Effect of latitude on the variability of eddy-driven jets

The effect of latitude on eddy-driven jet variability is investigated by stirring the non-divergent vorticity equation on the sphere. Jet variability is shown to be more persistent at equatorward latitudes compared to poleward latitudes, and the leading pattern of variability changes from a shift to a pulse as the jet is located closer to the pole. Both effects are shown to be due to the sphericity of the earth, which influences eddy propagation and inhibits wavebreaking on the poleward flank of the jet, decreasing the positive feedback between the eddies and the mean flow. Similar eddy propagation arguments can explain the behaviour of the eddy-driven jet in the presence of a strong subtropical jet. These results can help us understand why the low-phase NAO is more persistent than the high-phase and why AR4 models with jets too far equatorward relative to observations over-predict the timescale of Southern Annular Mode events.