Developing Wave Packets in the North Pacific Storm Track

Abstract

Developing wave packets in the western North Pacific storm track are diagnosed observationally. An abrupt upstream edge to baroclinic wave activity over the western North Pacific facilitates comparisons between the observational results and previous theoretical predictions on the spatiotemporal evolution of an impulse disturbance. Results show that surface cyclogenesis events are preceded by a sharply peaked wave packet that originates poleward of the Himalaya Plateau and develops rapidly across the North Pacific to North America. Composite wave-packet structure is broadly consistent with linear theory for idealized models such as Eady's. The longitude?height structure of the mature packet reveals deep growing waves with horizontal wavelengths of approximately 4000 km near the packet peak. Downstream from the peak, amplitude decays exponentially, and wavelength decreases approximately linearly to about 2500?3000 km at the leading edge. Meridional potential vorticity gradients are concentrated near the tropopause. In contrast to linear theory, the packets show an abrupt upstream edge and no evidence of upstream development. As the packet travels through the along-stream variations of the Pacific jet stream, the packet-peak and leading-edge group velocity vary. These accelerations change the packet length and suggest that the Pacific jet may act to focus the packets. A sample of North Atlantic storm track events reveals similar results and suggests that the Atlantic storm track is often seeded by wave packets that originate over the western North Pacific Ocean. In contrast, Atlantic packets refract equatorward and become trapped on the subtropical jet to the south of Himalaya Plateau, suggesting perhaps less potential for seeding disturbances in the Pacific storm track.