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- Author or Editor: Walter A. Robinson x
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Abstract
A baroclinic mechanism for the positive eddy feedback on the zonal index is proposed. The author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies. The transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation, but over time, the action of surface drag leads to enhanced baroclinicity at these latitudes. The steady-state response has positive baroclinicity at the latitude of eddy generation, if the eddies propagate away from this latitude before they dissipate. This reinforcement of the low-level baroclinicity provides a positive feedback, if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low-level baroclinicity. The proposed mechanism explains observed and modeled features of zonal index variations: the frequency and drag dependence of eddy feedback, the bandedness in latitude of zonal wind variations, and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time.
Abstract
A baroclinic mechanism for the positive eddy feedback on the zonal index is proposed. The author considers the quasigeostrophic response of the zonally averaged flow to forcing by baroclinic eddies. The transient response to eddy forcing is a largely barotropic acceleration of the westerly flow and a reduction of the baroclinicity at the latitudes of eddy generation, but over time, the action of surface drag leads to enhanced baroclinicity at these latitudes. The steady-state response has positive baroclinicity at the latitude of eddy generation, if the eddies propagate away from this latitude before they dissipate. This reinforcement of the low-level baroclinicity provides a positive feedback, if it is assumed that baroclinic eddies are generated more vigorously in regions of stronger low-level baroclinicity. The proposed mechanism explains observed and modeled features of zonal index variations: the frequency and drag dependence of eddy feedback, the bandedness in latitude of zonal wind variations, and the tendency for anomalies in the zonally averaged zonal wind to drift poleward over time.
Abstract
In this paper an atmospheric jet is considered self-maintaining if the overall effect of baroclinic eddies is to preserve or enhance its westerly shear with height. Observations suggest that the wintertime jets in Earth’s atmosphere are self-maintaining. This has implications for the intrinsic variability of these jets—the annular modes—and for how the extratropics respond to tropical warming.
The theory of quasigeostrophic eddy–zonal flow interactions is employed to determine how a jet can be self-maintaining. Whether or not a jet is self-maintaining is found to depend sensitively on the meridional distribution of the absorption of wave activity.
The eddy driving of the jet in a simple two-level model of the global circulation is examined. It is found that, with approximately wintertime settings of parameters (a radiative equilibrium equator–pole temperature contrast of 60 K), the midlatitude jets in this model are self-maintaining. The jet is not self-maintaining, however, when the radiative equilibrium equator-to-pole temperature contrast is reduced below a critical value (∼24 K temperature contrast). Eddy amplitudes are also greatly reduced, in this case. The transition to a self-maintaining jet, as the radiative equilibrium temperature contrast is increased, suggests a set of feedback mechanisms that involve the strength of the baroclinicity in the jet center and where baroclinic eddies are absorbed in the subtropics. A barotropic eastward force applied to the model Tropics causes a poleward shift in the latitudes of greatest eddy absorption and induces a transition from a non-self-maintaining to a self-maintaining jet.
Self-maintaining behavior ultimately disappears, as the equator–pole thermal contrast, and thus the eddies, are strengthened. The flow is then highly disturbed and no longer dominated by wavelike baroclinic eddies.
Abstract
In this paper an atmospheric jet is considered self-maintaining if the overall effect of baroclinic eddies is to preserve or enhance its westerly shear with height. Observations suggest that the wintertime jets in Earth’s atmosphere are self-maintaining. This has implications for the intrinsic variability of these jets—the annular modes—and for how the extratropics respond to tropical warming.
The theory of quasigeostrophic eddy–zonal flow interactions is employed to determine how a jet can be self-maintaining. Whether or not a jet is self-maintaining is found to depend sensitively on the meridional distribution of the absorption of wave activity.
The eddy driving of the jet in a simple two-level model of the global circulation is examined. It is found that, with approximately wintertime settings of parameters (a radiative equilibrium equator–pole temperature contrast of 60 K), the midlatitude jets in this model are self-maintaining. The jet is not self-maintaining, however, when the radiative equilibrium equator-to-pole temperature contrast is reduced below a critical value (∼24 K temperature contrast). Eddy amplitudes are also greatly reduced, in this case. The transition to a self-maintaining jet, as the radiative equilibrium temperature contrast is increased, suggests a set of feedback mechanisms that involve the strength of the baroclinicity in the jet center and where baroclinic eddies are absorbed in the subtropics. A barotropic eastward force applied to the model Tropics causes a poleward shift in the latitudes of greatest eddy absorption and induces a transition from a non-self-maintaining to a self-maintaining jet.
Self-maintaining behavior ultimately disappears, as the equator–pole thermal contrast, and thus the eddies, are strengthened. The flow is then highly disturbed and no longer dominated by wavelike baroclinic eddies.
Abstract
Zonally averaged flows in general circulation models exhibit strong sensitivity to the strength of the surface friction, subgrid-scale diffusion, and gravity wave drag. A commonly observed effect is that the midlatitude jets shift poleward as the drag on the zonal wind is decreased. In the present two-level primitive equation model the jet moves poleward with decreasing surface friction and with increasing subgrid-scale diffusion. The barotropic component of the jet shows much greater sensitivity than does the baroclinic component. Experiments using different values of friction for the eddies and for the zonal means reveal that the jet latitude is primarily controlled by the drag on the zonal means.
The shift in the latitude of the jet is derived from the altered equilibrium response of the zonal wind to forcing by eddies when the friction is changed and the change in meridional structure of the eddy momentum fluxes in response to the modified zonal wind. The latter effect is also displayed by linear baroclinic modes.
Abstract
Zonally averaged flows in general circulation models exhibit strong sensitivity to the strength of the surface friction, subgrid-scale diffusion, and gravity wave drag. A commonly observed effect is that the midlatitude jets shift poleward as the drag on the zonal wind is decreased. In the present two-level primitive equation model the jet moves poleward with decreasing surface friction and with increasing subgrid-scale diffusion. The barotropic component of the jet shows much greater sensitivity than does the baroclinic component. Experiments using different values of friction for the eddies and for the zonal means reveal that the jet latitude is primarily controlled by the drag on the zonal means.
The shift in the latitude of the jet is derived from the altered equilibrium response of the zonal wind to forcing by eddies when the friction is changed and the change in meridional structure of the eddy momentum fluxes in response to the modified zonal wind. The latter effect is also displayed by linear baroclinic modes.
Abstract
The invertibility of potential vorticity is used to investigate how flows in the middle atmosphere are determined. The distribution of potential vorticity associated with an observed flow is computed. The inversion problem, calculating the flow from the potential vorticity, is solved repeatedly, including and excluding different portions of the potential vorticity. This procedure reveals which bits of potential vorticity are important in determining the flow at a time and location of interest.
We consider the minor warming of January 1979, using data obtained by the Limb Infrared Monitor of the Stratosphere (LIMS). The results indicate that the middle stratospheric flow is dominated by potential vorticity that is local both in height and latitude during this period, while flows in both the lower stratosphere and mesosphere are less locally dominated. That portion of the flow responsible for wave-mean flow interactions in the middle stratosphere is also induced by potential vorticity that is nearly local in height.
Calculations of the downward influence of the rapid rearrangement of stratospheric potential vorticity during sudden warming episodes show induced tropospheric geopotential tendencies implying pressure increases over the pole of meteorologically significant magnitudes.
Abstract
The invertibility of potential vorticity is used to investigate how flows in the middle atmosphere are determined. The distribution of potential vorticity associated with an observed flow is computed. The inversion problem, calculating the flow from the potential vorticity, is solved repeatedly, including and excluding different portions of the potential vorticity. This procedure reveals which bits of potential vorticity are important in determining the flow at a time and location of interest.
We consider the minor warming of January 1979, using data obtained by the Limb Infrared Monitor of the Stratosphere (LIMS). The results indicate that the middle stratospheric flow is dominated by potential vorticity that is local both in height and latitude during this period, while flows in both the lower stratosphere and mesosphere are less locally dominated. That portion of the flow responsible for wave-mean flow interactions in the middle stratosphere is also induced by potential vorticity that is nearly local in height.
Calculations of the downward influence of the rapid rearrangement of stratospheric potential vorticity during sudden warming episodes show induced tropospheric geopotential tendencies implying pressure increases over the pole of meteorologically significant magnitudes.
Abstract
A wave-1 minor warming is simulated in a mechanistic, global, primitive equation model. The modification of the zonal flow by the rearrangement of potential vorticity on a middle stratospheric isentrope is compared in a fully nonlinear model and in a model with only one wave and the zonal flow (a quasi-linear model). The permanent rearrangement of potential vorticity during the wave episode is more intense and more localized meridionally in the fully nonlinear model, which is able to capture the process of planetary wave breaking in some detail.
Additional experiments reveal that the differences between the quasi-linear and nonlinear models persist for a broad range of wave amplitudes, and that the quasi-linear model can qualitatively reproduce the modification of the zonal flow by the wave when the diffusive dissipation of the wave is enhanced. These results are discussed in the context of the theory of barotropic Rossby waves in shear flows, and in comparison with recent numerical simulations of the middle atmosphere.
Abstract
A wave-1 minor warming is simulated in a mechanistic, global, primitive equation model. The modification of the zonal flow by the rearrangement of potential vorticity on a middle stratospheric isentrope is compared in a fully nonlinear model and in a model with only one wave and the zonal flow (a quasi-linear model). The permanent rearrangement of potential vorticity during the wave episode is more intense and more localized meridionally in the fully nonlinear model, which is able to capture the process of planetary wave breaking in some detail.
Additional experiments reveal that the differences between the quasi-linear and nonlinear models persist for a broad range of wave amplitudes, and that the quasi-linear model can qualitatively reproduce the modification of the zonal flow by the wave when the diffusive dissipation of the wave is enhanced. These results are discussed in the context of the theory of barotropic Rossby waves in shear flows, and in comparison with recent numerical simulations of the middle atmosphere.
Abstract
Reasoning in terms of potential vorticity provides physical explanations for the dissipative destabilization of external Rossby waves and for the acceleration of the zonal mean jet during baroclinic life cycles. Both explanations depend upon the equivalence between surface temperature and a sheet of potential vorticity located just above the surface.
Abstract
Reasoning in terms of potential vorticity provides physical explanations for the dissipative destabilization of external Rossby waves and for the acceleration of the zonal mean jet during baroclinic life cycles. Both explanations depend upon the equivalence between surface temperature and a sheet of potential vorticity located just above the surface.
Abstract
The response of linear planetary wave 2 to changes in the isentropic zonally symmetric distribution of potential vorticity (PV) is investigated numerically. Wave 2 is sensitive to the width and position of a region where the meridional derivative of PV is weak, denoted the “surf zone”, in the middle stratosphere. A narrow surf zone leads to an amplification of wave 2, and confines some of its Eliassen-Palm wave activity in high latitudes. For wider surf zones the wave activity is concentrated near the associated critical surface, and the amplitude of the wave decreases. Large changes in the wave amplitude and in the distribution of its activity are associated with the subtle changes in the zonal winds produced by modest modifications in the distribution of PV.
Basic states that include regions of reversed meridional gradients of PV lead to wave overreflection and strong poleward focusing of wave activity. The amplitude of wave 2 is enhanced in the presence of negative gradients, with large responses occurring for eastward traveling waves.
Abstract
The response of linear planetary wave 2 to changes in the isentropic zonally symmetric distribution of potential vorticity (PV) is investigated numerically. Wave 2 is sensitive to the width and position of a region where the meridional derivative of PV is weak, denoted the “surf zone”, in the middle stratosphere. A narrow surf zone leads to an amplification of wave 2, and confines some of its Eliassen-Palm wave activity in high latitudes. For wider surf zones the wave activity is concentrated near the associated critical surface, and the amplitude of the wave decreases. Large changes in the wave amplitude and in the distribution of its activity are associated with the subtle changes in the zonal winds produced by modest modifications in the distribution of PV.
Basic states that include regions of reversed meridional gradients of PV lead to wave overreflection and strong poleward focusing of wave activity. The amplitude of wave 2 is enhanced in the presence of negative gradients, with large responses occurring for eastward traveling waves.
Abstract
Interactions between stationary planetary waves are investigated in the context of severely truncated quasi-geostrophic dynamics in a midlatitude beta-channel. Such interactions are solely a consequence of dissipation and are mediated by waves with smaller meridional scales. Linear numerical experiments with wave 1 and 2 basic states indicate that wave 1 is amplified in a wave 2 basic state, while the behaviour of a wave 2 disturbance in a wave 1 basic state is sensitive to the relative phases of the waves. At some phases, wave 2 is confined below the region of law wave 1 amplitudes.
The dynamics of these interactions are diagnosed using the potential enstrophy budgets of the waves. These budgets are more sensitive to the wave-wave interactions than are the amplitudes of the waves themselves. Nonlinear experiments show behavior that is a combination of the linear results with an amplification of wave 1 and a strong dependence on the relative phases of waves 1 and 2 in the stratosphere.
Abstract
Interactions between stationary planetary waves are investigated in the context of severely truncated quasi-geostrophic dynamics in a midlatitude beta-channel. Such interactions are solely a consequence of dissipation and are mediated by waves with smaller meridional scales. Linear numerical experiments with wave 1 and 2 basic states indicate that wave 1 is amplified in a wave 2 basic state, while the behaviour of a wave 2 disturbance in a wave 1 basic state is sensitive to the relative phases of the waves. At some phases, wave 2 is confined below the region of law wave 1 amplitudes.
The dynamics of these interactions are diagnosed using the potential enstrophy budgets of the waves. These budgets are more sensitive to the wave-wave interactions than are the amplitudes of the waves themselves. Nonlinear experiments show behavior that is a combination of the linear results with an amplification of wave 1 and a strong dependence on the relative phases of waves 1 and 2 in the stratosphere.
Abstract
The quasi-geostrophic form of the Eliassen-Palm flux is calculated for steady linear planetary wave 1 in a realistic zonal flow with dissipation. This flux shows a region of divergence in the polar stratosphere that is similar to that found in time averages of observations. Comparison with the full primitive equation form of the Eliassen-Palm flux indicates that in the model this divergent region is spurious and results from the overestimation of the momentum flux divergence when it is calculated from geostrophic winds. An approximate expression for the Eliassen-Paim flux in terms of balanced winds is suggested as an alternative to the quasi-geostrophic form.
Abstract
The quasi-geostrophic form of the Eliassen-Palm flux is calculated for steady linear planetary wave 1 in a realistic zonal flow with dissipation. This flux shows a region of divergence in the polar stratosphere that is similar to that found in time averages of observations. Comparison with the full primitive equation form of the Eliassen-Palm flux indicates that in the model this divergent region is spurious and results from the overestimation of the momentum flux divergence when it is calculated from geostrophic winds. An approximate expression for the Eliassen-Paim flux in terms of balanced winds is suggested as an alternative to the quasi-geostrophic form.
