Rossby Wave Scales, Propagation, and the Variability of Eddy-Driven Jets

The eddy-driven jet is located in the midlatitudes, bounded on one side by the pole and often bounded on the opposite side by a strong Hadley-driven jet. This work explores how the eddy-driven jet and its variability persist within these limits. It is demonstrated in a barotropic model that as the jet is located at higher latitudes, the eddy length scale increases as predicted by spherical Rossby wave theory, and the leading mode of variability of the jet changes from a meridional shift to a pulse. Looking equatorward, a similar change in eddy-driven jet variability is observed when it is moved equatorward toward a constant subtropical jet. In both the poleward and equatorward limits, the change in variability from a shift to a pulse is due to the modulation of eddy propagation and momentum flux. Near the pole, the small value of beta (the meridional gradient of absolute vorticity) and subsequent lack of wave breaking near the pole account for the change in variability, whereas on the equatorward side of the jet the strong subtropical winds can affect eddy propagation and restrict the movement of the eddy-driven jet or cause bimodal behavior of the jet latitude. Barotropic quasilinear theory thus suggests that the leading mode of zonal-wind variability will transition from a shift to a pulse as the eddy-driven jets move poleward with climate change, and that the eddy length scale will increase as the jet moves poleward.