A Dynamical Interpretation of the Poleward Shift of the Jet Streams in Global Warming Scenarios

The role played by enhanced upper-tropospheric baroclinicity in the poleward shift of the jet streams in global warming scenarios is investigated. Major differences between the twentieth-and twenty-first-century simulations are first detailed using two coupled climate model outputs. There is a poleward shift of the eddy-driven jets, an increase in intensity and poleward shift of the storm tracks, a strengthening of the upper-tropospheric baroclinicity, and an increase in the eddy length scale. These properties are more obvious in the Southern Hemisphere. A strengthening of the poleward eddy momentum fluxes and a relative decrease in frequency of cyclonic wave breaking compared to anticyclonic wave breaking events is also observed. Then, baroclinic instability in the three-level quasigeostrophic model is studied analytically and offers a simple explanation for the increased eddy spatial scale. It is shown that if the potential vorticity gradient changes its sign below the midlevel (i.e., if the critical level is located in the lower troposphere as in the real atmosphere), long and short wavelengths become respectively more and less unstable when the upper-tropospheric baroclinicity is increased. Finally, a simple dry atmospheric general circulation model (GCM) is used to confirm the key role played by the upper-level baroclinicity by employing a normal-mode approach and long-term simulations forced by a temperature relaxation. The eddy length scale is shown to largely determine the nature of the breaking: long (short) wavelengths break more anticyclonically (cyclonically). When the upper-tropospheric baroclinicity is reinforced, long wavelengths become more unstable, break more strongly anticyclonically, and push the jet more poleward. Short wavelengths being less unstable, they are less efficient in pushing the jet equatorward. This provides an interpretation for the increased poleward eddy momentum fluxes and thus the poleward shift of the eddy-driven jets.