The Arctic frontal zone as seen in the NCEP-NCAR reanalysis

Calculations of a thermal front parameter using NCEP-NCAR reanalysis data o ver the period 1979-98 reveal a relative maximum in frontal frequencies dur ing summer along northern Eurasia from about 60 degrees to 70 degreesN, bes t expressed over the eastern half of the continent. A similar relative maxi mum is found over Alaska, which is present year-round although best express ed in summer. These high-latitude features can be clearly distinguished fro m the polar frontal zone in the midlatitudes of the Pacific basin and colle ctively resemble the summertime "Arctic frontal zone'' discussed in several early studies. While some separation between high- and midlatitude 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 troposp here. The regions of maximum summer frontal frequency correspond to preferr ed 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 the y may have an impact on the sea-ice circulation. Development of the summert ime Eurasian frontal zone and the summertime strengthening of the Alaskan f eature appear to be largely driven by differential heating between the cold Arctic Ocean and warm snow-free land. Frontal activity also shows an assoc iation 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. While such a link age is not discounted here, a vegetation forcing is not required in this co nceptual model.