Decadal relationship between european blocking and the North Atlantic Oscillation during 1978-2011. Part II: A theoretical model study

In Part I of this study, it is revealed that decadal variations of European blocking, in its intensity, duration, and position, during 1978-2011 are modulated by decadal changes in the frequency of North Atlantic Oscillation (NAO) events associated with background Atlantic conditions. In Part II, reanalysis data are analyzed to first show that a T-bone-type structure of the climatological-mean blocking frequency in the Euro-Atlantic sector roughly results from a combination of the blocking frequency distributions along the southeast-northwest (SE-NW) direction associated with negative-phase NAO (NAO^-) events and along the southwest-northeast (SW-NE) direction associated with positive-phase NAO (NAO⁺) events. A nonlinear multiscale interaction (NMI) model is then used to examine the physical processes behind the blocking frequency distributions. This model shows that the combination of eastward- and westward-displaced blocking frequency patterns along the SW-NE and SE-NW directions associated with NAO⁺ and NAO^- events leads to a T-bone-type frequency distribution, as seen in reanalysis data. Moreover, it is found that the westward migration of intense, long-lived blocking anomalies over Europe following NAO⁺ events is favored (suppressed) when the Atlantic mean zonal wind is relatively weak (strong). This result is held for the strong (weak) western Atlantic storm track. This helps explain the findings in Part I. In particular, long-lived blocking events with double peaks can form over Europe because of reintensification during the NAO⁺ decay phase, when the mean zonal wind weakens. But the double-peak structure disappears and becomes a strong single-peak structure as the mean zonal wind strengthens.