dashed line represents  standard deviations.
Figure 3. Composites of daily NAO indices for strong and weak winter mean AR and SBL, in which the solid line represents strong AR and SBL, and the dashed line denotes weak AR and SBL: (a) composite of NAO events for the different AR strength and (b) composite of NAOevents for the different SBL strength.
Figure 4. Composites of the daily Atlantic ridge (AR) and Scandinavian blocking (SBL) indices for NAO (NAO) events without transition and for NAO to NAO (NAO to NAO) transition events for P1 (P2), in which the dashed (solid) line represents the NAO events without transition (transition events): (a) P1 and (b) P2.
Figure 5. Time sequences of composited geopotential height anomalies at 300 hPa for NAO to NAO transition events within P1 and for NAO to NAO transition events within P2, in which the dashed (solid) line denotes the negative (positive) value: (a) NAOto NAOtransition (the contour interval (CI) is 40 gpm) and (b) NAO to NAO transition (CI=40 gpm). The dark (light) shading indicates positive (negative) value regions that exceed the 95% confidence level for a two-sided student’s t-test.
Figure 6. Time sequences of the amplitude of each wave component for wavenumber 1-3 in the composited geopotential height anomalies for NAO transition events, in which Lag 0 denotes the maximum amplitude of the NAO anomaly before the transition: (a) NAO to NAO transition in P1 and (b) NAO to NAO transition in P2.
Figure 7. Hovmöller diagrams of the composite geopotential height anomaly averaged from  to  as a function of longitude and time (Lag days): (a) NAO to NAO transition and (b) NAO to NAO transition. The thick bar denotes the propagation of the maximum.
Figure 8．Time variation of the zonal position of the maximum amplitude of the large-scale high-latitude negative (positive) anomaly over the European continent and its downstream region for the composite NAO anomaly from the NAO to NAO (NAO to NAO) transition, in which “-30” and “30” denote “ ” and “ ” respectively, and the solid (dashed) curve represents wavenumber 1 (wavenumber 2) : (a) NAO to NAO transition events and (b) NAO to NAO transition events.
Figure 9. As Fig.8, except for the composite NAO anomaly for NAO events without transition: (a) NAO and (b) NAO.
Figure 10. Difference of the composited barotropic zonal wind averaged from Lag-15 to Lag-10 between the transition and non-transition events within P1 (a) and P2 (b), in which the shading indicates the region above the 80% confidence level for a two-sided student’s t-test. Figure 11. As Fig.10, except for the eddy kinetic energy at 300-hPa.          
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Figure 1. Time sequences of composited geopotential height anomalies at 300 hPa for non-transition NAO events: (a) NAO anomaly (the contour interval (CI) is 40 gpm) and (b) NAO anomaly (CI=40 gpm). The dark (light) shading indicates positive (negative) value regions that exceed the 95% confidence level for a two-sided student’s t-test.
Figure 2. Time series of normalized winter (DJF) mean AR and SBL indices, in which the dashed line represents standard deviations.
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