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Surface friction is important to a vortex moving toward Taiwan but was ignored in several previous studies. The change of the potential vorticity comes from friction in the shallow-water equation, hence, it was applied to study the westbound vortex influenced by the central mountain range (CMR) blocking and surface friction, which is defined as friction coefficient multiplied by the square root of topographic elevation. Without surface friction, the movement of vortex is not affected by the CMR due to the conservation of potential vorticity. With small surface friction, the simulated vortex first deflects southward slightly, then rebounds gently north due to the effect of channel flow, as the previous studies. With moderate or large surface friction, when the vortex approaching Taiwan, it deforms and creates two wind maxima; one due to effect of channel flow and the other on the east of the vortex, because the slowdown vortex is pushed by the mean easterly flow behind. Meanwhile, the vortex and two wind maxima rotate cyclonically. Hence, the vortex can deflect north or south, or form a loop, that depends on the strength and location of the wind maxima. If the circulation of the vortex moves around the northern tip of Taiwan, it can induce a significant secondary vortex on the lee side. On the other hand, the secondary vortex, triggered by the flow passing over the CMR, is rather weak. This paper may provide the formation of asymmetric inner flow and the deflection of the vortex, which may be difficult to define in a more complicated atmospheric model.

The vortex moving toward Taiwan and the influence of the central mountain range