The Structure of Baroclinic Wave Packets

Abstract

In this paper, the three-dimensional structure of baroclinic wave packets is studied using composites from ECMWF reanalysis data. Eddy variances and covariances associated with wave packets are examined in terms of the eddy energy budget and wave activity and fluxes, and the large-scale, phase-independent flow component associated with wave packets is studied using composites of the low zonal wavenumber component of the flow. The results suggest that the sources of upper-level eddy energy?wave activity associated with the wave packets are located over the central and upstream portion of the wave packet, and the downstream part of the wave packet is entirely maintained by radiation of energy?wave activity fluxes from the upstream end. The results are entirely consistent with previous studies of the energetics of individual wave evolution cases. The composites also show that the basic flow response to eddy transports associated with the wave packets consists mainly of a poleward shift of the jet near the center of the wave packet. One of the goals of this study is to find out how the structure of wave packets is maintained against the linear tendency of dispersion. In the 10 Southern Hemisphere summer seasons examined, between two and six coherent wave packets that basically maintain their structure for at least 7 days are found for each season. However, previous hypotheses that the upstream ends of wave packets are stabilized by enhanced barotropic dissipation due to the formation of a barotropic jet, or that wave packets are maintained by nonlinear self-focusing of wave activity, are not supported by the data analyses. The results presented here suggest that alternative mechanisms will be needed to explain the zonal confinement of wave packets against the linear tendency of lateral spreading.