Geographical and Long-term Variations in the Western North Pacific Typhoons

Abstract

   In order to examine geographical and long-term variations in the frequency of typhoons, empirical orthogonal functions are calculated from a matrix of five-year running means of the annual frequency for 14 squares of 10° latitude-longitude covering the western North Pacific during the years from 1951 to 1980. The first three eigenvectors account for about 82% of the total variance, and this degree of accuracy is sufficient for the purpose of representing frequency patterns.
   The first eigenvector, explaining 45% of the total variance, indicates that frequencies in the South China Sea and those in regions to the southeast of Okinawa and around the Carolines are inversely proportional to each other. The second eigenvector shows the variations of frequencies in regions to the southeast of Japan and to the south of Okinawa. The third eigenvector depicts the variances in regions around the west of Carolines and the southern Philippines.
   To clarify the distinctive periods in the long-term variation of the geographical frequency of typhoons, a cluster analysis was made by using the Euclidean distance as a measure between two sets of the amplitude coefficients. According to the cluster analysis of the amplitude coefficients of the first three eigenvectors, there are three periods with the characteristic frequency pattern of typhoons. These are 1953-1959, 1963-1968, and 1970-1973.
   The frequency of typhoons in the South China Sea and in regions to the southeast of Japan were lowest in 1953-1959. The middle of the 1960s (1963-1968) has higher frequencies except in lower latitudes. The frequency in the South China Sea was highest and those in regions to the southeast of Okinawa and around the Carolines were lowest in 1970-1973.