Tropospheric jet variability in different flow regimes

A transition in the variability properties of the jet stream is studied using a two-layer quasi-geostrophic model. The eddy energy is increased by controlling two parameters that affect baroclinic instability, causing the jet to shift poleward and leading to a stronger and less persistent annular mode. The transition in the jet variability properties as the eddy energy increases is associated with a transition from a merged (subtropical and eddy-driven) jet regime to an eddy-driven jet regime, and is accompanied by a transition in the eddy spectral properties. In the merged jet regime the spectrum is dominated by a single synoptic-scale wave mode, whereas in the eddy-driven jet regime the spectrum includes a wider range of planetary- and synoptic-scale waves. An eddy–mean flow feedback mechanism explains the relation between the transition in the eddy spectrum and the jet variability properties. In the merged jet regime synoptic-scale waves maintain the jet close to its climatological position at all times and act as a positive feedback on the weak annular mode, increasing its persistence. In the eddy-driven jet regime the jet fluctuates between an equatorward-shifted state, which is similar to the merged jet, and a poleward-shifted state, where the eddy-driven jet is separated from the subtropical jet. The low persistence of the annular mode in the eddy-driven jet regime is due to a negative feedback by planetary-scale waves, which grow barotropically on the poleward flank of the jet during the equatorward-shifted jet state. The results of this study highlight the need to capture the dynamics of planetary-scale waves correctly in general circulation models in order to prevent the emergence of an overly persistent annular mode.