The wave spectrum and zonal-mean-flow maintenance in different flow regimes of the jet stream are studied using a two-layer modified quasigeostrophic (QG) model. As the wave energy is increased by varying the model parameters, the flow transitions from a subtropical jet regime to a merged jet regime and then to an eddy-driven jet regime. The subtropical jet is maintained at the Hadley cell edge by zonal-mean advection of momentum, while eddy heat flux and eddy momentum flux convergence (EMFC) are weak and concentrated far poleward. The merged jet is narrow and persistent and is maintained by EMFC from a narrow wave spectrum, dominated by zonal wavenumber 5. The eddy-driven jet is wide and fluctuating and is maintained by EMFC from a wide wave spectrum. The wave–mean flow feedback mechanisms that maintain each regime are explained qualitatively. The regime transitions are related to transitions in the wave spectrum. An analysis of the wave energy spectrum budget and a comparison with a quasi-linear version of the model show that the balance maintaining the spectrum in the merged and subtropical jet regimes is mainly a quasi-linear balance, whereas in the eddy-driven jet regime nonlinear inverse energy cascade takes place. The amplitude and wavenumber of the dominant wave mode in the merged and subtropical jet regimes are determined by the constraint that this mode would produce the wave fluxes necessary for maintaining a mean flow that is close to neutrality. In contrast, the equilibrated mean flow in the eddy-driven jet regime is weakly unstable.

An abrupt transition from a merged jet regime to a subtropical jet regime is analyzed using a two-layer modified quasigeostrophic (QG) spherical model. Unlike the common version of QG models, this model includes advection of the zonal mean momentum by the ageostrophic mean meridional circulation, allowing for a relatively realistic momentum balance in the tropics and subtropics. The merged jet is a single jet inside the Ferrel cell created by a merging of the subtropical and eddy-driven jets, and the subtropical jet is a mainly thermally driven jet at the Hadley cell edge. The maintenance of each type of jet depends on the dominant baroclinic modes. In the merged jet regime, the spectrum is dominated by intermediate-scale (wavenumbers 4-6) fast waves at the midlatitudes that grow close to the jet maximum. In the subtropical jet regime, the spectrum is dominated by long (wavenumbers 1-3) slow westward-propagating waves at high latitudes and somewhat weaker intermediate-scale slow waves at the midlatitudes. In the subtropical jet regime, waves equilibrate at weaker amplitudes than in the merged jet regime. A mechanism is found that explains why baroclinic instability is weaker in the subtropical jet regime, although the vertical shear of the mean flow is stronger, which has to do with the lower-level potential vorticity (PV) structure. The relevance of these results to the real atmosphere seams to hold in local zonal sections but not for the zonal mean.

Comprehensive climate models exhibit a large spread in the magnitude of projected poleward eddy‐driven jet shift in response to warming. The spread has been connected to the radiative response to warming. To understand how different radiative assumptions alone affect the jet shift in response to warming, we introduce a new clear‐sky longwave radiation hierarchy that spans idealized (gray versus four bands; without or with interactive water vapor) through comprehensive (correlated‐k) radiation in the same general circulation model. The new hierarchy is used in an aquaplanet configuration to explore the impact of radiation on the jet stream response to warming, independent of mean surface temperature and meridional surface temperature gradient responses. The gray radiation scheme produces a split jet and its eddy‐driven jet shifts equatorward as climate warms, whereas the storm track shifts equatorward then poleward. Including four longwave bands leads to a merged jet that shifts slightly poleward with warming, and the storm track shifts monotonically poleward. Including interactive water vapor makes the poleward jet shift comparable to the jet shift with comprehensive radiation and interactive water vapor. These jet and storm track differences are linked to the radiation response of the stratospheric temperature, the tropopause height, and the meridional gradient of the radiative forcing to warming. Dynamically, the equatorward jet shift with the gray scheme is consistent with reduced wave reflection on the poleward flank of the jet, whereas the poleward jet shift with the other schemes is consistent with increased eddy length scale that favors equatorward wave propagation.

Abstract. Atmospheric jet streams are typically separated into primarily “eddy-driven” (or polar-front) jets and primarily “thermally driven” (or subtropical) jets. Some regions also display “merged” jets, resulting from the (quasi-)collocation of the regions of eddy generation with the subtropical jet. The different locations and driving mechanisms of these jets arise from very different underlying mechanisms and result in very different jet characteristics. Here, we link the current understanding of dynamical jet maintenance mechanisms, mostly arising from conceptual or idealized models, to the phenomena observed in reanalysis data. We specifically focus on developing a unitary analysis framework grounded in dynamical systems theory, which may be applied to both idealized models and reanalysis, as well as allowing for direct intercomparison. Our results illustrate the effectiveness of dynamical systems indicators to diagnose jet regimes.

Coupled climate models project that extratropical storm tracks and eddy-driven jets generally shift poleward in response to increased CO2 concentration. Here the connection between the storm-track and jet responses to climate change is examined using the Eliassen–Palm (EP) relation. The EP relation states that the eddy potential energy flux is equal to the eddy momentum flux times the Doppler-shifted phase speed. The EP relation can be used to connect the storm-track and eddy-driven jet responses to climate change assuming 1) the storm-track and eddy potential energy flux responses are consistent and 2) the response of the Doppler-shifted phase speed is negligible. We examine the extent to which the EP relation connects the eddy-driven jet (eddy momentum flux convergence) response to climate change with the storm-track (eddy potential energy flux) response in two idealized aquaplanet model experiments. The two experiments, which differ in their radiation schemes, both show a poleward shift of the storm track in response to climate change. However, the eddy-driven jet shifts poleward using the sophisticated radiation scheme but equatorward using the gray radiation scheme. The EP relation gives a good approximation of the momentum flux response and the eddy-driven jet shift, given the eddy potential energy flux response, because the Doppler-shifted phase speed response is negligible. According to the EP relation, the opposite shift of the eddy-driven jet for the different radiation schemes is associated with dividing the eddy potential energy flux response by the climatological Doppler-shifted phase speed, which is dominated by the zonal-mean zonal wind.