North Atlantic Blocking Variability and Role of the Atlantic Multidecadal Oscillation

Spatial and temporal variability of the winter (December-March) atmospheric blocking in the North Atlantic region are examined for 1900-2010 from the 20th Century Reanalysis (20CR). Extremely consistent interannual variability of the North Atlantic blocking is found in the all 56 individual members as well as the ensemble mean field of the 20CR. DJFM number of blocking days exhibit a strong interannual-to-multidecadal variability with four centers of action: (1) near Greenland (Greenland Blocking; GB), (2) around the British Isles (European Blocking; EB), (3) near Azores (Azores Blocking; AB), and (4) over the North Sea (Northern European Blocking; NEB). GB and AB are anti-correlated (r=-0.6) and associated with the North Atlantic Oscillation (NAO). In the years with more frequent GB, the negative phase of the NAO is favored (r=-0.8), while the positive NAO is correlated with enhanced AB (r=0.6). On the other hand, the EB and NEB are significantly correlated with the seasonal mean circulation associated with the Eastern Atlantic Pattern. Statistically significant influence from sea-surface temperature (SST) variability associated with the Atlantic Multidecadal Oscillation (AMO) is found when the positive AMO leads by 3-10 years the anomalous blocking pattern with less (more) frequent AB (GB). This anomalous blocking pattern projects well onto the second EOF pattern of the winter blocking days and associated with the negative NAO. However, this relationship with the AMO is substantially asymmetric and thus only robust in the positive AMO phase. The mechanism of the AMO impact on the blocking will be discussed. Furthermore, the corresponding atmospheric blocking variability is examined in climate models and the impact of mean biases of the ocean and atmosphere will be discussed.