Large‐scale indices for assessing typhoon activity around Taiwan. (14th September 2018)

• Record Type: Journal Article
• Authors: Tu, Jien‐Yi; Chen, Jau‐Ming
• Is Part Of: International journal of climatology. Volume 39:Number 2(2019)
• Page Start: 921
• Page End: 933
• Publication Date: 2018-09-14
• Subjects: index -- sea surface temperature -- typhoon -- western Pacific subtropical high; Climatology -- Periodicals; Climat -- Périodiques; Climatologie -- Périodiques; 551.605
• DOI: 10.1002/joc.5852
• Languages: English
• ISSNs: 0899-8418

Abstract

Abstract : 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. Abstract : (a) The domain of typhoon impact zone surrounding Taiwan. (b) Seasonal cycle of the typhoon count per decade affecting Taiwan averaged over the period 1979–2016. The 5‐day running mean of the typhoon count represents the seasonal cycle. The vertical dashed lines from left to right indicate the dates: June 15, September 1, and October 15, respectively. (c) Monthly climatology of 500 hPa, 5, 870 gpm contour for June, July, August, September, and October, respectively. The JJASO‐mean contour is shaded. The rectangular box denotes the areas: 120°–130°E, 25°–30°N and 120°–130°E, 20°–25°N.