Influence of diabatic heating on the maintenance of the midlatitude jet

Diabatic heating due to latent heat release in the storm tracks plays an important but poorly understood role in the maintenance of the zonal-mean midlatitude circulation. To examine how the midlatitude circulation is maintained in the presence of diabatic heating on either side of the jet, a dry model is used to apply mid-tropospheric perturbations to the radiative equilibrium temperature profile to which the model is relaxed, constituting an intermediate step between an externally-imposed diabatic heating and a setup that allows for full diabatic feedbacks. By applying transient 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-stable structure where the eddies maintain a region of concentrated baroclinicity, latitudinally shifted away from the region of the heating perturbation. However, the initial response is generally very different. When the heating is poleward of the jet, there is an initial thermal-wind adjustment to the heating and weakened eddy heat fluxes, followed later by weakened eddy momentum fluxes aloft that maintain an equilibrated equatorward shift of the jet. Conversely, when the heating is equatorward of the jet, the evolution is more complex with an initial strengthening of the eddy heat fluxes and a weakening of the Hadley cell to balance the heating, in addition to a thermal-wind adjustment that immediately modifies the critical latitudes and thus, the eddy momentum fluxes. These momentum-flux changes encourage a poleward propagation of the anomalies from the subtropics to midlatitudes over 40 days, where they straddle the jet and ultimately yield an equilibrated poleward jet shift. When the heating is at the jet core, the circulation simply weakens rather than exhibiting any latitudinal shift. These mechanisms of self-concentration of the baroclinicity are discussed and compared with previously proposed mechanisms of jet self-maintenance.