In strictly two-dimensional geostrophic turbulence, it is known that energy cascades upscale as enstrophy cascades downscale. In a baroclinic atmosphere, such a constraint does not appear to exist. Yet the pheonomenon of "upscale energy cascade" appears to be present in numerical models and in observational data, where it is often found that although the "energy injection scale" is at around wavenumbers 11-12, the peak in transient energy and heat flux occurs upscale, near wavenumbers 5-6. It has been assumed that the pathway of energy transfer is through nonlinear wave-wave interaction, akin to the upscale-energy cascade in two-dimensional geostrophic turbulence. We will show, using model simulation and observation, that the energetics in a baroclinic atmosphere are very different from those of a barotropic atmosphere, and that the dominant energy pathway is quasilinear. Wave-wave transfer, although exists, is not the primary source of energy for the longer waves. Nonlinearity serves only the secondary role of saturating the waves at the "energy injection scale", thus allowing the longer waves to extract energy from the potential energy of the mean flow. The resulting conceptual picture for quasilinear and nonlinear energy transfers is actually much simpler in the baroclinic case than that of the two-dimensional turbulence.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics