Impact of the Intraseasonal Variability of the Western North Pacific Large-Scale Circulation on Tropical Cyclone Tracks
Weather and forecasting
The life cycle of the Southeast Asia-western North Pacific monsoon circulation is established by the northward migrations of the monsoon trough and the western Pacific subtropical anticyclone, and is reflected by the intraseasonal variations of monsoon westerlies and trade easterlies in the form of an east-west seesaw oscillation. Previous studies have shown that the characteristics of tropical cyclone tracks are affected by the intensity of monsoon westerlies and trade easterlies and the
... rlies and the anomalous circulation patterns associated with these monsoon flows, but not in the context of the monsoon life cycle. Thus, an effort was made in this study to disclose the impact of monsoon life cycle on tropical cyclone tracks. A majority of straight-moving (recurving) tropical cyclones appear during weak (strong) monsoon westerlies and strong (weak) trade easterlies. The monsoon condition associated with straight-moving tropical cyclones is linked to the intensified subtropical anticyclone, while that associated with recurving tropical cyclones is coupled with the deepened monsoon trough. Composite circulation difference between the flows associated with the two types of tropical cyclone tracks shows a vertically uniform short-wave train along the North Pacific rim, as portrayed by the Pacific-Japan Oscillation. During the extreme phases of the monsoon life cycle, the anomalous circulation pattern east of Taiwan resembles this anomalous short-wave train. A vorticity budget analysis of the strong monsoon condition reveals a vorticity tendency dipole with a positive zone to the north and negative zone to the south of the deepened monsoon trough. This meridional juxtaposition of vorticity tendency propagates the monsoon trough northward. During the strong monsoon condition, the interaction of a tropical cyclone with the monsoon trough intensifies the north-south juxtaposition of vorticity tendency and deflects the tropical cyclone northward. On the other hand, during the weak monsoon condition, the interaction between a tropical cyclone and the subtropical high results in a northwestward motion steered by the intensified trade easterlies. An accurate forecast of tropical cyclone tracks requires an accurate prediction of the monsoon trough and the subtropical anticyclone variations coupled with the monsoon life cycle.