Studies Of The Impact Of Tropical Cyclone Activities Over Western Pacific On The Meridional Movement Of Subtropical High

Tropical cyclone(TC)and western Pacific subtropical high(WPSH)are important weather and climate systems over East Asia-West Pacific.Their activities can significantly affect the weather and climate conditions in East Asia.The research into the influence of TC activities on the meridional movement of WPSH can deepen the understanding of the characteristics and mechanism of the meridional movement of WPSH,contributing to the improvement of the cognition of the interaction between TC activities and WPSH.In addition to this,this study is of important theoretical significance and practical application value for revealing the climatic effects of TC activities in the Western Pacific,assessing the relative contribution of TC activities to the evolution of the East Asia-Western Pacific atmospheric circulation and improving the ability of disastrous weather forecasting and climatic prediction of China.This paper focuses on the influences of TC activities on the meridional movement of WPSH,analyzing the overall consistency of the meridional movement of WPSH by using the FNL data from NCEP,and the TC optimal path data of the Joint Typhoon Warning Center(JTWC)of the United States is used for discussing the interaction between the TC activities and WPSH meridional movement.On this basis,the mesoscale non-hydrostatic numerical model WRF is used to carry out TC Megi(2010)case simulation experiment,and the phenomenon of TC energy dispersion affecting large-scale circulation through the vortex Rossby waves is discussed through dynamic theoretical analyzes.Moreover,the propagation characteristics of the vortex Rossby waves in asymmetric and symmetric basic flows are also discussed.Besides,sensitive numerical experiments,ensemble numerical experiments and TC remove experiments are carried out to further analyze the characteristics and possible mechanism of TC activity affecting the meridional motion of WPSH.Conclusions are as follows:(1)The meridional movement of WPSH is consistent overall,and the WPSH ridge line moves synchronously with its main body centroid.Furthermore,there is a correlation between the Western Pacific TC activity and the meridional movement of WPSH,and high TC activities always correspond to a more northwards position of WPSH.Results of numerical experiment show that the disturbance caused by TC activities has a wave propagation characteristic,which can affect the environmental field and cause the meridional movement of WPSH.(2)The direction of TC energy dispersion is related to the motion state of TC,and the energy dispersion of TC through the vortex Rossby wave is one of the main ways in which TC can affect the large-scale circulation.The energy dispersion paths of the vortex Rossby wave are different in symmetric and asymmetric TC basic flow.Dynamics analysis shows that the asymmetry of TC basic flow can change the TC vortex Rossby wave ray slope by affecting the radial and tangential group velocities of vortex Rossby wave,leading to the change of the dispersion path of TC energy,and ultimately affect the influence of TC energy dispersion on large-scale circulation.(3)There is difference in the characteristics TC activities with different intensities whose influence on the environmental field is also different,and the meridional movement of WPSH ridge caused by TC with different intensities is different,too.The outward propagation disturbance caused by TC activity is related to the intensity and location of TC activities,and generally speaking,the greater the intensity of TC is,the greater the influences of TC activity on atmospheric dynamic and thermal conditions by exciting outward propagation disturbance are,and the more significant the WPSH ridge movement is.When TC is located in the south of WPSH and moving westward,it always causes WPSH to move northward,but when TC is located in the west of WPSH and moving northward,it always causes WPSH to move southward.(4)There is a good relationship between the TC-induced abnormal zonal wind near the ridge line and the movement of WPSH ridge line.The temperature meridional gradient anomaly caused by TC can lead to the abnormal zonal wind near the WPSH ridge line varying with height,which is possibly the reason for the distribution difference of WPSH ridge line at different levels.When the positive temperature gradient anomaly caused by TC activity occurs near the WPSH ridge,the easterly anomaly near the ridge will increase with height under the constraint of thermal wind,and the WPSH ridge will then move northward,but when the negative temperature gradient anomaly occurs near the WPSH ridge,the westerly anomaly near the ridge will increase with height,and the WPSH ridge will move southward.(5)The difference of vorticity anomalies on different sides of WPSH ridge caused by TC activity corresponds well with the meridional movement of WPSH.When the difference of vorticity anomalies between the north and south sides of WPSH ridge is positive,it will cause WPSH to move southward;when the difference of vorticity anomalies between the north and south sides of WPSH ridge is negative,it will cause WPSH to move northward.In addition,diagnostic analysis of the vorticity equation shows that different terms in the vorticity equation play different roles in causing the movement of WPSH.Overall,when the meridional movement of WPSH occurs,the advection term and the vertical advection term in the vorticity equation are always favorable for the maintenance and further development of the meridional movement of WPSH.