Effects Of Subseasonal-scale Guided Airflow On Tropical Cyclone Tracks In The Northwest Pacific

Intraseasonal oscillation is the most significant oscillation signal between the synoptic-scale variability and the seasonal variability.It is one of the most active atmospheric modes in the western North Pacific(WNP)in summer,and plays an important role in the tropical cyclone(TC)movement.Some TCs near the East Asian coast do not land in the East China mainland,but move northward along the east coast of China.When these TCs move to the middle latitudes,they move in different directions.Among them,some TCs continue to move northward,mainly affecting Northeast China,South Korea and North Korea,while others turn northeastward,mainly affecting Japan.As a TC moves northward through the east coast of China and moves to the middle latitudes,a key operational forecast challenging is whether it will keep a northward movement,or recurve northeastward to affect Japan.In addition,straight northward moving TCs which have small latitudinal moving distances are not common in the WNP and are difficult to forecast.In this paper,we analyze the influence of the intraseasonal-scale steering flows on these TCs using the reanalysis data,and use the Weather Research and Forecasting(WRF)model to simulate and diagnose the straight northward track of TC Sanba(2012).The main conclusions are summarized as follows:(1)After TCs near the East Asian coast move northward through the east coast of China and move to the middle latitudes,the northward component of the steering flow is mainly contributed by the intraseasonal-scale circulations.Some TCs move northeastward due to the eastward steering of the climatological background flow.In August,the subtropical high system in the WNP is strong and extends westward,which makes it easier for the TCs near the east coast of China to move northward and affect Northeast China,South Korea and North Korea.In September,more TCs move northeastward to affect Japan,which is related to the eastward withdrawal of the subtropical high system.Several individual cases that stood out from the majority of the group are analyzed.For those cases,the intraseasonal flow plays an important role in affecting the movement of the TCs mainly through the wave train,in which a cyclonic circulation is located to the north-west(north)and an anticyclonic circulation to the south-east(east)of northeastward(northward)moving TCs.(2)Examining each of the individual straight northward moving TC cases in the WNP reveals that these TCs interacted with three types of intraseasonal background flows during their northward journey.The first type is the monsoon gyre pattern,in which the northward moving TC is embedded in a closed cyclonic monsoon gyre circulation and moves northward with it.The second type is the wave train pattern,in which a(an)cyclonic(anticyclonic)vorticity circulation is located to the west(east)of the TC center.The northward moving TC moves northward under the steering of the south wind associated with the low-frequency wave train.The third type is the mid-latitude trough pattern,in which the northward TC center is located in the maximum vorticity region of the trough.(3)Sanba(2012),the strongest TC of the year worldwide,moved almost straight northward near the east coast of Asia during its lifetime and passed through different background flows from low to high latitudes.Our analysis indicates that the intraseasonal timescale wave train with east-west oriented circulations had the largest contribution to the movement of Sanba.Effects of the intraseasonal steering on Sanba's movement are investigated with numerical simulations using the WRF model.In the control simulation,total fields from the NCEP reanalysis are used as initial and boundary conditions,and the northward motion of Sanba is well captured.In the sensitive experiments,the intraseasonal component is removed and the simulated TC moves westward,northwestward,or northeastward.Vorticity budget analyses indicate that the horizontal vorticity advection term changes and forces the simulated Sanba to move westward,northwestward,or northeastward with the intraseasonal component filtered out.