冬季における北極前線帯の位置について

In the preceeding studies which determined the location of the Arctic frontal zones on a hemispheric scale, the definition of the Arctic fronts was vague and the locations of the frontal zones varied by authors. The purpose of this paper is to define the Arctic front, to identify it on daily surface synoptic weather charts, to clarify its frequency distribution and to determine the wintertime location of the Arctic frontal zones. The basic weather charts used for this study were “Täglicher Wetterbericht” published by Deutcher Wetter-dienst for winter months (December to February) from 1958 to 1962.<br> First, to clarify the relations between the surface positions of fronts and the thermal and wind fields of upper isobaric levels, isotherms at 850, 700 and 500mb levels (Fig. 2), isotachs at 300 and 500mb levels (Fig. 3) and a vertical north-south cross section (Fig. 4) were analys-ed in the European sector. The 500mb level analyses were extended to the whole area of the weather charts, Northern Hemisphere except the Pacific and South Asia regions (Fig. 5). As a result, the Arctic fronts could be defined as fronts which were accompanied with the Arctic front jet streams running intermittently in the circumpolar region. The temperature of this jet streams at the 500 mb level was -28 to -44°C, the mean was -34.6°C and the standard deviation was 2, 7°C in the case of January, 1960 (Fig. 6). These results meant that the Arctic front jet streams were discernible from the Polar front jet streams according to its flow pattern and temperature.<br> Then, all the Arctic fronts appearing on every daily surface synoptic weather charts were classified into the Arctic fronts and the Polar fronts based on the flow pattern and tempera-ture of the jet streams at 500mb level. The total numberr of the Arctic fronts in each five degree square mesh was counted by months. Five year mean monthly frequency distribution (Fig. 7) and locations of the axes of the maximum frequency of the Arctic fronts in each year were shown (Fig. 8). As a result, the Arctic fronts were shown to have the highest frequency extending from Barents Sea to the central inland part of Siberia. This zone was named “Atlantic-Eurasian Arctic frontal zone”. Apart from this frontal zone, a secondary axis extended from central Siberia westward to Baltic Sea. The second highest frequency appeared in the northwestern part of North America, along the mountain range of Alaska and the northern periphery of the Rockies. In the northeastern part of North America, the Arctic fronts tended to concentrate except in February, though its frequency was relatively low (below 20%).<br> Based on Figs. 7 and 8, winter months' locations of the Arctic frontal zones were deter-mined (Fig. 9). Fig. 9 showed that two principal Arctic frontal zones were recognised in winter, and these were located approximately along the great-circle.