Interannual variation of seasonal changes of precipitation and moisture transport in the western North Pacific

Interannual variation of seasonal changes of precipitation and moisture transport in the western North Pacific from June to August is studied using global monthly precipitation data, and NCEP-NCAR (National Center for Environmental Prediction-National Center for Atmospheric Research) reanalysis for 20 years from 1979 to 1998. According to the meridional shift of the intense precipitation area and precipitation amounts in June and August to the east of the Philippines, the seasonal changes of precipitation are categorized into Types A and B. Type A is characterized by the northward shift of intense precipitation area and the increase of precipitation from June to August. Type A is further classified into two: Sub-Types A1 and A2. The former is characterized by the gradual increase of precipitation and the northward shift of the intense precipitation area from 6 degrees N to 16 degrees N. The latter is characterized by a relatively intense precipitation in June, decrease of precipitation in July, significant increase of precipitation with a maximum in August, and the northward shift from 13 degrees N to 22 degrees N. Type B is characterized by a seasonal change, with almost no precipitation increase nor northward shift from June to August. Difference in the precipitation amount between Types A and B corresponds to the westerly moisture flux originating from the Indian Ocean, and the southerly moisture flux which comes across the equator at the lower level. It is found that the westerly and southerly moisture flux in Type B are much smaller than those in Type A, especially in August. The westerly moisture flux in Sub-Type A2 is larger than that in Sub-Type A1. The seasonal changes of westerly moisture flux show gradual increase in Sub-Type A1, and significant increase in June and August and decrease in July in Sub-Type A2. They are corresponding to those of the precipitation amount, respectively. The large westerly moisture flux and the strong moisture convergence to the east of the Philippines in August in Type A, are associated with the eastward extension of the trough from the Eurasian continent. The northward shift of the intense precipitation area corresponds to the intensification of the eastward extended trough.