LAUSR

Variability of tropical cyclone (TC) genesis frequency in the western North Pacific and South Pacific ocean basins in the interdecadal scale is studied. It is demonstrated that the TC genesis frequency in these ocean basins experienced an abrupt decrease near the end of the 20th century. The decreased occurrence of TC genesis in the recent two decades is mainly located in the low-latitude regions at the eastern side of the two ocean basins. It is also shown that a significant part of the decreased TC genesis occurred during October to December, i.e., the post-peak season in the western North Pacific ocean basin and the pre-peak season in the South Pacific ocean basin. The interdecadal trend of variation of the TC genesis frequency in these adjacent two ocean basins seems to be mainly due to a common mechanism, i.e., the variation of the atmospheric vorticity. In contrary to the decrease of the total TC genesis, the intense typhoon occurrence frequency experienced an interdecadal increase during the same period. This trend for intense typhoon is particularly clear in the western North Pacific ocean basin. The zonal distribution of the increased number of the intense typhoon occurrence shows a similar pattern with the increased value of sea surface temperature. It is then suggested that the variation of the intense typhoon occurrence frequency in the western North Pacific ocean basin is related to a change of the La Nina-like sea surface warming pattern.