Strong Alaskan and Siberian anticyclones

Strong anticyclones are an important component of the winter climate across the continental landmasses of the Northern Hemisphere especially over northwestern North America and Siberia. Cold air masses associated with these features can move into populated mid-latitude regions and can create a host of problems and thus it is critical to gain an understanding of the processes responsible in the formation of the strongest anticyclones, which often are characterized as being associated with the coldest surface air masses.

Composite analyses of individual strong Alaskan and Siberian anticyclones using the ERA-40 dataset for the winter 1978/79–2001/02 are performed to diagnose key dynamical and thermodynamical features relevant to the development of these systems. The anticyclones are identified using regionally-defined SLP thresholds of 1050 hPa and 1060 hPa, respectively.

The results indicate that atmospheric blocking is an important component in the development of these surface anticyclones as their formation occurs downstream of anomalous upper-tropospheric ridges. The dynamical impact of the ridge associated with the strong Alaskan anticyclone is found to be more impressive than that of the strong Siberian anticyclone. Another key aspect of the development of these strong anticyclones is the role of radiational cooling. The cooling of the near-surface air mass is shown to be important to both anticyclones, but cooling due to radiative effects dominates in the Siberian anticyclone. We also demonstrate that elevated terrain plays a significant role in the local maintenance of cold air mass.