An Observational Study of the Energy-Transfer between the Seasonal Mean Flow and Transient Eddies

Abstract

The large-scale energetics of the Northern Hemisphere atmospheric motions are computed in the frequency domain using a 5-year data set from the ECMWF operational analyses. The geographical distributions of kinetic energy (KE) for the fast transient (periods shorter than 10 days) assume the shape of elongated bands indicating the structure of the storm tracks. The slow transients (periods longer than 10 days) exhibit local maxima of KE over the eastern regions of the major oceans. The slow transient available potential energy (APE), on the other hand, displays maxima over the American and Asian continents. Both baroclinic and nonlinear conversions are responsible for the maintenance of low-frequency disturbances when hemispherically-integrated quantities are considered. Low-frequency disturbances over the land are maintained primarily through baroclinic energy conversions, while those over the oceans are maintained primarily through a barotropic energy transfer from the seasonal mean flow and through a nonlinear energy transfer from high frequency eddies. With the flow separated in terms of three frequency "bands", namely, the seasonal mean, the low- and the high-frequency eddies, the energy cycle is a rather simple one. During summer, (i) the APE flows from lower to higher frequency bands, (ii) the baroclinic conversion transforms APE to KE for all frequency bands, and finally, (iii) KE flows from higher to lower frequencies (including the time-mean flow). For the winter season, the energy flows in the same direction, except for one transfer, the exchange of KE between the time-mean flow and the slow transients. The direction is from the time-mean flow to the slow transients, which is presumably due to the time-mean motions being barotropically unstable during the winter.