Initial Zonal Flow Research Articles

The Predictability of Stratospheric Warming Events: More from the Troposphere or the Stratosphere?

Abstract The roles of the stratosphere and the troposphere in determining the predictability of stratospheric final warming and sudden warming events are evaluated in an idealized atmospheric model. For each stratospheric warming event simulated in the model, a number of forecast experiments are performed from 10 or 20 days prior to the warming onset with perturbations in the troposphere and in the stratosphere separately. It is found that the stratosphere affects predictions of warming onset primarily by providing the initial state of the zonal winds, while the tropospheric initial conditions have a large impact through the generation and propagation of planetary waves. These results correspond to the roles played by the initial zonal flow and the evolution of eddy forcings in a zonally symmetric model. The initial stratospheric zonal flow has some influence on stratospheric wave driving, but in most cases this does not significantly affect the timing of the warming, except when the initial condition is close to the onset date. These results highlight the role of the troposphere in determining stratospheric planetary wave driving and support the importance of tropospheric precursors to the stratospheric warming events.

Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions

Abstract Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere–NAO connection.

Self‐limiting feedback between baroclinic waves and a NAO‐like sheared zonal flow

The eddy‐mean flow interaction associated with the North Atlantic Oscillation (NAO) is examined by using the baroclinic wave life cycle experiments. When a sheared zonal flow perturbation akin to the NAO‐related dipole wind anomaly is added to the basic state, momentum fluxes due to baroclinic waves tend to reinforce the initial zonal flow dipole in the upper troposphere both for the anticyclonic and cyclonic shears. The eddy feedback is stronger for the anticyclonic shear because the node of the dipole flow is asymmetric about the basic jet, suggesting that the positive NAO is more favored by the eddy feedback. For the zonal wind anomaly with extremely large amplitude, the baroclinic wave breaking cannot efficiently intensify the zonal flow dipole, indicating a self‐limitation in the positive eddy feedback to the NAO‐like zonal flow anomaly.

Barotropic Decay of Baroclinic Waves in a Two-Layer Beta-Plane Model

Abstract A two-layer quasi-geostrophic model is used to study the effects of a meridionally sheared zonal flow on the life cycle of a weakly unstable baroclinic wave. In most of the cases analyzed, the fluid is inviscid with the exception of scale-selective fourth-order horizontal diffusion. The initial zonal flow is identically zero in the lower layer. The character of the eddy life cycle in the limit of weak supercritically is shown to depend on whether or not the meridional shell in the upper layer is strong enough to produce a critical latitude for the wave. If the shear is sufficiently weak, the wave undergoes periodic amplitude vacillation characterized by symmetric growth and baroclinic decay. However, when the meridional shear is strong enough to allow for the existence of a critical layer, the flow undergoes an asymmetric life cycle which resembles that found by Simmons and Hoskins in a primitive equation model on the sphere: the wave grows baroclinically but decays barotropically toward a wave-f...

Zonal winds near Venus' cloud top level: A model study of the interaction between the zonal mean circulation and the semidiurnal tide

A primitive equation wave-mean flow interaction model, designed by J. R. Holton and used originally to study Earth's middle atmosphere, has been adapted to Venus in order to clarify our understanding of the interaction between the semidiurnal tide and the thermally driven mean meridional circulation near the cloud top level. With or without the tide the model produces midlatitude jets whose structure is insensitive to vertical shear of the background angular velocity above and below the cloud top level, but it is sensitive to background angular velocity at the cloud top level. When this background angular velocity is close to that of Venus, the latitudes and speeds of these jets are similar to the latitudes and speeds of jets at the Venus cloud top level as inferred from observed temperatures and the cyclostrophic balance condition. In agreement with the hypothesis of Fels and Lindzen, the model tide accelerates the equatorial zonal wind near the cloud top level and decelerates it at higher levels. The tidal vertical wavelength, maximum amplitude, dissipative decay length, and zonal flow accelerations are sensitive functions of background angular velocity, in agreement with elementary gravity wave theory. In the equatorial cloud top region, tidal acceleration is comparable in magnitude to the decelerative effects of vertical advection and the model's Rayleigh friction damping. For sufficiently rapid initial zonal flow near the cloud top level, the area-weighted global mean cloud top level zonal wind increases with time over a 50-day model run as a result of tidal acceleration. Agreement between the model tide and the observed tide, or the tide determined in the more detailed calculations of Pechmann and Ingersoll, is best when the background angular velocity at the jet level is about 30% larger than that observed.

An Investigation of the Equatorial Orographic–Dynamic Mechanism

A barotropic model over an equatorial beta-plane is used to investigate the response when a uniform zonal current crosses an isolated hypothetical mountain centered at the equator. The bounded derivative initialization method is applied to suppress gravity-inertia motions. A quasi-stationary trough is generated at the top of the orography. Its intensity is greater in the easterly case than in the westerly case of the initial zonal current. A diagnostic equation is derived to interpret this difference in the solution. The quasi-stationary orographic divergence field which is positive on the windward slope and negative on the leeward slope (for both easterly and westerly cases of the initial zonal current) is responsible for maintaining the trough. The difference in the intensity of the trough is due to the contribution of the beta-u effect, which changes sign with the direction of the initial zonal flow.

A Comparison of Observed and Simulated Properties of Sudden Stratospheric Warmings

Review of observational data and dynamical numerical simulations of stratospheric warmings. Classes of warmings, major and minor (major if poleward movement of planetary-scale thermal systems entails reversal of polar circulation at 10 mb or below), trajectories of warm cells, vertical and horizontal scale of warm-air systems, the time-scale of warming, initial zonal flow conditions prior to a warming, circulation reversals, and details of the energy budget before and after a warming are discussed. The 1963 and 1973 types of warmings are contrasted: the strong baroclinic conversion of eddy potential to eddy kinetic energy was not repeated in the latter, but both events were preceded by very large fluxes from the troposphere. Numerical model simulations by various authors are compared and evaluated.

A numerical model of the sudden stratospheric warming mechanism

A simple numerical model is developed to study the planetary wave and zonal flow interaction that is thought to be responsible for a sudden stratospheric warming. The wave is generated by switch-on forcing at the base of an isothermal atmosphere and propagates vertically into an initial zonal flow profile that subsequently evolves in time in response to vertically structured horizontal heat flux by the wave. The principal model assumptions are β plane geometry and neglect of latitudinal dependence of the evolving zonal flow in the wave equation. Results indicate that the mechanism responsible for the warming event in a numerical treatment by Matsuno that is not based on these simplifying assumptions is contained in this model. Integrations for different values of model parameters show the dependence of certain features of the warming on dissipation and on the nature, amplitude, and duration of the wave forcing. We then use the model to analyze in detail the behavior of oscillating potential vorticity transport arising from linear superposition of the forced wave and transient free modes and establish this as the mechanism that triggers the nonlinear feedback that produces the warming event. The amplitude of this oscillation is the decisive factor in whether or not a warming occurs. A further set of model runs shows the dependence of the time it takes for a warming to occur on the initial zonal flow profile. These show that resonant forcing of the wave, occurring when the initial zonal flow configuration is such that one of the free modes is stationary, is not particularly favorable for the production of a warming.

A numerical integration of a two-level quasi-geostrophic model with variable Rossby parameter

Circulation patterns were obtained from a two-level quasi-geostrophic model, in which the Rossby parameter varied with latitude. The initial basic state was characterized by a constant zonal flow at both reference level. The results we re compared with the pattern obtained in a series of experiments with a similar model, in which the Rossby parameter was assumed to be constant. the initial zonal flow in these experiments was assumed in one case to be constant with latitude, in the second casse to have a symmetrical jet centered on the central latitude, and in the thrid case to have its maximum in the southern half of the region. the results of the experiment with a variable Rossby parameter have the closest resemblance to the patterns observed in the real atmospehre. In particular, during the mature stages of the run, the distribution of the zonallyaveraged eddytransport of momentum corresponded quite well to patterns actually observed in the atmosphere.