Characterization Of Vertical Wind Shear In The Tropical Cyclone Environment Of The Northwest Pacific

As an important environmental factor affecting tropical cyclone intensity changes,different magnitudes,directions,depths and heights of vertical wind shear may have different effects on tropical cyclone intensity changes.In this thesis,we investigate the magnitude,direction,depth and height of vertical wind shear associated with the tropical cyclones in the western North Pacific from 1980 to 2018,and analyze the relationships between these shear characteristics and tropical cyclone intensity changes.The interannual change in shear magnitude is also taken as a focus to further reveal the primary circulation features affecting the interannual change in shear magnitude in the western North Pacific.The environmental vertical wind shear of tropical cyclones in the western North Pacific is dominated by shallow shear.The correlation analysis between the depth of shear and the change of non-dimensional intensity of tropical cyclones indicates that the greater the depth of shear,the more detrimental to the intensification of tropical cyclones.The frequency of upper-level shear is the highest,and the special variation of the shear height in different seasons may affected by the westerlies.There is no significant correlation between shear height and tropical cyclone nondimensional intensity change,which means the shear height makes less impact on the change of nondimensional intensity change.South-westerly and north-east shear are the two main shear directions of the environmental vertical wind shear of tropical cyclones in the western North Pacific.The differences in the effect of different shear orientations on the nondimensional intensity changes of tropical cyclones are relatively small,but the upper-and lower-level systems around tropical cyclones corresponding to the environment of different shear orientations such as westerly trough,inverted-trough and monsoon trough may still affect the nondimensional intensity changes.The first three modes affecting the interannual change in the magnitude of zonal vertical wind shear in the western North Pacific during the active tropical cyclone season pertain to El Ni(?)o,IOD,and PNA,respectively.In El Ni(?)o years,warm sea surface temperatures in the central and eastern equatorial Pacific excites an atmospheric Rossby wave response,and the western North Pacific is dominated by counter-cyclonic shear.In IOD positive phase years,colder sea surface temperatures in the East Indian Ocean excite cold Kelvin waves that propagate eastward in the lower troposphere,and the western North Pacific is regulated by counter-cyclonic shear.The third mode shows counter-cyclonic zonal vertical shear in the lowlatitude region of the western North Pacific under the influence of PNA.The first three modes that contribute to the interannual change in the magnitude of zonal vertical wind shear in the western North Pacific during the inactive season of tropical cyclones are related to La Ni(?)a,AMM,and AO,respectively.In La Ni(?)a years,colder sea surface temperatures in the central and eastern equatorial Pacific excites an atmospheric Rossby wave response,and cyclonic shear exists in the western North Pacific region.In AMM positive phase years,the upper-level(lowerlevel)westerlies(easterlies)in the western North Pacific are significant,and anomalous westerly shear thereby exists in the western North Pacific.In AO positive phase years,anomalous anticyclones exist in the North Pacific troposphere during winter,so the low and mid-latitudes of the western Pacific are affected by westerly and easterly wind shear,respectively.