A Near-term Typhoon Risk Model for the Western North Pacific Basin

Tropical Cyclone (TC) best-track data analysis reveals active and quiet periods of TC seasons in the Western North Pacific (WNP) basin. Landfalling TCs in the East Asian countries have also shown such multi-decadal variability. From the available historical information, no significant long-term trend has been detected so far in basin-wide activity. Natural climatic variability such as El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) alters atmospheric and oceanic conditions for genesis and intensification of typhoons, leading to multi-decadal variability with active and quiet seasons for extended periods of time in the WNP basin. Since no statistically significant long-term trend is found in the TC activity, it is reasonable to conclude that decadal TC variability is largely contributed by sea surface temperature (SST) changes in the WNP basin through ENSO and PDO. Out of 26 named TCs that form annually in the WNP basin, about 13 to 14 named TCs make landfall each year, of which 7 to 8 are typhoons. Therefore, above normal or below normal TC activity for an extended period of time in the basin is likely to have significant impacts on the East Asian typhoon risk. In this study an effort is made to assess the impact of multi-decadal TC variability on the East Asian countries, based on observational and modeling techniques.

This study is based on an analysis of the U. S. Navy's Joint Typhoon Warning Center (JTWC) TC best-track data for the period 1945-2011 and on EQECAT's basin-wide Asia Typhoon catastrophe risk model. The EQECAT Asia Typhoon model calculates probabilistic distributions of losses using stochastic methods. The model is comprised of approximately 150,000 simulated TC tracks.

The standardized anomaly of the number of basin-wide TCs and TCs making landfall in the Philippines, Malaysia, Thailand, Vietnam, Hong Kong, China, Taiwan, South Korea, and Japan during 1945-2011 show active and quite periods. TC activity was below normal before 1960, during the 1970s and early 1980s, and since the late 1990s. During these quiet periods, the basin-wide TC activity was about 8% below normal. TC activity was above normal during the 1960s and early 1970s, and in the mid-1980s to late 1990s. About 21% more TCs formed during these active periods than in the quiet periods. In the active periods, 21% more TCs made landfall than in the quiet periods. A 26% increase in the number of landfalling major typhoons is observed during the active periods relative to the long-term average (1945-2011).

A basin-wide near-term typhoon frequency model is derived for southern, central, and northern regions. In general, typhoon risk is likely to reduce (increase) during quiet (active) periods for all modeled countries. During the quiet periods with fewer major typhoon landfalls, the average annual loss (AAL) is likely to be significantly reduced in the Philippines and Japan; the opposite is likely during the active periods. Moderate changes in risk are likely in China, Hong Kong, and Taiwan where the rates of TC landfalls during quiet and active periods are not substantially different from the long-term mean. The changes in the risk are likely to be minimal in Malaysia, Thailand, and South Korea during quiet and active periods.