Southward Deviation of the Polar Frontal Zone in East Asia in the Summers of the Early 19th Century

The period 1825-1840 is identified as the cooler period during the late Little Ice Age in East Asia, on the basis of the fact that summers often were notably cool in northern Japan and central China.<br> Extremely cool summers could be caused by the southward deviation of the polar frontal zones which are related to the southward deviation of cold air masses.To understand the effect of the polar frontal zone on the summer climate in the cooler period during the Little Ice Age, its average position in the period 1825-1840 (a cooler period) was compared with that in 1815-1824 (a non-cooler period).<br> The daily positions of polar fronts are reconstructed with the following procedure:<br> (1) Daily weather distribution maps in Japan for historical years (1811-1840) were reconstructed on the basis of old diaries. The same kind of maps for recent years (1975-1984) were also drawn using meteorological observational data.<br> (2) Weather maps for recent years which were similar to the individual weather maps for his-torical years were all selected.<br> (3) In order to select a suitable map from recent years which was the most similar to the cor-responding map in historical years, the following two points were considered, One is the similarity of the day-to-day changes in weather distribution pattern between the historical map and the pre-sent-day one. The other point is whether the descriptions of wind direction, hotness, and coolness on the historical weather distribution map agrees well with those in the selected synoptic maps for recent years.<br> The best-fitted map thus selected was regarded as the map which had the same synoptic situation as the corresponding day in the historical period. Therefore, the positions of polar fronts which were taken from the synoptic maps at 9:00 A. M. on selected days were used as the position on the historical days.<br> Polar front positions were classified according to the degrees of latitude. Four categories were identified along 140 E: above 42.5°N (type A), 42.5°N--37.5°N (type B), 37.5°N-32.5°N (type C), and below 32.5°N (type D). The following results were obtained by counting the frequencies of each type.<br> The northward shift of polar frontal zones in July for 1825-1840 was 10 days later than that in 1815-1824. Therefore, in the period 1825-1840 it rained a great deal in southern Japan. In the mid-summer season (August) the polar fronts in 1825-1840 sometimes shifted to the south. In early autumn (September), the time of migration of the polar frontal zone to the south was the same in both 1825-1840 and 1815-1824. Otherwise the polar fronts in 1825-1840 in September tend to be located in the south of Japan.<br> The dryness and wetness pattern over East Asia was reconstructed to discuss the connection with the polar front position described above. The wetness degrees in many places over China were quoted from the Central Meteorological Service of China (1981), and in some parts of Japan the wetness degree was identified based on the frequency of rainy days for June to September. These data for each year were averaged for 1815-1824 and for 1825-1840. In 1815-1824 northern China shows a wet pattern, while in 1825-1840 Japan and central China show a wet pattern. We believe that the wet pattern for 1825-1840 was caused by the southward deviation of the polar frontal zone. The cool summers in 1825-1840 were caused by the same phenomenon.