Response Of Tropical Cyclone Genesis Over The Western North Pacific To Anomalous SST

By using the data of Joint Typhoon Warning Center(JTWC) in the period of 1970-2012, the atmospheric reanalysis data, and the generalized equilibrium feedback assessment (GEFA) method,we separated each ocean’s influence to the tropical cyclone genesis number(TCGN), and presented an assessment of the TCGN response to major ocean variability modes based on sea surface temperature anomalies (SSTA); then compared the advantages and disadvantages of the equilibrium feedback assessment (EFA) and GEFA, detected how the interaction between each SSTA modes effects on the TCGN; furthermore, in order to explaining the phenomenon of the TCGN’s rising trend during 1970-1994 and the downward trend during 1994-2012, GEFA is used to analyze the TCGN response to major ocean variability modes which contains the linear trend. The main conclusions are the following:(1)By using the method of GEFA, TCGN during June, July, September, October(JASO, the most active season of TC) is significantly forced by the first SSTA EOF mode of north Atlantic(also named NA1,95% confidence level) and the tropical Indian Ocean Monopole mode(IBOM, also named TIO1,90% confidence level). The feedback intensities of NA1 and TIO1 are-0.379 and-0.41 respectively. Though the relative variance contributions of IOBM(20.56%) is more than NA1(16.55), NA1 has a bigger effect on TCGN during the years when the TCGN is less than the climate. The spatial distribution of TCGN is close to the environment factors, like 850hPa relative vorticity,600hPa relative humidity, vertical wind shear between 200hPa and 850hPa and vertical gradient of pseudo potential temperature between 200hPa and 925hPa. Besides, when the SST in TIO has a negative IOBM force, the TIO cold SSTA can excite cold equatorial Kelvin wave to the east over the WNP and the westerly wind, leading to anomalous cyclonic vorticity and convergence. And negative NA1 SSTA means a divergence circulation in the upper air of WNP, promoting the vertical motion, anomalous cyclonic vorticity and convergence in the tropics off the equator in the WNP TC genesis region.(2) By using the method of EFA, TCGN is significantly forced by the second SSTA EOF mode of the tropical Pacific (TP2), the second SSTA EOF mode of the north Pacific (NP2), IBOM, NA1 and second SSTA EOF mode of the north Atlantic (NA2). It is the interaction between each SSTA modes that causes the difference between EFA and GEFA. And in the study of comparing the Multi factor line regression method and GEFA, the results indicate that GEFA can make accurate estimates on the random variation of the atmosphere.(3) The TCGN sequence has a rising trend during 1970-1994 and a downward trend during 1994-2012. It is found that TP2 has a significant contribution to the rising trend, besides IOBM and NA1 have significant contributions to the downward trend. In addition, the rising trend affects most in the areas of 10°-20°N,140°-150°E, and the downward trend affects most both in the South China Sea and the areas on the eastern side of 150°E. It is consistent with the low-level relative vorticity’s and the mid-level relative humidity’s response to the significant SSTA modes.