Modulation of the Atlantic Multidecadal Oscillation (AMO) on the Winter Blocking Activity over the Ural Mountains

The impacts of the Atlantic Multidecadal Oscillation (AMO) on the blocking activity over the Ural Mountains during boreal winter (December-January-February) are investigated using three sets of reanalysis datasets and numerical experiments with an atmospheric global circulation model (AGCM), CAM5. The observational analyses suggest that AMO plays an important role in modulating blocking activity over the Ural region. The AMO in its different phase results in a dipole-like anomaly in Ural blocking frequency, with sign-reversed anomalies along the latitude of about 60°N. An index defined as the difference of the blocking frequency in the north minus that in the south is highly correlated with the AMO index (0.82). On the aspect of atmospheric circulation, the blocking anomalies substantially result from a northward (southward) shift of the anticyclone, which usually dominates the Ural region, and a southward (northward) shift of the associated westerly. The result is robust because of a high consistency, not only among the three sets of reanalysis datasets but also with AGCM experiments with the AMO-associated SSTA forcing. The mechanism that maintains the blocking-related atmospheric responses is diagnosed by investigating the roles of AMO-forced anomalous transient vorticity forcing via a linear baroclinic model (LBM). The results suggest that the blocking-associated height anomalies over the Ural region are largely maintained by anomalous transient vorticity forcing induced by the AMO-related SSTA.