Long-term features of tropical cyclones affecting Taiwan

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-evel 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.