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J. R. Ziemke and J. L. Stanford

Abstract

Stratospheric disturbances on the 35–60 day time scale are investigated with particular emphasis on the Southern Hemisphere. The data used are stratospheric brightness temperatures from 90 to 1.5 hPa covering seven 6-month southern winter periods (from April 1980 to March 1987).

Global time-lag correlation plots are constructed from which tropical/extratropical connections, three-dimensional wave structure, and propagation characteristics are studied. Horizontal correlation patterns at 90 hPa reveal a strong connection between the Indonesian tropics and the winter extratropics. Vertical correlation patterns in the southern winter extratropics reveal westward tilt with height and vertical propagation of 35–60 day zonal wavenumber 1 perturbations, from tropospheric regions up to great heights, at least as high as the stratopause region. Disturbances are found to propagate from 90 hPa to 1.5 hPa in generally 6 to 9 days. In contrast, the vertical correlation plots in the tropics indicate little or no vertical propagation or westward tilt with height.

Statistical coherence studies provide supporting evidence that the observed extratropical disturbances may result from tropical forcing, with the ensuing connection favoring a period close to 50 days.

The observations are in qualitative agreement with model calculations featuring a tropical forcing and suggest, at least for the disturbances captured by the broad vertical weighting functions of the satellite instruments used, that the low frequency disturbances move out of the tropical latitudes before propagating vertically to the stratopause region.

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J. R. Ziemke and J. L. Stanford

Abstract

Careful spectral, correlation and coherence analyses of low-frequency fluctuations in global geopotential height data are presented. Attention is paid to proper statistical assessments. The main points are:

1) one-to-two month oscillating quasi-stationary wavetrains have recently been reported in the extratropical Southern Hemisphere troposphere, as far south as the edge of Antarctica. However, only weak correlations were observed with the supposed tropical forcing region, leading to the question of whether the wavetrain is a response to tropical forcing or possibly due to in situ instabilities on 1–2 month time scales. The present paper clears up this enigma with analyses of other tropical datasets which reveal clear correlation between low latitude source regions and the SH extratropical troposphere.

2) An earlier investigation found strong correlations between 1–2 month oscillations in the upper stratosphere and tropical troposphere, yet no vertical propagation was found directly above the tropics. This is explained in the present work with evidence of temperature fluctuations propagating initially quasi-horizontally towards higher latitudes from the Indonesian tropical troposphere, along the bottom of the tropopause to near 35°S. At this latitude, stratospheric winter westerlies allow vertical propagation of the 1–2 month perturbations up to the middle stratosphere where the wavetrain arches equatorward and upward to the stratopause.

3) Finally, Eliassen–Palm flux diagnostics for the SH stratosphere reveal that while the 1–2 month perturbations occasionally cause significant forcing of the zonal mean wind Ū, on the long term average only about 10% of ∂Ū/∂t can be attributed directly to these low-frequency eddies.

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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

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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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

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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X. H. Gao and J. L. Stanford

Abstract

No abstract available.

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X. H. Gao and J. L. Stanford

Abstract

Equatorial low-frequency oscillations with periods of 1–2 months are being intensively studied by many investigators. A strong equatorial “dipole” pattern is observed in which atmospheric variables such as temperate, wind, and pressure are out of phase between the Indian Ocean-Indonesia region and the western Pacific. While it is generally thought that the oscillations make a complete circuit around the earth from their excitation region in the equatorial Indian Ocean-western Pacific, the signal is more difficult to observe over the South America-Atlantic-Africa sector. Using analyses of four year of satellite-derived microwave radiance data, evidence is presented here for the possibility of a feedback route in the Southern Hemisphere. The observed propagation path extends from the central equatorial Pacific across lower South America and heads equatorward after passing south of Africa. The response route finally reenters the equatorial Indian Ocean with the correct phase to enhance the primary equatorial dipole structure. The Southern Hemisphere propagation path migrates northward in April-September and southward in October-March. The largest correlation with the equatorial dateline region occurs at the turning point of the feedback route, in the South Atlantic. This propagation path appears to constitute a feedback mechanism which could aid in stabilizing the low frequency oscillations through positive feedback.

The correlations are shown to be statistically significant by several methods, including Monte Carlo simulations.

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J. L. Stanford, J. R. Ziemke, and S. Y. Gao

Abstract

Stratospheric circulation is investigated by further analyses of three years of Stratospheric And Mesospheric Sounder (SAMS) data. Eddy effects on constituent transport are investigated with two transport formulations the transformed Eulerian mean formulation and the effective transport formulation. The transformed Eulerian mean formulation, together with calculated residual mean winds (*, *), is used to delineate regions, or times, of significant eddy contributions to constituent transport. Significant regions are found in the stratosphere in all seasons, not only in the Northern Hemisphere (NH) winter high latitudes (where contributions from nonlinear and nonsteady perturbations in sudden and final warming events are expected), but also in the midlatitude, middle stratosphere in autumn and at winter-summer low latitudes near the stratopause. Questionably large calculated ρ0−1Δ · M magnitudes near the stratopause during solstice suggest that the use of residual winds calculated here may be inadequate for describing mass transport at these heights. The effective transport formulation is used, involving individual calculations of the effective-transport velocity (†, †) for both CH4 and N2O. With CH4, the tracer continuity equation is diagnosed term-by-term for January, April, July, and October months. Both gases reveal a descent in the tropics during northern spring (associated with a “double peak” in mixing ratio measured along latitude) and upwelling in the summer tropics of the stratosphere during solstice. The NH summer upwelling appears to cause larger CH4 increases than corresponding Southern Hemisphere (SH) summer tropical upwelling.

The effective transport calculations involving CH4 show that in July most local change in mixing ratio in the upper stratosphere is attributable to mean advection. In the upper stratosphere and lower mesosphere during NH summer-autumn, pulses of enhanced mixing ratio (related to an apparent coupling of semiannual and annual circulation components) propagate from low northern latitudes into both hemispheres. The behavior is similar to that predicted by diabatic models, but under equinox conditions. The calculated (†, †) fields for June–July exhibit this cellular feature for both CH4 and N2O. In the extratropics at stratopause heights, particularly in the SH, large local amplitudes of the semiannual component may be attributable in part to this mean meridional advection from summer to winter hemisphere. Each year, both CH4 and N2O show features consistent with significant NH autumnal vertical and meridional transport at high latitudes in the lower mesosphere.

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Elizabeth M. Stone, William J. Randel, and John L. Stanford

Abstract

The transport of passive tracers in idealized baroclinic wave life cycles is studied using output from the National Center for Atmospheric Research Community Climate Model (CCM2). Two life cycles, LCn and LCs, are simulated, starting with baroclinically unstable initial conditions similar to those used by Thorncroft et al. in their study of two life cycle paradigms. The two life cycles LCn and LCs have different initial horizontal wind shear structures that result in distinctive nonlinear development. In terms of potential vorticity–potential temperature (PV–theta) diagnostics, the LCn case is characterized by thinning troughs that are advected anticyclonically and equatorward, while the LCs case has broadening troughs that wrap up cyclonically and poleward.

Four idealized passive tracers are included in the model to be advected by the semi-Lagrangian transport scheme of the CCM2, and their evolutions are investigated throughout the life cycles. Tracer budgets are analyzed in terms of the transformed Eulerian mean constituent transport formalism in pressure coordinates and also in isentropic coordinates. Results for both LCn and LCs show transport that is downgradient with respect to the background structure of the tracer field, but with a characteristic spatial structure that maximizes in the middle to high latitudes. For the idealized tropospheric tracers in this study, this represents a net upward and poleward transport that enhances concentrations at high latitudes. These results vary little with the initial distribution of the constituent field. The time tendency of the tracer is influenced most strongly by the eddy flux term, with the largest transport occurring during the nonlinear growth stage of the life cycle. The authors also study the transport of a lower-stratospheric tracer, to examine stratosphere–troposphere exchange for baroclinic waves.

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