RELATIONSHIP BETWEEN THE VARIATION OF PRECIPITATION DISTRIBUTION AND OF 500mb FLOW PATTERN IN WINTER OVER NORTH AMERICA

Abstract

Precipitation distribution is in general fairly complex, because many factors are involved in it. To classify such complicated distribution a principal component analysis is a useful means. This paper aims to clarify the relationship between the variation of precipitation distribution and that of upper flow pattern, which is considered to govern the variation of precipitation distribution in winter (January and February) over North America, Monthly data during 1951_??_1978 were analysed for this purpose. Besides, some new findings with regard to the recent climatic fluctuations were obtained. The results are summarized as follows: 1. Four principal components were obtained by the principal component analysis of month ly precipitation for 38 stations (Fig. 3). The first and the second components mainly explain precipitation fluctuation of the eastern part of the United States and that of the western coast of North America respectively. Both the third and the fourth components explan more local fluctuations. 2. The values of correlation coefficients between component scores and 500 mb height prove that the first two components are closely related to the 500 mb height distribution (Fig. 5). 3. On the other hand, four components were obtained with regard to monthly mean height of 500 mb for 39 stations (Fig. 6). Both the first and the second components determine either meridional or tonal flow type (Fig. 7), and the third and the fourth components mainly explain meridional movement of 500 mb contour lines. 4. Months of high component score were chosen with respect to the first two components, e. e. four types of precipitation anomaly distribution (1 WA, 1 WB, 2 WA 2 WB) and composite maps of 500 mb height distribution were made for these four types. These maps indicate that four types of precipitation anomaly distribution are prescribed by the four different 503 erent 50 mb flow patterns respectively (Fig. 9). 5. The first two components of 500 mb height give four 500 mb flow types (1Wa 1Wb, 2Wa 2Wb ; Fig. 7). They almost coincide with the four different 500 mb flow patterns (Fig. 9) which are considered to prescribe the four different precipitation anomaly distribution types. 6. Four types of precipitation anomaly distribution correspond to four different 540 mb flow types (1 WA-1Wb, 1 WB-1 Wa, 2.WA-2Wa, 2 WB-2 Wb) . The dominant feature of 1 WA-1 Wb (Fig. 9-a) is the prevailing tonal flow with stronger middle latitude westerlees. The eastern part of the United States is a positive precipitation anomaly area. In the case of 1 WB1 Wa (Fig. 9-b), the meridional flow is dominant with a ridge over the Rocky Mountains and a trough over the eastern part of North America or the adjacent Atlantic Ocean. The eastern part of the United States is a negative precipitation anomaly area. The 2 WA-2Wa type (Fig. 9-c) is characterized by the dominant meridional flow. A ridge is formed over the area from Alaska to the Pacific Ocean and a trough is over the eastern part of North America. As for precipitation, the vicinity of 45-N of the western coast is a negative anomaly area, while Alaska and the northeastern part of Canada is a positive anomaly area. In the case of 2WB-2Wb (Fig. 9-d), the zonal flow is dominant but with weaker middle lattitude westerlies. Characteristics of precipitation distribution are contrary to the case of 2 WA-2 Wa. 7. Examination of time series of 500 mb height component scores (Fig. 8) elucidates the following tendency of flow pattern. The zonal flow was prevalent in the former half of the 1950's, whereas the meridional flow appeared in the former half of the 1960's and in the latter half of the 1970's. The circumpolar vortex has expanded southward since about 1963. 8. Judging from time series of precipitation component scores (Fig. 4), precipitation was intend to decrease in the eastern part of the United States through the investigated period.