An Inactive Period of Western North Pacific Tropical Cyclone Activity: 1998-2011

The tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation and the period 1998-2011 is identified as an inactive period. During this period, the number of TC formations in the southeastern part of the WNP has a significant decrease, which is related to its decadal variation. A decrease in TC genesis is also observed in the main development region, which is related a downward trend in this region.

The effects of vertical wind shear and subtropical high on the variations of TC activity on multi-decadal time scales have been examined. A vertical wind shear index, defined as the mean magnitude of the difference of the 200-and 850-hPa horizontal zonal winds in the region 10o-17.5oN, 150o-180oE, is highly correlated with the annual number of tropical storms 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 is calculated as the area enclosed by the 5880 gpm line of the 500-hPa geopotential high within the region 0o-40oN, 100o-180oE. This index shows a significant upward trend and stronger-than-normal subtropical high is generally observed in during this inactive period. Thus, the strong vertical wind shear and strong subtropical high lead to the atmospheric conditions not favorable for TC genesis, which therefore partly explain the low TC activity in the period 1998-2011.