On the self-maintenance of midlatitude jets

In this paper an atmospheric jet is considered self-maintaining if the overall effect of baroclinic eddies is to preserve or enhance its westerly shear with height. Observations suggest that the wintertime jets in Earth's atmosphere are self-maintaining. This has implications for the intrinsic variability of these jets-the annular modes-and for how the extratropics respond to tropical warming. The theory of quasigeostrophic eddy-zonal flow interactions is employed to determine how a jet can be self-maintaining. Whether or not a jet is self-maintaining is found to depend sensitively on the meridional distribution of the absorption of wave activity. The eddy driving of the jet in a simple two-level model of the global circulation is examined. It is found that, with approximately wintertime settings of parameters ( a radiative equilibrium equator-pole temperature contrast of 60 K), the midlatitude jets in this model are self-maintaining. The jet is not self-maintaining, however, when the radiative equilibrium equator-to-pole temperature contrast is reduced below a critical value ( similar to 24 K temperature contrast). Eddy amplitudes are also greatly reduced, in this case. The transition to a self-maintaining jet, as the radiative equilibrium temperature contrast is increased, suggests a set of feedback mechanisms that involve the strength of the baroclinicity in the jet center and where baroclinic eddies are absorbed in the subtropics. A barotropic eastward force applied to the model Tropics causes a poleward shift in the latitudes of greatest eddy absorption and induces a transition from a non-self-maintaining to a self-maintaining jet. Self-maintaining behavior ultimately disappears, as the equator-pole thermal contrast, and thus the eddies, are strengthened. The flow is then highly disturbed and no longer dominated by wavelike baroclinic eddies.