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- Author or Editor: J. R. Ziemke x
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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.
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.
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.
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.
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 (v̄*, w̄*), 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
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 (v̄†, w̄†) 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.
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 (v̄*, w̄*), 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
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 (v̄†, w̄†) 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.
