A composite study of sub-monthly circulation features over East Asian monsoon area during late summer

Abstract: The circulation features with a 7-30 day time scale over East Asia during late summer seasons (July-August) for 1991-2001 have been examined using the filtering and composite techniques. There exists a wave pattern propagating northnorthwestward from the tropical region 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. Therefore, it is suggested that these wave patterns could be related to the tropical cyclones over the western North Pacific. It is proposed that the northnorthwestward 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 monsoon trough/subtropical ridge area where the wind is weaker. Further studies indicate that the fluctuation of the monsoon trough/subtropical ridge system coincides with the wave patterns and the typhoon tracks.