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William J. Randel

Abstract

Observational characteristics of the 2-day wave, a westward-propagating zonal wave 3 oscillation in the summer subtropical upper stratosphere and mesosphere, are studied based on five years of National Meteorological Center (NMC) operational stratospheric analyses. These data show episodic occurrence of the 2-day wave in the upper stratosphere in January (centered near 20°S) and July–August (centered near 20°N). These episodes are strongly correlated with observed reversals of the zonal mean potential vorticity gradient near the core of the summer easterly jet, consistent with previous suggestions that the 2-day wave is generated by an in situ instability of this jet. On the other hand, the horizontal and vertical structure of the waves is very similar to that calculated by Salby for a global normal-mode Rossby wave. The combination of normal-mode structure and instability signature suggests that the 2-day wave is a near-resonant mode forced by dynamical instability.

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William J. Randel

Abstract

Eight years of Solar Backscatter Ultraviolet ozone data are examined to study zonal mean ozone variations associated with stratospheric planetary wave (warming) events. These fluctuations are found to he nearly global in extent, with relatively large variations in the tropics, and coherent signatures reaching up to 50° in the opposite (summer) hemisphere. These ozone variations are a manifestation of the global circulation cells associated with stratospheric warming events; the ozone responds dynamically in the lower stratosphere to transport, and pholochemically in the upper stratosphere to the circulation-induced temperature changes. The observed ozone variations in the tropics are of particular interest because transport is dominated by zonal-mean vertical motions (eddy flux divergences and mean meridional transport are negligible), and hence, substantial simplifications to the governing equations occur. The response of the atmosphere to these impulsive circulation changes provides a situation for robust estimates of the ozone-temperature sensitivity in the upper stratosphere.

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William J. Randel

Abstract

Westward-propagating Rossby normal-mode planetary waves are documented in stratospheric ozone data using Solar Backscatter Ultraviolet (SBUV) satellite measurements. These modes are evidenced by enhanced spectral power and near-global coherence for westward-traveling zonal wave 1 oscillations with periods of 5–10 days. The ozone waves have maxima in high latitudes of the middle stratosphere (due to transport) and over midlatitudes in the upper stratosphere (due to photochemistry). These modes are nearly continuous throughout the eight years of SBUV observations, with maximum global coherence during the equinoxes. The upper-stratospheric waves are symmetric (in phase) between hemispheres, even for modes previously identified as antisymmetric in geopotential height. This behavior is due to differing wave vertical structure in each hemisphere: the planetary temperature waves are nearly in phase in the upper stratosphere, even thogh the height waves are out of phase. The observed ozone waves are furthermore compared to calculations based on linear wave transport and photochemistry, incorporating derived wind and temperature fields. Good agreement is found, showing that normal modes provide an idealized context to study the linear wave behavior of trace constituents in the real atmosphere.

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William J. Randel

Abstract

Spatial structure and temporal evolution of synoptic time scale variations of the tropospheric zonal mean flow are studied by extensive cross-correlation analyses, and the degree to which observations agree with two-dimensional, adiabatic theory is determined. Observational data from seven years of operational daily global analyses are studied, along with data from the NCAR general circulation model.

Observed zonal mean zonal wind and temperature tendencies are compared with analyzed adiabatic forcing terms. Although significant correlations are found throughout the extratropics, there are significant equation residuals in both operational analyses and model data. Model momentum residuals result from calculational inaccuracies (interpolation to pressure surfaces and spectral aliasing of nonlinear terms) and biases introduced by once daily sampling; diabatic terms are also important for the daily thermodynamic balance.

Coherent wave-zonal mean flow interactions are revealed via cross-correlation analyses, including fluctuations in zonal mean temperature, three-dimensional winds, and quadratic wave quantities. Equatorward propagating wavelike patterns in the meridional plane are observed for both zonal wind and temperature tendencies. These patterns result from midlatitude baroclinic-wave life cycles, and the signatures associated with wave growth and decay are revealed with novel detail. New observed features of baroclinic wave life cycles shown here include coherent fluctuations of the extratropical mean meridional circulation (Ferrel cells), and equatorward propagation of midlatitude wave activity (Rossby-wave radiation) as far as the equator.

An individual case study is presented to show the variability associated with a particular event.

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William J. Randel

Abstract

Planetary wave propagation in the southern winter troposphere and stratosphere is studied in an attempt to trace the origins of upward propagating disturbance. Daily geopotential girds from 1000 to 1 mb are analyzed for two 120-day winter seasons. A cross-correlation analysis technique is developed which allows coherent wave structure to be traced in time. Significant correlations are observed between the troposphere and stratosphere at finite time lags, indicative of vertically propagating waves. The observed vertical propagation time scales between the middle troposphere and middle stratosphere are on the order of 4 days for zonal wavenumber 1 (k=1), 1–2 days for k=2, and 1 day for k=3.

The cross-correlation analysis also delineates the meridional and vertical structures of the transient (in time) planetary waves. Zonal wavenumber 1 fluctuations exhibit a vertical out-of-phase relationship between the midlatitude troposphere and atmosphere. Three out-of-phase maxima in latitude are observed in the troposphere, separated by 25–30° latitude, whereas a singe broad latitudinal maximum is found in the stratosphere. Wavenumbers 2 and 3 exhibit similar overall structures, quite distinct from that of k=1. Two out-of-phase maxima in latitude are observed in the troposphere, separated by 30–35° latitude, and the stratospheric variance is found to be coherent and in phase with that in the high latitude troposphere.

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William J. Randel

Abstract

Several methods of obtaining horizontal wind fields in the extratropical stratosphere from geopotential height data are evaluated and compared to geostrophic estimates, with focus on the poleward fluxes of momentum and heat and on the resulting Eliassen–Palm (EP) flux divergence estimates. Winds derived from a coupled iterative solution of the zonal and meridional momentum equations (“balance” winds) are proposed and tested, in addition to winds derived from linearizing these equations about the zonal mean flow (“linen” winds). Comparison of the different analysis methods are made for a general circulation model simulation of the Northern Hemisphere (NH) winter stratosphere, and for NH and Southern Hemisphere (SH) winter observational data.

The balance and linear wind estimates of poleward momentum flux are similar and substantially smaller than geostrophic values in the high-latitude stratosphere; neglect of local curvature effects is the primary cause of the geostrophic overestimate. The relative errors are larger in the southern winter stratosphere due to the stronger polar night jet. Poleward beat flux estimates are not substantially changed. Use of the improved wind fluxes results in a sizable reduction in the EP flux divergence in the high-latitude stratosphere.

Comparison with model winds suggests that the balance method is the superior analysis technique for evaluating local winds, particularly in the NH winter where local nonlinear effects can be important. Based on observed balance winds, estimates are made of the relative importance of rotational versus divergent motions in the winter stratosphere.

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William J. Randel
and
John L. Stanford

Abstract

Recently reported medium-scale wave dominance of the Southern Hemisphere summer circulation is studied using NMC geopotential height fields for the 1978–79 summer. These features, corroborated by independent analyses of satellite microwave measurements, are apparent in meridional thermal winds derived from the NMC grids.

In the time mean, we observe strong medium-scale waves which extend throughout the troposphere and lower stratosphere, in agreement with Kalnay et al. (1981). These zonally asymmetric features attain a maximum in low latitudes, exhibiting an equivalent barotropic vertical structure with maximum amplitude near the tropopause.

A longitudinal phase versus time plot from daily analyses of zonal wavenumbers 4–7 (which contain the majority of the time variance) reveals periodic variations in both phase and amplitude: wave 5 frequently dominates, exhibiting eastward phase progression with period near 10 days. During other times, shorter scale waves (waves 6–7) exhibit enhanced amplitudes south and east of Africa, showing considerably faster eastward movement. Waves 6 and 7 both show remarkably regular eastward movement throughout the 90 day record, with periods near 5 and 4 days, respectively. The traveling waves exhibit maximum amplitude near the tropospheric jet core, often with an equivalent barotropic vertical structure.

The amplitude of the medium-scale waves is observed to vary with approximately the same time scale as the period of their phase progression (10–15 days). The zonal wind exhibits fluctuations of amplitude and time scale which suggest that the medium-scale waves may grow at the expense of the zonal mean kinetic energy. Episodes of latitudinal phase structure indicative of barotropic energy exchange with the mean wind field are observed. An exceptionally clear case of stationary-transient wave interference is observed.

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William J. Randel
and
John L. Stanford

Abstract

Medium-scale waves (zonal wavenumbers 4–7) frequently dominate Southern Hemisphere (SH) summer midlatitude circulation patterns This work is an observational study that focuses on their temporal and spatial characteristics, along with detailing the forcing mechanisms responsible for their formation.

Medium-scale wave characteristics for three SH summers (1978/79 through 1980/81) are discussed. It is shown that the time-mean medium-scale wave structure is consistent with the basic state linear wave propagation characteristics. The energetics of the medium-scale waves are studied using the transformed Eulerian-mean formalism of Plumb. It is found that wave-zonal mean exchange is a valid concept for describing the SH summer atmospheric circulation, and that the flow vacillates between periods of highly perturbed and zonally symmetric states, with a time wale on the order of 10–20 days. These vacillations result from nonlinear baroclinic instabilities, and the medium-scale waves exhibit well-defined life cycles of baroclinic growth, maturity, and barotropic decay.

The observational characteristics of the medium-scale waves are discussed in terms or Northern Hemisphere observational studies, modeled baroclinic waves, and laboratory annulus experiments. It is argued that the zonal symmetry of the SH summer atmosphere is responsible for many of the observed wave characteristics.

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William J. Randel
and
John L. Stanford

Abstract

Eastward moving, baroclinically forced, medium-scale waves are frequently observed to dominate the Southern Hemisphere summer circulation. In addition, strong quasi-stationary medium-scale waves were also observed during the summer of 1978/1979. In this paper we present the results of an observational study for several weeks of this time period, during which the stationary and transient waves are found to exhibit clear linear interference characteristics. Energetic analyses indicate that the medium-scale waves grow barotropically and decay baroclinically during this period, although these interference induced contributions are secondary to the usual baroclinic growth-barotropic decay life cycle characteristics observed by Randel and Stanford. Data analyses and simple model calculations are presented which demonstrate that the observed baroclinic decay results from equatorward heat flux associated with the differing vertical structures of the stationary and transient waves. An interference induced feedback mechanism between the medium-scale waves and the zonal-mean flow is discussed.

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William J. Randel
and
John L. Stanford

Abstract

Medium-scale waves (zonal wavenumbers 4–7) frequently dominate Southern Hemisphere summer circulation patterns. Randel and Stanford have studied the dynamics of these features, demonstrating that the medium-scale waves result from baroclinic excitation and exhibit well-defined life cycles. This study details the evolution of the medium-scale waves during a particular life cycle. The specific case chosen exhibits a high degree of zonal symmetry, prompting study based upon zonally averaged diagnostics. An analysis of the medium-scale wave energetics reveals a well-defined life cycle of baroclinic growth, maturity, and barotropic decay. Eliassen-Palm flux diagrams detail the daily wave structure and its interaction with the zonally-averged flow.

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