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Abstract

The equator-to-pole temperature gradient has traditionally been understood as the primary driver of the midlatitude stormtracks, which derive their kinetic energy in the process of transporting sensible heat down the gradient. Latent heat, however, accounts for an estimated 30-60\% of the meridional energy transport, a portion which is likely to increase in a warmer world. The contribution of latent heat to the energetics is complicated in that it is inefficient: only a fraction of the transported latent heat is converted into kinetic energy. Currently, there is no complete theory to explain the relationship between meridional energy transport and kinetic energy generation by midlatitudes eddies. We use a two-layer moist quasi-geostrophic model to develop a theory of how the energetic output of the midlatitude atmosphere depends on the relative humidity structure. By varying the surface evaporation rate, we show that the system only reaches the maximum possible energetic output in the saturated limit, producing substantially less kinetic energy at lower evaporation rates. We quantify this reduction in kinetic energy production in terms of a moist conversion efficiency. Using a Moist Energetic framework, we identify that precipitation dissipation and the diffusion of moisture in subsaturated regions account for the reduction in energetic output. We then show that the moist conversion efficiency can be diagnosed from the distribution of humidity.