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

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).