Climate Characteristics of Tropical Cyclone Number Change in Mid-1990s over North Western Pacific

Based on the CMA tropical cyclone (TC) best track data and NECP reanalysis dataset, variation characteristics of the number of TC from 1949 to 2013 over north western Pacific (including the South China Sea) are analyzed. We find the TC number has changed from more to less around 1995 abruptly. Comparative analysis shows that environmental factors which are necessary for the formation of TC also changed after mid-1990s. After 1995, sea surface temperature in the western equatorial Pacific is above normal, and there is a La Nina-like pattern in the tropical Pacific. However, in the same period, vertical upward movement decreases, vertical shear of tropospheric east wind increases, and sea level pressure rises, all of which are unfavorable to TC's generation and work together to make TC numbers decrease significantly. In addition, in two separate periods before and after 1995, we also compare oceanic and atmospheric environment of typical years when more and less TCs occur respectively. The analysis results show that, SST in the tropical Pacific has a relatively important effect on TC's formation before 1995, but the influence of the wind environment is more prominent in the generation of TC after 1995.