The Role of Wave Packets in Wave–Mean Flow Interactions during Southern Hemisphere Summer

Abstract

In this study, reanalysis data produced by the European Centre for Medium-Range Weather Forecasts for 14 Southern Hemisphere (SH) summer seasons have been analyzed. All cases of hemispheric transient eddy kinetic energy (TEKE) maxima have been identified, and the evolution of the local energetics and planetary-scale flow anomalies accompanying these TEKE growth/decay episodes are composited. The longitude?time evolution of the composite energetics shows the clear signature of a wave packet propagating eastward at a group velocity of about 27° longitude per day and undergoing a life cycle of growth and decay, with the energetics within a volume close to the wave packet center dominating the hemispheric mean energetics. When individual cases are examined, 52% are found to resemble the composite and have the energetics life cycle dominated by the evolution of a single wave packet, and an additional 21% are found to be dominated by the evolution of two wave packets having similar amplitudes. Only the remaining 27% can be regarded as having experienced TEKE growth and decay throughout much of the hemisphere. The zonal mean flow and eddy feedback anomalies (i.e., reduction in the meridional temperature gradient due to the effects of the eddy heat fluxes, as well as increase in the barotropic shear due to a narrowing of the midlatitude jet through the effects of the eddy momentum fluxes) associated with the cases dominated by the evolution of a single wave packet are also found to be dominated by anomalies close to the wave packet center. The fact that hemispheric wave growth/decay is often dominated by the evolution of a single wave packet has interesting dynamical consequences when the climatological basic flow is not zonally symmetric. When a wave packet propagates over regions of enhanced baroclinicity, it can extract more energy from the mean flow via baroclinic conversion, leading to its preferential growth. On the other hand, when a wave packet propagates over regions of weak baroclinicity, baroclinic conversion is suppressed; hence any packet growth must be due to other processes. By examining the location of wave packet peaks when hemispheric TEKE is at a maximum, it is observed that hemispheric mean TEKE peaks much more frequently when the dominant wave packet is located downstream of the region with strongest baroclinicity. In addition, the growth in TEKE for these cases is usually dominated by an increase in baroclinic conversion. In contrast, for the small number of cases in which the hemispheric mean TEKE maximum occurs when the dominant wave packet is located downstream of the region with weakest baroclinicity, the growth of the hemispheric TEKE is instead dominated by a reduction in barotropic dissipation.