This study explores sub-monthly variation for the circulation and convection in the western North Pacific during July and August. The sub-monthly variability is dominated by a wave-like pattern that propagates north-northwestward from the northeast of Papua New Guinea, to the East China Sea. The wavelength and phase speed are about 4000 km and 5 m s(-1), respectively. This feature fluctuates in a 7-30 day timescale, with the spectral peak at 12.5 days. The vertical structure exhibits a barotropic structure below 500 hPa, which gradually tilts northwestward in upper levels near Taiwan and southern Japan. For more than 70% of cases which reveal the wave-like fluctuation, recurving tropical cyclones are found to occur concurrently with the wave-like pattern. These recurving tropical cyclones are likely part of the wave-like pattern, and may not be regarded as isolated vortices. The wave-like pattern occurs in a large-scale flow, which is characterized by an enhanced monsoon trough extending eastward into the Philippine Sea, and a strong ridge protruding westward from the subtropical high. It is suggested that the cyclonic phase of the wave-like pattern provides a favorable environment for the genesis, growth, and movement of the recurving tropical cyclones. The monsoon trough/subtropical ridge fluctuation, the wave-like pattern, and tropical cyclones, which represent phenomena in three different time and spatial scales, form a multi-scale system that affects the sub-monthly variability of the circulation, and convection, in the western North Pacific during the boreal summer.
