This study demonstrates the multiscale nature, from synoptic to intraseasonal time scales, of the atmospheric flow in the tropical western North Pacific. The multiscale features include intraseasonal oscillations (ISO), northwestward-propagating submonthly wave patterns, and recurving tropical cyclones (TCs). In the ISO westerly phase, the wave pattern was better organized and the TCs were clustered near the cyclonic circulation of the wave pattern during the genesis, development, and propagation. On the other hand, the wave pattern and TCs were weak and poorly organized in the ISO easterly phase. The distinct characteristics between the westerly and easterly phases could be attributed to the ISO modulation on the monsoon trough and the subtropical anticyclonic ridge. The ISO in the westerly phase provided a favorable background (e.g., enhanced monsoon trough and moisture confluent zone) for the wave-TC pattern development, while the ISO in the easterly phase provided a less favorable environment.

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 À1 , 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.

Propagation and maintenance mechanisms of the tropical cyclone/submonthly wave pattern in the western North Pacific are explored. The wave pattern exhibited an equivalent barotropic structure with maximum vorticity and kinetic energy in the lower troposphere and propagated northwestward in the Philippine Sea in the intraseasonal oscillation (ISO) westerly phase and north-northeastward near the East Asian coast in the easterly phase. The mean flow advection played a dominant role in the propagation in both phases.Barotropic energy conversion is the dominant process in maintaining the kinetic energy of the pattern. The wave pattern tended to occur in the confluent zone between the monsoon trough and the anticyclonic ridge, where the kinetic energy could be efficiently extracted from the westerly mean flow associated with the monsoon trough. The individual circulation circuit embedded in the pattern was oriented northeastsouthwest (east-west) to have optimal growth and propagation during the ISO westerly (easterly) phase.When tropical cyclones (TCs) developed in a development-favorable background flow provided by the submonthly wave pattern, they in turn enhanced the amplitudes of the vorticity and kinetic energy of the submonthly wave pattern by more than 50% and helped extract significantly more energy from the background ISO circulation. This TC feedback was much more significant in the ISO westerly phase because of the stronger clustering effect on TCs by the enhanced monsoon trough.

This study introduces a modified Pacific–Japan (PJ) index that exhibits a substantial periodicity of 5–16 days in the East Asian summer monsoon region. The quasi-periodic fluctuations of the PJ index can indicate changes in the large-scale circulation systems. In the PJ high phase, the wave pattern propagates northwestward from the western North Pacific tropics to an area near northern Luzon and is then forced to move westward because of a stationary, anomalous high pressure system over southern Japan. The tropical cyclones (TCs) associated with the anomalous low pressure systems tend to follow a straight-moving propagation route through the northern South China Sea. The anomalous cyclonic flow causes heavy rainfall in eastern Taiwan. However, in the PJ low phase, the wave pattern and TCs follow a recurving propagation route toward higher latitudes. The circulation pattern typically brings heavy rainfall to northern Taiwan in the PJ low phase. Therefore, wave patterns under the influence of the quasi-periodic fluctuations of the PJ pattern affect rainfall because of the changing propagation routes of the wave patterns, as well as the TC tracks.

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