The Effect Of Intraseasonal Oscillation On The Abrupt Northward Turning Typhoon Tracks In Western North Pacific

The ARW-WRF model is used to investigate the sharp northward turn of super Typhoon Megi (2010) after it moved westward and crossed the Philippines. The NCEP analyzed fields during this period are separated into a slowly varying background flow component, a 10-60-day low frequency component representing the monsoon gyre, and a 10-day high-pass filtered component representing Megi and other synoptic-scale motion. It appears that the low-frequency (10-60-day) monsoon gyre interacted with Megi and affected its track. To investigate the effect of the low-frequency mode on Megi, numerical experiments were designed. In the control experiment, the total fields of the analysis are retained in the initial and boundary conditions, and the model is able to simulate Megi’s sharp northward turn. In the second experiment, the 10-60-day monsoon gyre mode is removed from the initial and lateral boundary fields and Megi moves westward and slightly northwestward without turning north. Tracks of the relative positions between the Megi and the monsoon gyre centers suggest that a Fujiwhara effect may exist between the monsoon gyre and Megi. The northward turning of both Megi and the monsoon gyre occurred when the two centers are close to each other and the beta drift was enhanced.A vorticity budget analysis was conducted. It is noted that the Megi moves toward to the maximum wavenumber-1 vorticity tendency. The sharp change of the maximum vorticity tendency direction before and after the track turning point is primarily attributed to the change of the horizontal vorticity advection. A further diagnosis shows that the steering of the vertically integrated low-frequency flow is crucial for the change of the horizontal advection tendency.Idealized numerical experiments are performed to study the interactions between a typhooon and monsoon gyre using a barotropic model. The result shows the redistribution of vorticity of monsoon gyre caused by typhoon and the energy dispersion may lead to typhoon abrupt northward turn. Typhoons with different size in different positions may give rise to different typhoon tracks.An idealized dataset that contains a background seasonal mean state, a reference intraseasonal oscillation (ISO) state and real-strength tropical cyclones (TCs) was constructed, in order to examine to what extent the presence of the real-strength TCs may impact the overall ISO strength over the western North Pacific. The averaged ISO strength (measured by combined 850hPa zonal and meridional wind fields) increases by 9% when the realistic TCs are included. The percentage increase of the overall ISO strength depends on the variable used. It is doubled (reduced to 3%) when the vorticity (stream function) is used as the estimated variable.20 cases of abrupt north turning typhoon are selected to test the importance ISO to the north turning of typhoon. The simulation result shows that among these cases about half of the sharp north turnings are caused by ISO, and though in the middle latitude may play a part in the other north turning cases.