Spatial and Temporal Variations of Tropical Cyclones at Different Intensity Scales over the Western North Pacific from 1945 to 2005

The tropical cyclone (TC) track data provided by the Joint Typhoon Warning Center (JTWC) of the U.S. Navy over the western North Pacific (including the South China Sea) from 1945 to 2005 are employed to analyze the temporal and spatial variations of TCs of different intensity scales. Most of the TCs occurred between 15 degrees and 25 degrees N, from the northern part of the South China Sea to the eastern part of the Bashi Channel until near 140 degrees E. Most of the severe and super typhoons occurred over waters from the eastern part of the Bashi Channel to about 140 degrees E. The TCs in a weakening or steady state take up a weak majority in the area west of 123 degrees E and north of 20 degrees N; those in an intensifying or steady state are mostly found in the area east of 123 degrees E and south of 20 degrees N. For severe tropical storms, typhoons, severe typhoons, and super typhoons, their average decaying rates are all greater than the respective average growing rates; for tropical storms, however, the average decaying rate is smaller than the average growing rate. Generally speaking, the stronger the TC, the faster the intensification (weakening) is. The. percentage of weak TCs is higher in June to August while that of strong TCs is higher in September to November. There are annual, interannual, and interdecadal variations in the observed number (every 6 h) and frequency of TCs at different intensity scales. As far as the long-term trend is concerned, the frequency and observed number of tropical storms have a significant linear increase, but the averaged intensity and number of TCs of other intensity categories do not exhibit such a significant linear trend. In El Nino years, the number and percentage of super typhoons are significantly higher, while the total number of tropical storms, severe tropical storms, typhoons, and severe typhoons is significantly lower, and the mean intensity of TCs is prominently stronger; in La Nina years, however, the opposite comes true.