How Much Do Tropical Cyclones Affect Seasonal and Interannual Rainfall Variability over the Western North Pacific?

摘要

The authors investigate the effects of tropical cyclones (TCs) on seasonal and interannual rainfall variability over the western North Pacific (WNP) by using rainfall data at 22 stations. The TC-induced rainfall at each station is estimated by using station data when a TC is located within the influential radius (1000 km) from the station. The spatial-temporal variability of the proportion of TC rainfall is examined primarily along the east-west island chain near 10 degree N (between 7 degree and 13 degree N) and the north-south island chain near 125 degree E (between 120 degree and 130 degree E). Along 10 degree N the seasonality of total rainfall is mainly determined by non-TC rainfall that is influenced by the WNP monsoon trough. The proportion of the TC rain is relatively low. During the high TC season from July to December, TC rainfall accounts for 30% of the total rainfall in Guam, 15%-23% in Koror and Yap, and less than 10% at other stations. In contrast, along 125 degree E where the WNP subtropical high is located, the TC rainfall accounts for 50%-60% of the total rainfall between 18 degree and 26 degree N during the peak TC season from July to October. In Hualien of Taiwan, TC rainfall exceeds 60% of the total rainfall. The interannual variability of the TC rainfall and total rainfall is primarily modulated by El Nino-Southern Oscillation (ENSO). Along 10 degree N, the ratio of TC rainfall versus total rainfall is higher than the climatology during developing and mature phases of El Nino (from March to the following January), whereas the ratio is below the climatology during the decaying phase of El Nino. The opposite is true for La Nina, except that the impact of La Nina is shorter in duration. Furthermore, in summer of El Nino developing years, the total seasonal rainfall increases primarily because of the increase of TC rainfall. In the ensuing autumn, an anticyclonic anomaly develops over the Philippine Sea and TC rainfall shifts eastward; as a result, the total rainfall over the Philippines and Taiwan decreases. The total rainfall to the east of 140 degree E, however, changes little, because the westward passage of TCs enhances TC rainfall, which offsets the decrease of non-TC rainfall. Along the meridional island chain between 120 degree and 130 degree E, the total rainfall anomaly is affected by ENSO starting from the autumn to the following spring, and the variation in TC rainfall dominates the total rainfall variation only in autumn (August-November) of ENSO years. The results from this study suggest that in the tropical WNP and subtropical East Asian monsoon regions (east of 120 degree E), the seasonal and interannual variations of rainfall are controlled by changes in nonlocal circulations. These changes outside the monsoon domain may substantially affect summer monsoon rainfall by changing TC genesis and tracks.