Using a primitive equation model with a spheric band domain,a numerical study is carried out of the effect on rainfall of 1991 Pacific SST anomaly in the leading region with the affecting mechanisms investigated.Evidence suggests that the SST abnormality has its impact confined largely to tropical and subtropical latitudes,and its direct influence is to mtxlify the sea-air exchange of sensible heat and water vapor,causing the variation in the temperature and flow fields and condensation feedback that will modulate precipitation pattern.