Journal of Climate 2012 ; e-View
Kin Sik Liu and Johnny C. L. Chan*
The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multi-decadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200- and 850-hPa horizontal zonal winds within (10°-17.5°N, 150°-180°E) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880 gpm line of the Jun-Oct 500-hPa geopotential height within (0°-40°N, 100°-180°E), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a stronger-than-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998-2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.