ON THE RELATIONSHIPS BETWEEN THE UNUSUAL TRACK OF TYPHOON MORAKOT (0908) AND THE UPPER WESTERLY TROUGH

In this paper, by carrying out sensitivity tests of initial conditions and diagnostic analysis of physical fields, the impact factors and the physical mechanism of the unusual track of Morakot in the Taiwan Strait are discussed and examined based on the potential vorticity (PV) inversion. The diagnostic results of NCEP data showed that Morakot's track was mainly steered by the subtropical high. The breaking of a high-pressure zone was the main cause for the northward turn of Morakot. A sensitivity test of initial conditions showed that the existence of upper-level trough was the leading factor for the breaking of the high-pressure zone. When the intensity was strengthened of the upper-level trough at initial time, the high-pressure zone would break ahead of time, leading to the early northward turn of Morakot. Conversely, when the intensity was weakened, the breaking of the high-pressure zone would be delayed. Especially, when the intensity was weakened to a certain extent, the high-pressure zone would not break. The typhoon, steered by the easterly flow to the south of the high-pressure zone, would keep moving westward, with no turn in the test. The diagnostic analysis of the physical fields based on the sensitivity test revealed that positive vorticity advection and cold advection associated with the upper-level trough weakened the intensity of the high-pressure zone. The upper-level trough affected typhoon's track indirectly by influencing the high-pressure zone.