Tropospheric jet variability in different flow regimes

Abstract

AbstractA 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.