Abstract:The causes of abrupt northward shifting, westward turning and extremely slow moving after entering the Taiwan Strait of Typhoon Morakot (0908) are analyzed by using the objective analysis data and numerical simulation methods. The results show: (1) The weakening and eastward withdrawing of the western Pacific subtropical high when the Typhoon moved close to the Taiwan Island made the typhoon slow down for its steering current obviously weakened. After the Typhoon entering the Taiwan Strait, the splitting, the subsequent rebuilding and the reinforcement are the main causes for its northward recurving and slow movement. (2) Typhoon Morakot has the Fujiwara effect of anticlockwise revolving around Typhoon Goni and Etau. Meanwhile, and the formation and development of Typhoon Etau delayed the westward extending of the western Pacific subtropical high. (3) Sensitivity tests to the Taiwan Island orography indicate that the terrain resulted in the asymmetric distribution of the pressure field, dense on the west and sparse on the east, and that the formation of a topographic low made the typhoon turn westward while traversing the Taiwan Island. (4) Typhoons tend to move towards the positive vorticity variation and warm/wet areas. The connecting line between positive vorticity variation center, positive equivalent potential temperature center, and typhoon center coincides with the direction of the typhoon’s abrupt Northward shifting and westward turning afterwards, which has denotative meaning for the shortrange forecast of typhoon tracks.
