A composite study of sub-monthly circulation features over East Asia during late summer

The sub-monthly variations of the circulation features over East Asia during late summer seasons (July-August) for 1991-2001 have been examined using the 7-30 day filtering and composite techniques. The composite results show that there exists a wave pattern propagating northwestward from the subtropical area to the eastern China Sea at a phase speed of 5m/s. The vertical structure of the wave pattern yields a warm-core and equivalent barotropic structure. After propagating further north, the vertical structure of the wave pattern is replaced by a baroclinic structure. Therefore, it is suggested that these wave patterns could be related to the tropical cyclones over the western North Pacific. It is thus proposed that the northwestward moving tropical cyclones tend to follow the tracks of these sub-monthly wave trains near this area. Moreover, these wave trains tend to occur around the confluent zone where the wind is weaker. Further studies indicate that the seasonal change of the confluent zone coincides with the wave patterns and the typhoon tracks.