The Impact Of Climate Change On Tropical Cyclone Activities

The impact of global warming on tropical cyclone(TC) genesis frequency and TC intensity are investigated.We focus mainly on climate variability of the both thermodynamic and dynamic necessary conditions which influence TC formation in response to global warming.We also investigated the relationship between ENSO events interdecadal variability and TC frequency variability under the global warming. Finally,TC intensity variation responding to global warming is discussed in this study.The research results show as follows:Firstly,the intertropical convergence zone(ITCZ) strength interdecadal/multidecadal variability over the tropical NP is investigated based on the NCEP/NCAR reanalysis data.The results from this study show that the NP ITCZ can be divided into three sections according to its wind convergence types and geographical positions,namely the ITCZ section locating at 100-140°E,10-20°N named ITCZ-â… ; the section locating at 140°E -140°W,5-15°N named ITCZ-â…¡and the one in the eastern NP locating at 140°W-90°W,10-18°N named ITCZ-â…¢.ITCZ three sections demonstrate their different long-term variability respectively,namely the ITCZ-â… weakened generally and both ITCZ-â…¡and ITCZ-â…¢strengthened with some fluctuations in July-October(JASO).Secondly,the zonal wind vertical shear long-term variation over the tropical NP is investigated.The results based on NCEP reanalysis show in the North Pacific,there are three key regions which display different characteristics in terms of TC formation and different wind shear.The results show that the variation of mean vertical shear locating at 100-140°E,5-20°become weakening,it is enhanced within 140-180°E,10-25°N area,and it strengthen in the basin of 130-90°W,7-18°N over the eastern Pacific during 1949-2008.Thirdly,sea surface temperature variation in tropical North Pacific(NP) is investigated based on the NCEP/NCAR reanalysis data It is found that the SST over the tropical Pacific are gradually increased during the past 60 years,suggesting that the warm pool has extended into central Pacific even into the equatorial eastern Pacific during the recent years if the definition of taking 28℃as the critical temperature for the warm pool is adopted.There produced obvious SST gradients in the central Pacific and east Pacific owing to inhomogeneous warming,a possible cause responsible for the heterogeneous variability in the ITCZ strength is suggested via differential SST gradients.We checked climate variability of these three TC formation factors based on coupled multi-model datasets in the World Climate Research Programmers's Coupled Model Intercomparison Project 3(WCRP CMIP3) of Intergovernmental Panel on Climate Change(IPCC) with the increasing CO2 concentration 1%per year climate scenario.The simulation results are similarly with the results based on the NCEP reanalysis data.The results for concerning the climate variabilities of TC formation conditions have related relationship with TC genesis frequency.Such as,in the past 40 years,sea surface has warmed quickly and mean SST increase about 0.6℃in the WNP(west to 140°E),while the frequency of TC under the background of global warming is decreases.The researches demonstrate that the following aspects may suppress the tropical cyclone formation and its frequency.(1) Vorticities within ITCZ locating at (10-20°N,100-140°E) is weakening;(2) The ITCZ and subtropical high ridge is shifting gradually equatorwards displacement.Both of the above causes are disadvantageous to tropical cyclone formation and development under the warming sea surface temperature background.However,TC frequency is increasing in the recent 40 years over EP basin,where ITCZ strength is strengthening remarkably and position is shifting polarward displacement which mainly possible causes are responsible for TC increase in this area though the zonal wind vertical shear enhancing at the same period dose not inhibit TC formation.In a summary,wherever,the TC formation is mainly detemined by the ITCZ strength and position,that is,when the ITCZ strength weaken and its position shifting eqautorward displacement,TC formation decrease even if other factors are favorable to TC formation,such as zonal wind vertical shear weakening and SST warming,which is the case in the WNP basin,while the ITCZ strength strengthen and its position equatorwards displacement,TC formation increase even if zonal wind vertical shear reinforce,which is the case in EP region.On the other hand,it is well known that the ENSO events have good relationship with TC genesis frequency.The research results reveal that ENSO events have obvious interdecadal variability during 1950-2007 based on the Multivariate ENSO Index(MEI), in which cold ENSO events are stronger and more frequent during the beginning of 1950's up to the late 1970's,the TC genesis frequency is quite high,and from mid 1970's up to now the warm ENSO events are and stronger more frequent under the global warming background which is the lower TC genesis frequency unanimously in this period.The spring MEI,especially that in Apr/May,has a good negative correlation with TC annual frequency with the correlation coefficient of-0.5 during 1950-2007.Fainally,as for how does global warming impact on TC intensity.The study reaches conclusions as follows:Firstly,the historical TC intensity observational data could not be used to determine its climate trend owing to the data quality issues with heterogeneity of technique for measuring and fixing the intensity.In this study,the impact of global warming on the theoretical maximum potential intensity(MPI) of tropical cyclones over the WNP is evaluated using the coupled multi-model datasets in the WCRP CMIP3 of IPCC.Our results show that there is no significant increasing trend in TC MPI responding to both SST and upper tropospheric temperature increasing slightly in the WPN for the 20^th century climate background,while considerable interdecadal fluctuations in MPI are found to be associated with the variation in the upper tropospheric temperature.In contrary,a consistent increasing trend in TC MPI is found over the whole basin owing to the increasing CO₂ concentration and SST.It is shown that with the doubled CO₂,the MPI measured in the sustained maximum near surface wind speed is increased by 5-10%in the WNP basin.Considerable fluctuations in MPI are again found and are inversely correlated with the fluctuations in the upper tropospheric temperature.