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M. Bithell, L. J. Gray, J. E. Harries, J. M. Russell III, and A. F. Tuck

Abstract

The degree to which the Southern Hemisphere polar vortex is isolated against horizontal (isentropic) mixing is investigated using data from the Halogen Occulation Experiment (HALOE), U.K. Meteorological Office (UKMO) potential vorticity (PV), and contour advection diagnostics. Measurements of methane and water vapor taken by HALOE during a disturbed period in the Southern Hemisphere springtime (21 September–15 October 1992) are interpreted in light of the prevailing synoptic meteorology. Daily fields of winds and PV are shown to be essential in the interpretation of the data. A climatological high pressure region is responsible for a distorted vortex, and a substantial “vortex stripping” event is present, associated with the early stages of vortex breakdown. This leads to significant temporal, zonal, and altitudinal variations in the distribution of tracers. The authors point out the difficulties this presents for the interpretation of solar occultation data, especially with regard to the use of zonal average time series. Longitude-height methane distributions from two days during the period are examined. Both days show substantial variations in abundance around a latitude circle. In particular, the authors investigate HALOE measurements at 77°S on 15 October 1992, which indicate an abundance of methane in the height region 600–2000 K (∼30-l mb) that is more typical of midlatitude air. Similar distributions, observed in the 1991 HALOE data, have previously been interpreted as evidence for the penetration of midlatitude air into the vortex. Gradients of potential vorticity and contour advection diagnostics are employed to examine whether the UKMO winds are consistent with this hypothesis in 1992. Although midlatitude air is able to penetrate poleward of the main jet core by advection processes alone, an essentially intact inner core of vortex air remains, which does not mix to any great extent with air from lower latitudes. The authors show that the high-latitude HALOE abundances that are typical of midlatitude air were observed in a region of extensive filamentation and mixing, rather than within the inner, more isolated, core.

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Xun Zhu, Jeng-Hwa Yee, E. R. Talaat, M. Mlynczak, and J. M. Russell III

Abstract

For migrating tides or fast-moving planetary waves, polarization relations derived from the linear wave equations are required to accurately derive the wind components from the temperature field. A common problem in diagnosing winds from the measured temperature is the error amplification associated with apparent singularities in the wave polarization relations. The authors have developed a spectral module that accurately derives tidal winds from the measured tidal temperature field and effectively eliminates the error amplification near the apparent singularities. The algorithm is used to perform a diagnostic analysis of tidal winds and the Eliassen–Palm (EP) flux divergence in the mesosphere and lower thermosphere (MLT) based on the zonal mean and tidal temperature fields derived from 6 yr of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite. The derived zonal mean wind and diurnal tidal amplitude reveal new insights into the mesospheric biennial oscillation (MBO) that exists in the MLT at both equatorial and midlatitude regions. The equatorial MBO in the zonal mean wind is present in the entire mesosphere from 50 to 90 km. The equatorial MBO in the temperature amplitude of the diurnal tide occurs near the mesopause region between 80 and 90 km and is largely coincident with the downward phase propagation of the equatorial MBO in the zonal mean wind, indicating a possible mechanism of wave–mean flow interaction between the two. On the other hand, the newly discovered midlatitude MBOs in zonal mean wind and the meridional wind in diurnal tide occur at different altitudes, suggesting possibly a remote forcing–response relationship. The acceleration or deceleration of the zonal mean wind due to EP flux divergence that is contributed by the migrating tides peaks at midlatitudes with a typical value of 10–20 m s−1 day−1 around 95 km.

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A. Gettelman, W. J. Randel, S. Massie, F. Wu, W. G. Read, and J. M. Russell III

Abstract

The interannual variability of the tropical tropopause region between 14 and 18 km is examined using observations of convection, winds, and tropopause temperatures from reanalyses and water vapor from satellites. This variability is compared to a simulation using the Community Climate Model version 3 (CCM3) general circulation model forced by observed sea surface temperatures. A coherent picture of the effect of the El Niño–Southern Oscillation (ENSO) on the tropopause region is presented in the NCEP–NCAR reanalyses and CCM3. ENSO modifies convection in the Tropics, and the temperature and circulation of the tropical tropopause region, in agreement with idealized models of tropical heating. CCM3 reproduces most details of these changes, but not the zonal mean temperature variations present in the analysis fields, which are not related to ENSO. ENSO also forces significant changes in observed and simulated water vapor fields. In the upper troposphere water vapor is at maximum near convection, while in the tropopause region water vapor is at minimum in the regions of convection and surrounding it. Convection, cirrus clouds, temperatures, and transport are all linked to describe the water vapor distribution and highlight the role of transport in the tropopause region.

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William J. Randel, Fei Wu, James M. Russell III, Aidan Roche, and Joe W. Waters

Abstract

Measurements of stratospheric methane (CH4) and water vapor (H2O) are used to investigate seasonal and interannual variability in stratospheric transport. Data are from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) spanning 1991–97. Profile measurements are binned according to analyzed potential vorticity fields (equivalent latitude mapping), and seasonal cycles are fit using harmonic regression analysis. Methane data from the UARS Cryogenic Limb Array Etalon Spectrometer and water vapor from the Microwave Limb Sounder are also used to fill in winter polar latitudes (where HALOE measurements are unavailable), yielding complete global seasonal cycles. These data reveal well-known seasonal variations with novel detail, including 1) the presence of enhanced latitudinal gradients (mixing barriers) in the subtropics and across the polar vortices, 2) strong descent inside the polar vortices during winter and spring, and 3) vigorous seasonality in the tropical upper stratosphere, related to seasonal upwelling and the semiannual oscillation. The observed variations are in agreement with aspects of the mean meridional circulation derived from stratospheric meteorological analyses. Interannual variations are also investigated, and a majority of the variance is found to be coherent with the equatorial quasibiennial oscillation (QBO). Strong QBO influence is found in the tropical upper stratosphere: the double-peaked “rabbit ears” structure occurs primarily during QBO westerlies. The QBO also modulates the latitudinal position of the tropical “reservoir” in the middle stratosphere.

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C.B. Leovy, C-R. Sun, M.H. Hitchman, E.E. Remsberg, J.M. Russell, III, L.L. Gordley, J.C. Gille, and L.V. Lyjak

Abstract

Data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) for the period 25 October 1978–28 May 1979 are used in a descriptive study of ozone variations in the middle stratosphere. It is shown that the ozone distribution is strongly influenced by irreversible deformation associated with large amplitude planetary-scale waves. This process, which has been described by McIntyre and Palmer as planetary wave breaking, takes place throughout the 3–30 mb layer, and poleward transport of ozone within this layer occurs in narrow tongues drawn out of the tropics and subtropics in association with major and minor warming events. Thew events complement the zonal mean diabatic circulation in producing significant changes in the total column amount of ozone.

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