Developing wave packets in the North Pacific storm track

An important property of observed storm tracks is the downstream extension of wave activity beyond regions of maximum baroclinicity. This extension occurs due to downstream development, where the growth of new cyclones is augmented by a flux of wave activity from an upstream cyclone. Furthermore, downstream development is often organized within a larger-scale coherent wave packet. Previous studies have shown the ubiquitous nature of downstream development, and climatological properties of observed wave packets have been well documented. Here we study the origin and evolution of observed wave packets based on a large sample population for a particular location. Effort is directed toward testing predictions from theoretical and numerical-modeling studies for the structure and propagation of the evolving packet.

The North Pacific storm track is selected for study since cyclogenesis events occur frequently in this location, and there exists a sharply defined edge to cyclogenesis due to the extensive upstream Eurasian land mass. This configuration provides conditions analogous to those assumed for idealized wave packet theories for the evolution of an isolated impulse. Based on a large sample of cases, we identify the disturbances that trigger the wave packets, as well as dominant rays that define the leading edge, trailing edge, and peak of the expanding wave packets. The results show that the packets are highly asymmetric and dominated by the downstream half; this result is qualitatively similar to the nonlinear packet behavior shown by Swanson and Pierrehumbert. An important practical consequence associated with the excitation of these packets is the potential influence on North American and European weather in the short and medium range, respectively. Results based on the sample population suggest that this potential often may not be realized due to a significant loss of wave activity to the subtropics.