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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

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

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

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

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 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

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 and Fei Wu
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William J. Randel and Fei Wu

Abstract

Long time records of stratospheric temperatures indicate that substantial cooling has occurred during spring over polar regions of both hemispheres. These cooling patterns are coincident with observed recent ozone depletions. Time series of temperature from radiosonde, satellite, and National Centers for Environmental Prediction reanalysis data are analyzed in order to isolate the space–time structure of the observed temperature changes. The Antarctic data show strong cooling (of order 6–10 K) in the lower stratosphere (∼12–21 km) since approximately 1985. The cooling maximizes in spring (October–December), with small but significant changes extending throughout Southern Hemisphere summer. No Antarctic temperature changes are observed during midwinter. Significant warming is found during spring at the uppermost radiosonde data level (30 mb, ∼24 km). These observed temperature changes are all consistent with model predictions of the radiative response to Antarctic polar ozone depletion. Winter and spring temperatures in Northern Hemisphere polar regions also indicate a strong cooling in the 1990s, and the temperature changes are coherent with observed ozone losses. The overall space–time patterns are similar between both hemispheres, suggesting that the radiative response to ozone depletion is an important component of the Arctic cooling as well.

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