Mixed Rossby–Gravity Waves and Western Pacific Tropical Cyclogenesis. Part I: Synoptic Evolution

A large-amplitude mixed Rossby–gravity wave packet is identified in the western Pacific using 6–10-day bandpass-filtered winds. Individual disturbances of 2300–3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden–Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east. West of 1508E, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby–gravity waves to off-equatorial tropical depression (TD)type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby–gravity waves remained outside the circulation centers, and three tropical cyclones formed on the edges of three consecutive cyclonic gyres as they moved off the equator. Although the expected Rossby–Kelvin wave structure was present in the background winds within the active MJO, tropical cyclone genesis did not occur within the trailing Rossby gyres, but 2500 km to the west and north. This case study provides evidence that equatorial modes, under the right conditions, can supply precursor disturbances for repeated formation of tropical cyclones. It is argued based on previous work in the literature that this sequence of events is not uncommon.