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. 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) west of the Iberian Peninsula (Southern European Blocking; SEB), and (4) over the North Sea (Northern European Blocking; NEB). GB and SEB 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 SEB (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 AMO leads the SEB by ~3 years. However, this relationship is robust only when the AMO is in its negative phase, i.e. colder-than-average North Atlantic SST. SEB is also shown to be associated with extensive changes in precipitation pattern across the western Europe. 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.