The Relationship between the Speed and the Latitude of an Eddy-Driven Jet in a Stirred Barotropic Model

A stirred barotropic model on a sphere is used to investigate the relationship between the speed and the latitude of an eddy-driven jet. When the wind speed is increased in the model, the jet shifts poleward, despite the fact that the stirring of vorticity remains statistically constant. The cause is found to be increasing meridional shear that results from increasing the wind speed in a meridionally confined region and reduces the absolute vorticity gradient on the flanks of the jet. This has two related consequences. The first is that wave propagation is discouraged, as a turning latitude is created where the absolute vorticity gradient tends to zero. On the sphere, this occurs first at high latitudes, thereby shifting wave dissipation toward the equator. The reduced high-latitude dissipation causes a poleward shift of the jet. The second consequence occurs when the vorticity gradient actually becomes negative, in which case the waves may overreflect where an instability is present, providing a high-latitude source of pseudomomentum. This may further encourage the jet to shift poleward. The relevance of the barotropic dynamics to more realistic atmospheres is unclear, but the intermodel variability of the poleward shift of the jet in response to increasing CO2 across a suite of state-of-the-art GCMs is consistent with the barotropic dynamics, suggesting that further investigation is warranted.