Baroclinic Adjustment and Frictional Control of Storm Tracks

The response of storm tracks introduces some of the largest uncertainties in climate model outputs, due to the complex and nonlinear underlying dynamics. To provide more insight into some of these nonlinear dynamics of storm tracks, this work studies the steady-state response of an idealised storm track to a large-scale extratropical thermal forcing and eddy dissipation in the context of a dry general circulation model.

A nonlinearity is found between the relative responses of the mean baroclinicity and baroclinic eddy fluxes, whereby mean baroclinicity responds more rapidly to eddy dissipation whereas baroclinic eddy fluxes respond more rapidly to the mean thermal forcing. The latter response is equivalent to the phenomena of baroclinic adjustment and eddy saturation, observed in the atmosphere and oceans, respectively. In such cases, forcing of the mean baroclinicity is counteracted by enhanced eddy intensity which reduces meridional temperature gradients and increases static stability. The response to eddy dissipation, on the other hand, is less well known. In this case changes in the eddy dissipation (either mechanical or thermal) are balanced by changes in the eddy growth rate.

The above nonlinearity in storm track responses is predicted by a two-dimensional model that was originally developed to emulate temporal nonlinear oscillations in baroclinicity and baroclinic eddy intensity on sub-seasonal timescales. Such nonlinear oscillatory relationship arises from a predator-prey relationship between baroclinic eddy intensity and baroclinicity. Despite the above nonlinearity in storm track responses, the associated barotropic jet shifts are of similar order of magnitude demonstrating that changes in baroclinicity can be as effective in changing the sign of the barotropic eddy momentum fluxes as the changes in the baroclinic eddy intensity.