Change Of Rapid Intensifying Western North Pacific Tropical Cyclones Related To Inter-annual And Inter-decadal Climate Variation

Using the tropical cyclone best track data from the Joint Typhoon Warning Center(JTWC)and the large-scale filed from the NCEP NCAR reanalysis data during 1970-2016,changes of the ratio of rapidly intensified tropical cyclones over the western North Pacific on the inter-annual and inter-decadal time scale and the associated plausible physical mechanism are explored in this study.Results shown that:(1)A significant increase in the proportion of tropical cyclones undergoing rapid intensification at least once during their lifetime(RITCs)over the western North Pacific(WNP)is observed since 1998 when an abrupt climate regime shift occurred.(2)Changes of large-scale atmospheric and oceanic conditions affecting TC activity are compared between two sub-periods: one before and one since 1998.Results suggest that both a significant decrease in the number of TCs and a nearly unchanged number of RITCs since 1998 caused a significant increase in the frequency of RITCs.The decrease in TC numbers is likely driven by considerably increased vertical wind shear and decreased low-level vorticity.In contrast,the unchanged RITC counts and thus increased ratio of RITCs during the recent decades are largely attributed to the dominance of a more conducive ocean environment with increased TC heat potential and warmer sea surface temperature anomalies.These associated decadal changes are closely associated with the recent climate regime shift.During the recent decades with a mega–La Ni?a–like pattern,stronger easterly trade winds have caused increased vertical wind shear and a weakened monsoon trough,thus hampering TC formation ability over the WNP.In addition,a steeper thermocline slope that hampered the eastward migration of warm water along the equatorial Pacific has generated a more favorable thermodynamic environment supporting TC rapid intensification over the WNP.(3)Considering the regional difference of ENSO impact on the WNP TCs,the WNP was divided into five sub-regions: South China Sea(SCS),SW-WNP;NW-WNP;NE-WNP and SE-WNP.The east-west distribution of TC genesis can be found during central-Pacific(CP)ENSO years,while TC genesis location show a southeast-westnorthward distribution during eastern-Pacific(EP)years.During CP El Ni?o years,more TCs experiencing RI occur over the central WNP,while more TCs occurs over the SE-WNP basin.During La Ni?a years,more TCs occurs over the W-WNP.Further analyses suggested that no significant difference of the ratio of RITC occurs over the W-WNP and E-WNP during CP ENSO years but more TCs with RI process over the E-WNP is found than that over the W-WNP for the EP ENSO years.(4)Moreover,the key factor affecting the inter-annual variability of RITC over the WNP is the mid-level relative humidity,while the ocean heat content is found to not be an important factor,although that is very important for the inter-decadal variation of RITCs over the WNP basin.