Long-term features of tropical cyclones affecting Taiwan

This study investigates the long-term variations of tropical cyclones (TCs) that affected Taiwan during 1970 - 2014 using wavelet analysis of TC activity, and identifies the corresponding large-scale environments related to such variations. Two significant oscillations in the numbers of TCs that affect Taiwan are found: interannual (3-5-year) and interdecadal (10-16-year). The distribution of TC formation locations and the corresponding large-scale environments associated with the positive and negative phases of the interannual and interdecadal oscillations are examined. In years when both the interannual and interdecadal oscillations are positive, the low-level relative vorticity around Taiwan is above normal, which therefore provides a more favorable environment for TC formation in the region near Taiwan. At the same time, the large-scale environment provides a steering flow over the above-normal formation region to steer the TCs towards Taiwan. In the negative phases, anomalies of low-level relative vorticity near Taiwan become negative so that TC formation is not favored. The steering flows over areas of favorable TC formation locations also do not tend to steer the TCs to move toward Taiwan. Thus, the positive and negative phases of the interannual and interdecadal variations of the number of TCs that affect Taiwan can be explained by those in the TC formation locations and the large-scale steering flow. However, none of these variations appears to be related to the El Niño- Southern Oscillation.