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  • Author or Editor: Fei Wu x
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William J. Randel
and
Fei Wu

Abstract

Temperature profiles in polar latitudes during summer reveal a strong and persistent inversion layer associated with the polar summer tropopause. This inversion layer is characterized by a temperature increase of ∼8 K in the first 2–3 km above the tropopause and is observed throughout summer polar latitudes in both hemispheres. Radiosonde and GPS radio occultation temperature observations are used to document characteristics of the inversion layer, including its seasonal variability and modulation by synoptic meteorological systems (cyclones and anticyclones). Previous analyses have suggested a radiative mechanism for formation and maintenance of tropopause inversions, related to water vapor and ozone near the tropopause. Fixed dynamical heating (FDH) calculations are used herein to investigate this behavior in polar regions, based on observed seasonally varying profiles of water vapor (from satellite measurements) and ozone (from ozonesondes). Water vapor exhibits a strong seasonal cycle throughout the troposphere and lowest stratosphere, with a pronounced summer maximum, which is primarily a result of the seasonally varying tropospheric temperatures. The FDH calculations suggest that enhanced summer water vapor leads to strong radiative cooling in a narrow layer near the tropopause, so that the radiative influence of water vapor provides a primary mechanism for the summer inversion layer.

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William J. Randel
and
Fei Wu

Abstract

Variability in tropical zonal mean temperatures over 10–30 km is analyzed based on high-quality, high-vertical-resolution GPS temperature measurements covering 2001–13. The observations are used to quantify variability spanning time scales of weeks to over a decade, with focus on behavior of the tropopause region and coupling with the upper troposphere and stratosphere. Large variations associated with the seasonal cycle, quasi-biennial oscillation (QBO), and El Niño–Southern Oscillation (ENSO) are isolated and removed, and residual time series are analyzed using principal components and spectrum analysis. The residual temperature exhibits maximum variance in the lower stratosphere, with a vertical structure similar to the seasonal cycle. Residual temperatures exhibit two dominant modes of variability: a “deep stratosphere mode” tied to high-latitude planetary wave forcing and a shallow “near-tropopause mode” linked to dynamically forced upwelling near the tropopause. Variations in the cold point tropopause (and by inference in global stratospheric water vapor) are closely tied to the near-tropopause mode. These coherent temperature patterns provide further evidence of distinct upper and lower branches of the tropical Brewer–Dobson circulation. Zonal mean temperatures in the lower stratosphere and near the cold point are most strongly coupled to the upper troposphere on time scales of ~(30–60) days, probably linked to the Madden–Julian oscillation (MJO). Enhanced temperature variance near the tropopause is consistent with the long radiative relaxation time scales in the lower stratosphere, which makes this region especially sensitive to low-frequency dynamical forcing.

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William J. Randel
and
Fei Wu

Abstract

Detailed structure of the global quasi-biennial oscillation (QBO) in ozone is analyzed using Stratospheric Aerosol and Gas Experiment II ozone and nitrogen dioxide data. Emphasis is placed on the midlatitude QBO, in particular its vertical structure and seasonal synchronization. The global QBO signal is isolated using a combination of singular-value decomposition and regression analyses, which combine to act as an accurate QBO digital filter. Results show that the midlatitude ozone QBO has a two-cell structure in the vertical (similar to that at the equator), with in-phase maxima in the lower and middle stratosphere. Both upper- and lower-level anomalies contribute important fractions to the midlatitude column amounts. The lower-level maxima have a broad latitudinal structure (˜15°–60°), and collocation with the strongest background gradients suggests that these anomalies result from mean vertical transport. The, middle stratosphere signal maximizes in the subtropics (10°–40°) and is likely due to nitrogen-related chemical effects (which are in turn due to transport variations). The vertically in-phase seasonal synchronization in midlatitudes is evidence of QBO modulation of the winter hemisphere circulation.

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William J. Randel
,
Rolando Garcia
, and
Fei Wu

Abstract

The dynamical balances associated with upwelling in the tropical lower stratosphere are investigated based on climatological 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis data. Zonal mean upwelling is calculated from momentum balance and continuity (“downward control”), and these estimates in the deep tropics are found to be in reasonable agreement with stratospheric upwelling calculated from thermodynamic balance (and also with vertical velocity obtained from ERA-40). The detailed momentum balances associated with the dynamical upwelling are investigated, particularly the contributions to climatological Eliassen–Palm (EP) flux divergence in the subtropics. Results show that the equatorward extension of extratropical waves (baroclinic eddies and, in the NH, quasi-stationary planetary waves) contribute a large component of the subtropical wave driving at 100 hPa. Additionally, there is a significant contribution to subtropical forcing from equatorial planetary waves, which exhibit a strong seasonal cycle (a reversal in phase) in response to latitudinal migration of tropical convection. The observed balances suggest that the strong annual cycle in upwelling across the tropical tropopause is forced by subtropical horizontal eddy momentum flux convergence associated with waves originating in both the tropics and extratropics.

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William J. Randel
,
Fei Wu
, and
Piers Forster

Abstract

Global characteristics of the extratropical tropopause inversion layer identified in radiosonde observations by Birner are studied using high vertical resolution temperature profiles from GPS radio occultation measurements. The GPS data are organized according to the height of the thermal tropopause in each profile, and a temperature inversion layer above the tropopause (with an average magnitude of 3–5 K) is found to be a ubiquitous, climatological feature. The GPS data show that the inversion layer is present for all seasons in both hemispheres, spanning the subtropics to the pole, and there is not strong longitudinal structure. Dependence of the inversion layer on upper-troposphere vorticity is studied; while anticyclones exhibit a substantially stronger inversion than cyclones (as expected from balanced dynamics), the inversion is evident for all circulation types. Radiative transfer calculations indicate that strong gradients in both ozone and water vapor near the tropopause contribute to the inversion. Significant absorption of both longwave and shortwave radiation by ozone occurs, warming the region above the tropopause. Water vapor near and immediately above the tropopause contributes to cooling, effectively enhancing the inversion.

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William J. Randel
,
Rolando R. Garcia
, and
Fei Wu

Abstract

Dynamical variability in the extratropical stratosphere occurs on a broad range of timescales, from daily to seasonal. Extratropical wave transience is correlated with variations in the mean meridional (Brewer–Dobson) circulation that links the Tropics and the extratropics. In this study, the variability of observed temperature and calculated vertical velocity in the tropical lower stratosphere is examined to isolate the imprint of forcing by extratropical waves. The influence of the waves is quantified by estimating zonal-mean tropical upwelling from the zonal-mean momentum balance on a daily basis; a large fraction of the variance of tropical upwelling occurs at periods of 10–40 days, forced by transient waves. In addition, significant coherence is found between calculated upwelling and observed temperatures in the tropical lower stratosphere on weekly to seasonal timescales. This relationship is quantitatively consistent with simple thermodynamic balance, and suggests that the large annual cycle of temperature near the tropical tropopause is mainly a result of the relatively long radiative timescales in that region. The results indicate that EP flux divergence due to extratropical waves is a major determinant of zonal-mean temperatures in the tropical lower stratosphere.

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William J. Randel
,
Mijeong Park
,
Fei Wu
, and
Nathaniel Livesey

Abstract

Near-equatorial ozone observations from balloon and satellite measurements reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ∼16 and 19 km, with approximately a factor of 2 change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with the well-known annual variation in tropical temperature. This study shows that the large annual variation in ozone occurs primarily because of variations in vertical transport associated with mean upwelling in the lower stratosphere (the Brewer–Dobson circulation); the maximum relative amplitude peak in the lower stratosphere is collocated with the strongest background vertical gradients in ozone. A similar large seasonal cycle is observed in carbon monoxide (CO) above the tropical tropopause, which is approximately out of phase with ozone (associated with an oppositely signed vertical gradient). The observed ozone and CO variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.

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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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William J. Randel
,
Fei Wu
,
Samuel J. Oltmans
,
Karen Rosenlof
, and
Gerald E. Nedoluha

Abstract

Interannual variations of stratospheric water vapor over 1992–2003 are studied using Halogen Occultation Experiment (HALOE) satellite measurements. Interannual anomalies in water vapor with an approximate 2-yr periodicity are evident near the tropical tropopause, and these propagate vertically and latitudinally with the mean stratospheric transport circulation (in a manner analogous to the seasonal “tape recorder”). Unusually low water vapor anomalies are observed in the lower stratosphere for 2001–03. These interannual anomalies are also observed in Arctic lower-stratospheric water vapor measurements by the Polar Ozone and Aerosol Measurement (POAM) satellite instrument during 1998–2003. Comparisons of the HALOE data with balloon measurements of lower-stratospheric water vapor at Boulder, Colorado (40°N), show partial agreement for seasonal and interannual changes during 1992–2002, but decadal increases observed in the balloon measurements for this period are not observed in HALOE data. Interannual changes in HALOE water vapor are well correlated with anomalies in tropical tropopause temperatures. The approximate 2-yr periodicity is attributable to tropopause temperature changes associated with the quasi-biennial oscillation and El Niño–Southern Oscillation.

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William J. Randel
,
Fei Wu
,
Richard Swinbank
,
John Nash
, and
Alan O’Neill

Abstract

Global circulation anomalies associated with the stratospheric quasi-biennial oscillation (QBO) are analyzed based on U.K. Meteorological Office (UKMO) assimilated wind and temperature fields. Zonal winds and temperatures from the assimilation are compared with Singapore rawinsonde data (the standard QBO reference time series), showing reasonable agreement but an underestimate of maxima in the UKMO analyses. Global structure of the QBO in zonal wind, temperature, and residual mean meridional circulation (derived from thermodynamic balance and mass continuity) is isolated, showing coherent tropical and midlatitude components. Important aspects of the QBO revealed in these data include 1) out of phase maxima in temperature (and vertical velocity) between the lower and upper stratosphere, and 2) strong seasonal synchronization of midlatitude anomalies. These characteristics are also evident in long records of satellite radiance measurements.

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