Influence of a local diabatic heating source on the midlatitude circulation

Diabatic heating due to latent heat release in the storm tracks plays an important but poorly understood role in the midlatitude circulation. To examine how localised diabatic heating affects the circulation, a dry model is used to apply midtropospheric perturbations to the radiative-equilibrium temperature profile to which the model is relaxed. This allows us to isolate the one-way causal influence of diabatic heating on the circulation without feedbacks of the circulation onto the heating. By applying switch-on perturbations at various latitudes, the mechanisms by which an equilibrated state is reached are examined. In all cases, the equilibrated circulation exhibits a dynamically insensitive jet structure with eddies maintaining a region of self-concentrated baroclinicity, latitudinally shifted away from the heating perturbation. However, the initial response is generally very different. For heating poleward of the jet, there is an initial thermal-wind adjustment and weakened eddy heat fluxes, followed later by weakened eddy momentum fluxes that maintain an equilibrated equatorward jet shift. For heating equatorward of the jet, the evolution is more complex with an initial strengthening of the eddy heat fluxes that dominantly balance the imposed heating, as well as a weakening of the Hadley cell. Further, the initial thermal-wind adjustment immediately modifies the subtropical critical lines and eddy momentum fluxes that encourage an (Formula presented.) 15° poleward propagation from the Subtropics to Midlatitudes, yielding an equilibrated poleward jet shift. The overall determining factor in balancing the imposed heating at equilibration is the local climatological heat budget: if the climatological eddy heat fluxes locally warm, such as in mid-to-high latitudes, then this effect weakens, whereas if the climatological overturning circulation locally warms, such as in the Subtropics, then this effect weakens. The insensitivity of the equilibrated jet profile to the imposed heating is remarkable given the maintenance mechanisms vary drastically according to heating latitude.