The impact of wave packets propagating across Asia on Pacific cyclone development

In this paper, ECMWF 40-yr reanalysis data have been examined to study the influence of upper-level wave packets propagating across Asia into the Pacific on surface cyclone development over the Pacific. Previous studies have shown that in winter, wave packets propagate across Asia over two branches-a northern branch over Siberia and a southern branch along the subtropical jet across southern Asia. Results presented here show that subsequent to the presence of wave packets on either branch, the frequency of occurrence of deep cyclones (defined as cyclones with central pressure below 960 hPa), as well as explosively deepening cyclones (those with a deepening rate of 1 Bergeron or more), are significantly enhanced. This enhancement also clearly follows the wave packet eastward as it propagates across the Pacific. Wave packets from the two branches are found to interfere with each other, such that if wave packets of the appropriate configuration are present on both the northern and southern branch, subsequent surface cyclone development over the western Pacific is further enhanced. Examination of the evolution of the anomalies suggests that these interferences can largely be explained by linear superposition of wave packets from the two branches. Examination of the evolution of the composite structure of wave packets that are followed by the development of a significant surface cyclone indicates that cyclones that develop as the northern packet propagates into the Pacific are phase locked with the upper-level trough and maintain a favorable westward tilt with height throughout their development, consistent with the hypothesis that cyclogenesis is triggered by the approach of the wave packet. In contrast, significant cyclones whose development are influenced by the southern packets initially develop west of the upper-level trough, and propagate eastward with a phase speed that is much faster than that of the upper-level trough, attaining a westward phase tilt with height only at the mature stage, suggesting that cyclogenesis for these cases is probably not triggered by the wave packet.