ANNUAL CHANGE IN FRONTAL ZONES IN THE NORTHERN HEMISPHERE

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From the viewpoint of synoptic climatology, detailed investigations of frequency of weather systems such as cyclones, anticyclones and their tracks have been made in recent years. However, the position of frontal zones in the Northern Hemisphere were dealt with only in the average condition of summer and winter. In this article the author aims to study the annual change in the positions of frontal zones over the Northern Hemisphere. The work was done for each month throughout the year taking an average of six years from 1952 to 1957. Daily Series of Synoptic Weather Maps and IGY World Weather Maps were used. The numbers of fronts except upper ones were counted in each of five degree square box regardless of their types. The total number in each box was divided by the number of days for each month and multiplied by 100 to give the percentage of frequency. In order to correct the areal size of the boxes, the values were modified according to the latitude length. The monthly situations are shown in Figs. 1-3. The following results can be pointed out: 1) Two frontal zones in the oceans exist all the year round and shift latitudinally. The maximum frequency of the Pacific Polar frontal zone appears along the southern coast of Japan in June. That of Atlantic Polar frontal zone appears over offshore of New Foundland in July. 2) Eurasian Polar frontal zone appears throughout the seasons but its movement is complicated. It is classified into three parts according to the period and the positions of occurrence. These are the Mediterranean branch, the Mongolian one and the main part of the frontal zone. The maximum frequency of them appears in January, May and August respectively. 3) The Atlantic Arctic frontal zone exists in autumn, winter and spring and develops most distinctively in November and March. 4) The Siberian-Canadian Arctic frontal zone appears in summer, but its eastern part exists in spring and autumn. 5) The North American Arctic frontal zone develops fully in midwinter over the north-western part of the U.S.A. 6) Summer frontal zone in North America appears in May and disappears in October. These annual changes in the frontal zones are summarized in Table 1. By taking account of the changes in the positions and in the maximum values, five stages are assumed in their annual changes. The first stage begins in March when the frontal zones over the oceans begin northward movements and the frontal zones that develop in winter distinctively become weak. The second stage begins in May. During this stage the summer frontal zones develop. The third is July and August when frontal zones, which exist in warm period, go up the northernmost and show the maximum frequency. The fourth stage starts from September. Frontal zones begin their southward movements and their frequency begin to decrease. Moreover, the Atlantic Arctic frontal zone already appears. The last stage be-gins in November. In this month, frontal zones go back to the positions of winter months except the Mediterranean branch of the Eurasia Polar frontal zone. These stages are also seen in the time-latitudinal isopleth of mean occurrence frequency of fronts. The four time-meridional cross sections along O°E, 140°E, 80°W and 130°W are prepared to consider the regional differences of the annual changes of frontal frequency. Five stages in the annual courses are also found in these figures except few months. Comparing these five stages with the mean time-meridional cross section of zonal wind speed shown in the study by Mintz (1954), good agreements are found. Furthermore, these relations are verified by the results of studies by Milier (1955) for 140°E and by Kochanshi (1955) for 80°W.
The Association of Japanese Geographersの論文

ANNUAL CHANGE IN FRONTAL ZONES IN THE NORTHERN HEMISPHERE

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