Analysis Of Super Typhoon Activity Over Western North Pacific

Using the tropical cyclone (TC) best track and intensity data of western North Pacific from the Joint Typhoon Warning Center (JTWC) of U.S., this study analyzes the statistical features of TC and STY (super typhoon) over western North Pacific (WNP) from 1965 to 2008 and described the temporal and spatial variability of TC and STY activities. The contrast between features of TC activity and STY activity are analyzed. The large-scale environmental parameters on TC activity are analyzed from vertical wind shear, sea surface temperature (SST), low-level relative vorticity and mid-level humidity by using the NCEP (the National Centers for Environmental Prediction) reanalysis data. The relation between STY activity and ENSO as well as the mechanism of ENSO affecting on STY activity were also investigated. The main results are as follow:1)There are about 30 TCs formed over WNP every year and 21% of them could reach the rank of STY. Both of TC frequency and STY frequency shows two peaks distribution.One of the two peaks locates to the east of Philippine sea, and the other locates in the north part of South China Sea.2)TC and STY activities have obvious seasonal variability. The most of TCs appear from July to October. There is a grate TC number in August while the minmum TC number is in February. The most of STYs appear from July to November. There is a grate STY number in September while the minmum TC number is in February and March. TC and STY active to the south of 15°N in February and March. From April to September, the area of TC and STY activity continuously migrated to the northwestward and the frequency of them was in an increasing trend. And then, the location of them migrated southeastward from October to December, and the frequency of them was in a decreasing trend during this time. Results showed that seasonal TC intensity change have three peaks(Apr, Jul and Nov) while seasonal TC number cycle have only one peak(Aug).This feature of TC intensity attributed to large-scale environmental parameters'seasonal variability. Low-level vortex and meridional vertical wind shear are more important than other five parameters, and they play important rules in the interannual variability of TC intensity.3) In the central Pacific warming years, TCs could stay in weak vertical shear and large positive Low-level vortex during their movements, and vice versa during central Pacific cooling (CPC) years. There are more STYs in CPW years than in CPC years. There is remarkable relationship between central Pacific ENSO and STY activity, while the relationship between eastern Pacific ENSO and STY activity is not remarkable.