The Arctic Frontal Zone as Seen in the NCEP/NCAR Reanalysis

Calculations of a thermal front parameter (TFP) using NCEP/NCAR reanalysis data over the period 1979-1998 reveal a relative maximum in frontal frequencies during summer along northern Eurasia from about 60-70 degrees north, best expressed over the eastern half of the continent. A similar relative maximum is found over Alaska year-round, although best developed in summer. These high-latitude features can be clearly distinguished from the polar frontal zone in the middle latitudes of the Pacific basin and collectively resemble the summertime "Arctic frontal zone" discussed in several early studies. While some separation between high and middle latitude frontal activity is observed in all seasons, the summer season is distinguished by the development of an attendant mean baroclinic zone aligned roughly along the Arctic Ocean coastline and associated wind maxima in the upper troposphere. The regions of maximum summer frontal frequency correspond to preferred areas of cyclogenesis and to where the summertime contribution to annual precipitation is most dominant. Cyclones generated in association with the Eurasian frontal zone often track into the central Arctic Ocean, where they may impact on the sea ice circulation. Development of the summer Eurasian frontal zone occurs in conjunction with a seasonal change in the large-scale circulation characterized by a zonal orientation of the isotherms. Over both Eurasia and Alaska, baroclinicity appears to be enhanced by differential heating between the Arctic Ocean and snow-free land. Frontal activity also shows an association with orography. Several studies have argued that the location of the summer Arctic frontal zone may be in part determined by discontinuities in energy exchange along the tundra/boreal forest boundary. However, a vegetation forcing is not required in our conceptual model. The potential roles of vegetation and orography are being further addressed through sensitivity studies using a regional climate model.