The Life Cycle of a Cyclone Wave in the Southern Hemisphere. Part I: Eddy Energy Budget

Abstract

The energetics of a Southern Hemisphere cyclone wave have been analyzed using ECMWF data and the results of a limited-area model simulation. An analysis of the energy budget for a storm that developed in the eastern Pacific on 4?6 September 1987 showed the advection of the geopotential height field by the ageostrophic wind to be both a significant source and the primary sink of eddy kinetic energy. Air flowing through the wave gained kinetic energy via this term as it approached the energy maximum and then lost it upon exiting. Energy removal by diffusion, friction, and Reynolds stresses was found to be small. The most important conclusion was that, while the wave grew initially by poleward advection of heat as expected from baroclinic theory, the system evolved only up to the point where this source of eddy energy and the conversion of eddy potential to eddy kinetic energy (typically denoted ??α?) was compensated for by energy flux divergence (dispersion of energy), mainly of the ageostrophic geopotential flux, va?. Energy exported in this fashion was then available for the downstream development of a secondary system. This finding seems to differ from the results of studies of the life cycle of normal-mode-type waves in zonal flows, which have been shown to decay primarily through transfer of energy to the mean flow via Reynolds stresses. However, this apparent inconsistency can be explained by the fact that while ageostrophic geopotential fluxes can also be very large in the case of individual normal modes, the waves export energy downstream at exactly the same rate as they gain from upstream. The group velocity of the 4?6 September storm, calculated from the ageostrophic geopotential height fluxes, showed that the energy packet comprising the system had an eastward group velocity slightly larger than the time-mean flow.