Thursday, December 6, 2012

New paper finds a significant decrease in Pacific tropical cyclones

A paper published today in The Journal of Climate examines tropical cyclone activity over the western North Pacific from 1960-2011 and finds "overall tropical cyclone activity shows a significant decrease" during the "recent inactive period" from 1998 to 2011. The paper adds to several others finding, contrary to the claims of climate alarmists, that global warming is correlated with a decrease in cyclone activity and decreased "extreme weather."

Inactive period of Western North Pacific Tropical Cyclone Activity in 1998-2011

Kin Sik Liu and Johnny C. L. Chan*
Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong
Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960-2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998-2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.
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.

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