HENRIK AUESTAD

Abstract:

<jats:title>Abstract</jats:title><jats:p>Midlatitude storms transport warm and moist air poleward and upward, releasing latent heat. Latent heating is thus organized by the circulation but then modifies temperature gradients and winds, constituting a nonlinear feedback. We define the latent heating feedback as the effects that arise from latent heating being coupled with the circulation. Because of its nonlinearity, the climatic effects of this feedback are difficult to isolate and remain poorly understood. By decoupling latent heating from the circulation in an atmospheric general circulation model, we show that the latent heating feedback enhances storm track eddy diffusivity, modifying eddy heat fluxes beyond changes in mean baroclinicity. Simultaneously, tracked storms occur at lower latitudes, intensify more, and propagate further poleward, while the subtropical jet strengthens as coupled latent heating preserves lower latitude baroclinicity. The feedback response supports the idea that diabatic effects cause the “too zonal, too equatorward” storm track biases in climate models.</jats:p>

Abstract:

Latent heating modifies the jet stream by modifying the vertical geostrophic wind shear, thereby altering the potential for baroclinic development. Hence, correctly representing diabatic effects is important for modelling the mid-latitude atmospheric circulation and variability. However, the direct effects of diabatic heating remain poorly understood. For example, there is no consensus on the effect of latent heating on the cross-jet temperature contrast. We show that this disagreement is attributable to the choice of spatio-temporal averaging. Jet representations relying on averaged wind tend to have the strongest latent heating on the cold flank of the jet, thus weakening the cross-jet temperature contrast. In contrast, jet representations reflecting the two-dimensional instantaneous wind field have the strongest latent heating on the warm flank of the jet. Furthermore, we show that latent heating primarily occurs on the warm flank of poleward directed instantaneous jets, which is the case for all storm tracks and seasons.