Features And Causes Of The Tropical Cyclone Frequency Generated In Dirrerent Period Over The South China Sea And The Western North Pacific

Based on the best track dataset for tropical cyclones (TC) over the Western North Pacific ocean (WNP) from Shanghai Typhoon Institute, the monthly average analysis data from the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) and the extended reconstructed sea surface temperature data from the National Oceanic and Atmospheric Administration (NOAA), classification studies were conducted in the paper on the temporal and spatial variations as well as the causes of the tropical cyclone (TC) frequency over the South China Sea (SCS) and WNP in different periods during1961～2010.The major results are summarized as follows:The whole year can be divided into an active period (June-November) and a calm period (December-May). There are three high frequency regions over the SCS and WNP during both periods, but the lacations of high TC frequency in the active period are roughly five degrees in latitude more northward than that in the calm period. During the recent50years, the TC frequency also shows an obvious interannual variability over the SCS and WNP in both periods, and a long term decrease trend, especially in active period, over the NWP, but not clear interdecadel variability over the SCS as revealed by the linear trend analysis. Analysis shows that there exists both3-5year short-period and nearly ten year long-period over the SCS, but only short-period and a abrupt change in1990s over the WNP. The empirical orthogonal function (EOF) analysis of the TC frequency indicates that, the first eigenvectors during both periods show a uniformly enhance or reduce of the TC frequency over the SCS and WNP, whilist the second eigenvector in the active period shows that the TC frequency over the WNP varies in opposite phase with that over the SCS, and the second eigenvector in the calm period shows that the TC frequency over the SCS and to the east of Philippines varies in opposite phase with that to the east of130°E.It is shown that the water vapor flux divergence exists a significant negative correlation with the TC frequency over the WNP in both periods and that over the eastern SCS and the vicinity of Philippines in the calm period, but only a weak negative correlation over the central and northern SCS in the active period. Composite analysis shows that, both convergence and divergence of the moisture anomalies can be found in the middle and lower levels over the SCS, but the SCS is located on the water vapor tranport channel, there is stronger (weaker) water vapor transportion with a strong local convergence (divergence) in the years of high (low) TC frequency, whereas the abnoamal stronger water vapor convergence in the central and eastern (western) parts of the WNP during the active (calm) period is favorable for the generation of TC therein. Further analysis indicates that in addition to water vapor conditions, variations in the500hPa geopotential height and the sea level pressure may also have significant impaction on the TC frequency over the WNP but not so for the SCS. From the climatology, the regions of the high TC frequency are well corresponding to the highest values of vertical upward movement near300hPa over the WNP, but for the SCS, the vertical upward movement there is weaker even in the years of higher TC frequency. The enviromental vertical wind shear also has an obvious impaction on the generation of TC, e.g., the smaller environmental vertical wind shears in the central and northern parts of the SCS and the eastern part of the WNP are responsible for the higher TC frequency therein. It shows that there is a negative correlation between the sea surface temperature (SST) over the central and eastern equatorial Pacific and the TC frequency over the SCS, which is more significiant in the clam periods; duing the active period the TC frequency over the WNP is significiantly and negatively (positively) correlated with the SST in the SCS and the western WNP (central and eastern equatorial Pacific); while duing the calm period, the TC frequency over the WNP is significiantly and negatively correlated with the SST over from the SCS to western Pacific and the eastern equatorial Pacific. Whereas the local SST shows little effect on the TC frequnency and sometime they even have negative correlation with each other, which may be due to the fact that when the TC frequency increases, the enhanced low level wind will strengthen the vertical mixing of the water near the sea surface, and deepen the thermocline layer which may cause more colder water upwelled from lower levels to the sea surface, thus lead to little change or even cold anomaly in lacal SST as compared to that in the other years.