Time and space structure of interannual variations in summer rainfall over China.

The time and space structure of interannual fluctuations of summer rainfall (May to September) for the period from 1951 to 1990 over China is described. First, a harmonic analysis is applied to the summer rainfall series. The variation with periods of two to four years (hereafter, refered to 2-4 year component) accounts for more than 40 percent of the total variance for all stations, and more than 70 percent of the total variance for 61 percent of stations. The three-year period seems most prevailent among this period band. An EOF (Empirical Orthogonal Function) technique is applied to the 2-4 year period series. The first two EOF modes account for 12.8 and 10.7 percent of the total variance, respectively. EOF mode 1 reveals the seesaw between the Yangtze River valley and the Northern part of China. The spatial pattern of EOF mode 2 is more complicated, but it may be characterized by the oscillation between the Mei-yu region including the Yangtze River valley, and the rest of the country. Although the contribution proportion of the two principle modes is not high, composite maps show that they present the situations (before applying the EOF analysis) well. These two EOF modes exhibit large amplitude modulations. The amplitudes or the squares of the time coefficients tend to become large or small alternatively. Correlation and composite analyses show that EOF mode 1 correlates with the Indian summer monsoon and the SOI (Southern Oscillation Index). This result agrees with that of preceding studies and suggests that the summer rainfall over China is associated with the ENSO (El Nino and Southern Oscillation) events on the time-scale of two to four years. EOF mode 1 seems to precede the anomaly of the SOI. This result supports the previous proposals that the ENSO signals in the eastern equatorial Pacific may originate in the central Asia or the Indian Ocean region. EOF mode 2 seems not to be related to the Indian monsoon and the ENSO.