梅雨降水量及び西太平洋における夏のモンスーンの年々及び長期変動とENSOとの関係

Abstract

Interannual and interdecadal variations of the Western North Pacific Monsoon (WNPM) and the East Asian Baiu rainfall are analyzed using 30-year (1963-1992) daily sea-level pressure, 30-year monthly and daily rainfall data for the East Asia, and 15-year (April 1978-December 1992) 5-day mean 1-degree latitude-longitude gridded GMS high-cloud-amount data. The relationship to the El Niño-La Niña cycles are investigated by using monthly sea surface temperatures (SST) data over a region defined by 4°N-4°S, 150°W-90°W in the equatorial Eastern Pacific (NINO3 SST) for a 30-year period from 1963 to 1992.An analysis of Interannual variations in intensity of the WNPM in late July shows high negative correlations with the NINO3 SST during a recent 15-year period from 1978 to 1992. In the La Niña (El Niño) years, strong (weak) WNPM are observed. For the Baiu rainfall, high positive correlations with the NINO3 SST are observed in July during this period. In the La Niña (El Niño) years, light (heavy) Baiu rainfall is observed. These correlations are the results of a delay of about 20 days in the onset dates of the WNPM and the peak dates of the Baiu rainfall between the La Niña and El Niño years. In the La Niña years, the early onset of the WNPM and early peak of the Baiu rainfall are observed. In the El Niño years, the late onset of the WNPM and late peak of the Baiu rainfall are observed.The Baiu rainfall also shows a high positive correlation with the NINO3 SST during the early 15-year period from 1963 to 1977. However, difference in the dates of peak of the Baiu rainfall between the La Niña and El Niño years are smaller.An analysis of the lag-correlations among the WNPM, the Baiu rainfall and the NINO3 SST shows that the SST leads the WNPM by 1 to 2 months. On the other hand, the Baiu rainfall leads (lags) the SST by 3 (2) months during a recent (early) 15-year period from 1978 to 1992 (1963 to 1977).An autocorrelation analysis of the NINO3 SST shows that the biennial oscillation mode was pronounced during a 15-year period from 1963 to 1977. In a recent 15-year period, the low-frequency mode was dominant. These changes in the periods of the SST oscillations may be related to the recent increase in the correlations among the NINO3 SST and the WNPM and Baiu rainfall.