Volume 2 (2006) Pages 104-107
Variability in tropical cyclone (TC) days in the western North Pacific (WNP) since the late 1970s is investigated based on two datasets. As an overall behavior, the intense TC days have increased for the last 30 years from both the Japan Meteorological Agency (JMA) dataset and the Joint Typhoon Warning Center (JTWC) dataset. Both datasets show that TC days with an intensity of Saffir-Simpson category 2 or higher have increased by 15-30% over the past 30 years. In terms of the detailed behavior of this increase in intense TC days, the contribution obtained from the JMA dataset is different from that of the JTWC. The JMA dataset reveals that the increase in moderately intense TC days contributes to the overall increase in intense TC days, while the JTWC dataset shows that the increase in extremely intense TC days has the dominant contribution. The difference between the two datasets becomes significant after 1987 when aircraft reconnaissance by the US Air Force was deactivated. The difference between the assessed contributions is due to different implementations of the Dvorak technique, the basis for TC intensity estimation at the JMA and the JTWC after the deactivation of aircraft reconnaissance.