Large‐scale indices for assessing typhoon activity around Taiwan

This study quantifies large‐scale indices associated with variations in the western Pacific subtropical high (WPSH) and tropical sea surface temperature (SST) to assess typhoon activity around Taiwan. A WPSH index is constructed for both summer and early autumn as the meridional difference of areal means between the northern (120°–130°E, 25°–30°N) and southern (120°–130°E, 20°–25°N) regions (north minus south) of the 500 hPa height. These are regions north and south of Taiwan. In the high index years, typhoons forming in the tropical western North Pacific tend to be steered by easterly/southeasterly flows at the southwestern boundary of a northwards‐shifted WPSH to move northwestwards towards Taiwan, leading to increased typhoon activity around Taiwan. In the low index years, a southwards‐shifted, eastwards‐retreated WPSH steers typhoons along its western periphery to move northwards away from Taiwan. The northwards shift of the WPSH and consequent enhanced typhoon activity around Taiwan associate with a meridional wave train induced by a zonal SST gradient in summer, but link with a Matsuno‐Gill‐type response induced by tropical SSTs in early autumn. As so, the summer SST index is defined as a zonal gradient of areal‐mean SSTs between the tropical western Pacific (150°–175°E, 10°–20°N) and the Bay of Bengal–South China Sea–Philippine Sea region (80°–130°E, 5°–20°N) (former minus latter). In early autumn, the SST index is averaged from the tropical western Pacific over 130°–165°E, 5°S–5°N. All WPSH and SST indices in summer and early autumn are significantly correlated (at the 99% level) with the number of typhoons affecting Taiwan.