Decreasing trend of tropical cyclone genesis frequency in July-August over the western North Pacific in the last 20 years

Abstract

This study analyzed the frequency of tropical cyclones (TCs) that occurred in each month of July to September, which is when most TCs occur during a year, in the western North Pacific for the last 20 years. Since the mid-1990s, the TC genesis frequency has tended to decrease during July to August but to increase during September. Therefore, the time series of the TC genesis frequency during July to August was analyzed. The average TC genesis frequency during July to August was 10TCs. Until 2006, a greater number of years had a TC genesis frequency more than 10TCs; however, from 2006, a greater number of years had a TC genesis frequency less than 10TCs. To determine why the TC genesis frequency showed this change in the mid-2000s, the difference between the average of July and August 2007 to 2014 (0714) and that of July and August 1995 to 2006 (9506) was analyzed. With regard to the TC genesis frequency, TCs occurred largely in the eastern sea of the Philippines during 0714 and in the distant eastern sea of the Philippines during 9506. With regard to the TC passage frequency, TCs came ashore at the southeastern area of China via the South China Sea from the Philippines during 0714 and tended to go north toward Korea and Japan through the East China Sea from the distant eastern sea of the Philippines during 9506. The cause of the differences in TC tracks between the two periods was verified through an analysis of the 850- and 500-hPa streamlines. During 0714, anomalous anticyclonic circulations were strengthened in most areas of the western North Pacific. Therefore, the TC genesis frequency during 0714 was lower than that during 9506 in the tropical and subtropical western North Pacific; in particular, in this area, anomalous easterlies were strengthened, which caused TCs to go toward the southeastern area of China.