Associations of Summertime Rainfall Anomalies over the Changjiang-Huaihe River Valley with the Interannual Variability of 30-60-day Oscillation Intensity in the Northwestern Pacific

Using the NCEP/NCAR reanalysis and in situ precipitation data from 1978 to 2007,the authors have investigated the associations of the interannual variability of 30-60-day intraseasonal oscillation(ISO)intensity with rainfall in the Changjiang-Huaihe River valley.The results demonstrate that the summer rainfall anomalies over the Changjiang-Huaihe River valley are significantly correlated with the changes of intensity of low-frequency oscillations in regions around the Taiwan Strait and the northwestern Pacific.An intensity index of the 30-60-day intraseasonal oscillations is defined to further examine the relationships of ISO with rainfall in eastern China.The phase composites of the low-frequency precipitation and circulation for the high-index years show that the cyclone and anticyclone appear alternatively in the northwestern Pacific,inducing the advancing or retreating of the subtropical high,and henceforth leading to the rainfall anomalies over the Changjiang-Huaihe River valley.In the low-index years,the rainfall over the Changjiang-Huaihe River valley is affected by changes of location and intensity of the western Pacific subtropical high on the intraseasonal time scale.However,precipitation anomalies in the low index years occur largely to the south of the Changjiang River valley other than in the Changjiang-Huaihe River valley.Further examinations have been carried out for both the non-30-60-day precipitation and intensity of 30-60-day oscillations,showing high correlations between them as expected.Nevertheless,both synoptic and biweekly perturbations in rainfall are possibly modulated by 30-60-day oscillations,which play very important roles in the interannual anomalies of rainfall over the Changjiang-Huaihe River valley.