Two Types of the Scandinavian Pattern: Their Formation Mechanisms and Climate Impacts

On the basis of daily data from the Japanese 55-year Reanalysis (JRA-55) for extended winters (December–March) from 1958/59 to 2014/15, this study examines the formation mechanisms and climate impacts of the subseasonal Scandinavian (SCA) pattern. Results indicate that the SCA pattern manifests itself as Rossby wave trains, arising from the initial height disturbances over the North Atlantic and propagating into the Scandinavian peninsula and central Siberia. One type of SCA may arise from a Rossby wave train over the North Atlantic that is closely coupled to an anomalous convective heating dipole and persists for about 2 weeks (convection-preceded SCAs). Another type of SCA arises from the weak height disturbances over the North Atlantic; the height disturbance over the Arctic also contributes to the SCA formation, with no significant convective heating anomalies being observed in the North Atlantic (convection-free SCAs). The results also indicate that both SCA types may cause strong climate anomalies in the Arctic and Eurasia that persist for about 2 weeks. The surface air temperature (SAT) anomalies assume a dipolar structure with one extremum located over the Greenland Sea through Barents Sea and the other extremum over the Eurasian continent. Associated with the SAT anomalies is a significant increase or decrease of sea ice cover over the Greenland Sea and Barents Sea, while over the Eurasian continent snow depth anomalies are found to occur over eastern Europe, western Asia, and the Russian Far East. Furthermore, as convection-free SCAs propagate vertically into the stratosphere, significant changes of intensity and air temperature of the stratospheric polar vortex are observed.