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Dennis L. Hartmann

Abstract

It is demonstrated that realistic zonal mean states on a sphere are unstable to perturbations of zonal wavenumbers 2 and 3. Moreover, these waves have significant growth rates and are of such a narrow meridional scale that they fall into the class of unstable modes first discovered by Charney. For zonal mean wind cross sections characteristic of the Southern Hemisphere, wavenumber 2 has an unstable mode with an e-folding time of about 9 days and a period of about 2 weeks. The growth rate of the unstable wave 2 is relatively insensitive to changes in wind structure and to topography representative of the Antarctic continent. The unstable waves extend well into the stratosphere, and it is suggested that the eastward traveling wave 2 and wave 3 components observed in the Southern Hemisphere may be baroclinically unstable modes of the Charney type. The relatively large growth rates of these waves are made possibly by a relatively narrow meridional scale in the troposphere. and their propagation into the stratosphere is enhanced because their meridional scale increases with increasing height in the stratosphere by a factor of 2 or more over its value in the troposphere.

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Dennis L. Hartmann

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Dennis L. Hartmann

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An attempt is made to relate tracer transport in the stratosphere to the requirements of atmospheric dynamics. The relationships are illustrated by considering the observed transport and budget of potential vorticity in the Southern Hemisphere. Eddy potential vorticity transports generally occur in association with upward fluxes of energy by the pressure-interaction mechanism. During the period studied, these transports were associated with significant but not major changes in the zonal mean flow. The eddy tracer transports inferred from the potential vorticity transport are consistent with the transports necessary to account for the distribution of atmospheric ozone. Dynamical considerations suggest that mixing length formulations for the mass transport by planetary waves are not strictly valid.

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Dennis L. Hartmann

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The use of satellite data of constructing synoptic analyses is appropriate in the stratosphere where the most important disturbances are very large scale and slowly moving. Nimbus 5 data are applied to a study of the structure of the stratosphere in the Southern Hemisphere during the period 1 July to 6 September, 1973. The eddy amplitudes and transports are highly variable in time and are greatest during active periods which occur approximately once a month. The structure of the waves identified by zonal Fourier analysis is also variable in time and is related to the stages of growth and decay of the wave amplitude. The characteristic scales of the disturbances are determined by space-time cross-spectral analysis. In the stratosphere the circulation is dominated by wavenumbers 1 and 2 which have time scales in excess of two weeks. Wavenumber 1 is primarily stationary and oscillates in time, while wavenumber 2 travels eastward. Also apparent is a wavenumber 3 component which travels eastward with a period of about 6 days and has its maximum amplitude in the mid-stratosphere.

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Dennis L. Hartmann

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Dennis L. Hartmann

Abstract

The effect of varying ozone column density on radiative-photochemical relaxation is examined. It is shown that neglecting variations in the total number of ozone molecules between a particular level and the sun can result in a spurious 50% increase in the damping rate of temperature perturbations. The relaxation rate of ozone perturbations, on the other hand, is decreased by as much as 50% if variations in extinction are neglected. The distribution and magnitude of the effect of varying column density on radiative-photochemical relaxation depends on elevation, latitude and season.

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Dennis L. Hartmann

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It is suggested here that the zonal flow vacillation of the Southern Hemisphere is caused by a mutual interaction between the barotropic shear of the zonal flow and the evolution of baroclinic eddies during the later stages of their lifecycle. An index of the zonal wind anomaly difference between 40° and 60°S is defined for a 13-yr record of European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. Periods are selected during which the zonal flow is at the extremes of its characteristic vacillation, which are distinguished by a broad westerly jet with minor maxima near 30° and 60°S and a narrower jet peaked near 40°S. The narrower 40°S jet has stronger cyclonic shear across the baroclinic zone. These changes are accompanied by shifts in poleward eddy momentum flux on the 500-hPa surface and potential vorticity fluxes on the theta surfaces that lie in the midtroposphere, which are both consistent with the maintenance of the zonal wind anomalies against friction. Series of synoptic maps of potential vorticity (PV) on isentropic surfaces suggest a very different evolution of the lifecycles of baroclinic waves in the two extreme composites. In the broad jet configuration, the vortex (region of high PV) is confined to polar latitudes, and cyclones that develop are relatively quickly distorted by anticyclonic zonal wind curvature. In the narrow midlatitude jet case, the vortex is expanded and cyclones develop within the region of cyclonic zonal wind curvature within the vortex. These cyclones tend to have a more isotropic shape and maintain their identity longer. In the later stages of their development they tend to roll downstream in the cyclonic curvature of the zonal flow and contribute to the maintenance of the expanded vortex structure. The different evolution of cyclones during the later stages of their lifecycles reinforces the zonal flow anomalies in such a way as to provide the mechanism for the vacillation. The different cyclone lifecycles found in the observations here correspond to differences found in idealized numerical experiments when similar differences in initial zonal wind structure are imposed.

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Dennis L. Hartmann

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A series of idealized nonlinear life cycle experiments is performed to compare changes in life cycle behavior caused by upper-level and near-surface meridional shear of the initial zonal wind. It is shown that both the eddy kinetic energy and the zonal flow accelerations produced during a cycle of growth and equilibration respond primarily to the meridional shear of the zonal wind near the surface and only weakly to shear near the tropopause. Near the critical shear for transition from anticyclonic to cyclonic life cycle behavior, the zonal flow accelerations are minimized and the eddy persistence is maximized. Above this critical shear, eddy breaking on the poleward side of the jet increases and strong cyclonic zonal wind shears are generated. The influence of baroclinic shears is minimized by using dipolar wind anomalies that are zero near the center of a basic baroclinic jet and by taking advantage of the fact that the life cycle response is very sensitive to small changes in the magnitude of initial meridional shear. The small baroclinic shears contribute to the differences in the sense that a cyclonic shear decreasing with height is slightly more effective in inducing cyclonic behavior than is a barotropic cyclonic shear with the same surface value. Upper-tropospheric eddy momentum fluxes by linear normal modes are also much more sensitive to lower-tropospheric meridional shear than to upper-tropospheric meridional shear. The primary response of normal modes to lower-tropospheric meridional shear is to change the momentum flux at all levels.

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Dennis L. Hartmann

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An eigenvalue analysis of the nondivergent barotropic model on a sphere and an initial value analysis of a baroclinic, quasi-geostrophic model on a sphere are used to study barotropically unstable zonal flows similar to those observed in the wintertime stratosphere. The unstable modes fall into two categories. Polar modes associated with regions of negative potential vorticity gradient on the polar flank of the stratospheric westerly jet are most unstable for zonal wavenumbers 1 and 2 which have significant growth rates and have periods on the order of 3–4 days and 1.5–2 days, respectively. Thew polar modes correspond to a wave observed by Venne and Stanford (1982). Mid-latitude modes associated with regions of negative potential vorticity gradient on the equatorward flank of the stratospheric westerly jet have much longer periods. For wind profiles near marginal stability the most unstable modes occur for the lowest zonal wavenumbers 1–3 and have periods which are on the order of a week or more for wavenumbers 1 and 2. It is suggested that these instabilities may interact strongly with planetary waves propagating upward from the troposphere producing an additional in situ source of energy for these waves.

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Dennis L. Hartmann

Abstract

The method of time series analysis is used to study rawinsonde data in middle latitudes. The spectra reveal baroclinically unstable traveling waves with a preferred period of four to five days and a large amount of variance at lower frequencies. The preferred period of the longer-period waves is roughly 30 days, although the character of the disturbances in this range is highly variable. For the waves of periods between three and seven days, phase relations are derived for the northward component of the wind, the temperature, and the height of constant pressure surfaces. The phase relations are characteristic of traveling baroclinic waves and possess reasonable statistical significance. No stable phases were obtained for the eastward component of the wind or for the longer-period oscillations.

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