Variability of eddy-driven jets and their interaction with the subtropical Hadley-driven jet

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 leading mode of variability of the jet changes from a meridional shift to a pulse, and the eddy-length scale increases, consistent with the Rhines-scale prediction. Looking equatorward, a similar change in eddy-driven jet variability is observed when a constant subtropical jet is moved toward the eddy-driven jet in midlatitudes. 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 and subsequent lack of wavebreaking near the pole accounts for the change in variability, whereas on the equatorward side of the jet, the strong subtropical winds affect eddy propagation and actually force the midlatitude jet poleward. This work suggests that the expansion of the Hadley cell may be enough to cause the eddy-driven jets to shift poleward with climate change, and that the leading mode of zonal-wind variability will transition from a shift to a pulse as the jet moves poleward.