Hans R. Schneider

Effects of the quasi‐biennial oscillation on the zonally averaged transport of tracers

The influence of the quasi-biennial oscillation (QBO) on the transport of long-lived tracers out of the tropics and the mechanism responsible for the QBO in subtropical ozone and its dependence on the seasonal cycle are examined with a two-dimensional model. The modeled QBO induces a meridional circulation which modulates transport of long-lived tracers out of the tropics. The induced circulation also produces a QBO in ozone in the subtropics by advection of ozone out of the tropics and down from higher altitudes. In agreement with observations, the subtropical anomalies in ozone are greatest in the winter season. This seasonal synchronization of the subtropical anomalies occurs because the induced circulation is stronger always in the winter hemisphere as a result of nonlinear momentum advection in the tropics and subtropics. Meridional transport in the model is enhanced by the QBO through an “upper” and a “lower” transport regime, in agreement with the analysis by Hitchman et al. [1994]. When there are descending westerly winds in the tropics in the model, transport out of the tropics is enhanced in the lower stratosphere. When there are descending easterlies, transport out of the tropics is enhanced in the middle stratosphere. This modulation of transport out of the tropics significantly influences the stratospheric distribution of long-lived tracers. Depending on the phase of the QBO, mixing ratio surfaces of long-lived tracers (such as N2O) in the extratropics can be displaced poleward by more than 10°.

Analysis of residual mean transport in the stratosphere: 1. Model description and comparison with satellite data

A coupled two-dimensional model of the dynamics, chemistry, and radiation of the stratosphere is described. The effects of Rossby wave mixing are parameterized by externally specified coefficients Kyy, which are used consistently in the zonal mean momentum equation and the diffusion term of the tracer transport equation. Rossby wave mixing is reduced in the tropics. Transport between the tropics and midlatitudes is controlled by advection in the model. It is shown that subtropical tracer gradients [e.g., Trepte and Hitchman, 1992; Randel et al., 1993] are produced as a consequence of the difference between advective timescales in the tropics and the diffusive time scale in midlatitudes. In the annual mean, the flow of air is directed from the tropics toward midlatitudes. However, advection between tropics and midlatitudes takes place in both directions at stratospheric altitudes at any given time. Calculated temperatures show some discrepancies with observations in the polar regions. Distributions of long-lived tracers are compared with observations made by instruments aboard the Upper Atmospheric Research Satellite and data from the Atmospheric Trace Molecule Spectroscopy Experiment. Tropical and midlatitude profiles calculated by the model are in reasonable agreement with observations. The sensitivity of calculated temperatures and distributions of long-lived tracers to the choice of large-scale diffusion coefficients, Kyy, is shown to be small due to the dual role of Kyy in determining diffusive tracer transport and providing a forcing term for the mean circulation [Holton, 1986].

Constraints on meridional transport in the stratosphere imposed by the mean age of air in the lower stratosphere

The sensitivity of mean age of air in the stratosphere to the vertical structure of mass exchange between the tropics and midlatitudes is examined with a two-dimensional model with consistent advective and diffusive transport. We use estimates of mean age of air and age spectra in the midlatitude lower stratosphere, derived from observations of CO 2 , to constrain transport from the tropics to midlatitudes. We show that to reproduce the latitudinal gradient in mean age in the lower stratosphere as well as the bimodal age spectra derived for the midlatitude lower stratosphere, the rate of horizontal transport from the tropics to midlatitudes between 20-30 km must be slower than that at higher and lower altitudes. The relative rates of meridional transport above and below this region determine the separation of the two peaks in the age spectra and the overall mean age of air in the lower stratosphere. Slow transport between 20-30 km can generally not be obtained by reducing only the diffusive component of transport. In the model, gravity wave drag must also be reduced in the lower stratosphere to prevent strong horizontal advective transport across the subtropics. We also show that adjusting the diffusive component of transport independently of the meridional circulation can produce mean ages different from those calculated in the fully coupled model by as much as 2 years.

An evaluation of the role of eddy diffusion in stratospheric interactive two-dimensional models

Abstract The effect of eddy diffusion in an interactive two-dimensional model of the stratosphere is reexamined. The model consists of a primitive equation dynamics module, a simplified HOx ozone model and a full radiative transfer scheme. The diabatic/residual circulation in the model stratosphere is maintained by the following processes: 1) nonlocal forcing resulting from dissipation in the parameterized model troposphere and frictional drag at mesospheric levels, 2) mechanical damping within the stratosphere itself, and 3) potential vorticity flux due to large scale waves. The net effect of each process is discussed in terms of the efficiency of the induced circulation in transporting ozone from the equatorial lower stratosphere to high latitude regions. The same eddy diffusion coefficients are used to parameterize the flux of quasi-geostrophic potential vorticity and diffusion in the tracer transport equation. It is shown that the ozone distributions generated with the interactive two-dimensional mode...

Analysis of residual mean transport in the stratosphere: 2. Distributions of CO2 and mean age

Distributions of CO2 and the mean age of stratospheric air are examined using the interactive two-dimensional model described in the companion paper. It is shown that the model can explain the distribution of age in the lower stratosphere and reproduce the correlations between CO2 and N2O observed at midlatitudes in the lower stratosphere. Seasonal characteristics of the measured correlations are captured by the model. The model uses externally specified coefficients Kyy to account for large-scale isentropic mixing in the stratosphere. The sensitivity of calculated distributions of CO2 to Kyy is be examined by comparing model runs using three different distributions of Kyy. It is shown that the CO2 measurements require Kyy to exceed a threshold value of approximately 105 m2 s−1 for most of the year. Once this threshold is exceeded, the slope of the CO2/N2 correlations depends only weakly on the magnitude and the structure of the distribution of Kyy.

A two-dimensional model with coupled dynamics, radiative transfer, and photochemistry. 2: Assessment of the response of stratospheric ozone to increased levels of CO2, N2O, CH4, and CFC

The impact of increased levels of carbon dioxide (CO2), chlorofluorocarbons (CFCs), and other trace gases on stratospheric ozone is investigated with an interactive, two-dimensional model of gas phase chemistry, dynamics, and radiation. The scenarios considered are (1) a doubling of the CO2 concentration, (2) increases of CFCs, (3) CFC increases combined with increases of nitrous oxide (N2O) and methane CH4, and (4) the simultaneous increase of CO2, CFCs, N2O, and CH4. The radiative feedback and the effect of temperature and circulation changes are studied for each scenario. For the double CO2 calculations the tropospheric warming was specified. The CO2 doubling leads to a 3.1% increase in the global ozone content. Doubling of the CO2 concentrations would lead to a maximum cooling of about 12°C at 45 km if the ozone concentration were held fixed. The cooling of the stratosphere leads to an ozone increase with an associated increase in solar heating, reducing the maximum temperature drop by about 3°C. The CFC increase from continuous emissions at 1985 rate causes a 4.5% loss of ozone. For the combined perturbation a net loss of 1.3% is calculated. The structure of the perturbations shows a north-south asymmetry. Ozone losses (when expressed in terms of percent changes) are generally larger in the high latitudes of the southern hemisphere as a result of the eddy mixing being smaller than in the northern hemisphere. Increase of chlorine leads to ozone losses above 30 km altitude where the radiative feedback results in a cooler temperature and an ozone recovery of about one quarter of the losses predicted with a noninteractive model. In all the cases, changes in circulation are small. In the chlorine case, circulation changes reduce the calculated column depletion by about one tenth compared to offline calculations.

A dynamical explanation for the asymmetry in zonally averaged column abundances of ozone between northern and southern springs

Abstract The observed zonally averaged column ozone shows a maximum at 90°N during the northern winter and spring and at 60°S throughout the southern winter and spring. This asymmetry is explained in the context of a zonally averaged model with coupled radiation, dynamics, and chemistry, together with consistently parameterized planetary wave driving and wave transport. It is shown that in the presence of weak wave driving, the penetration of the tropospheric circulation into the lower stratosphere and the characteristics of ozone chemistry are such that they produce a column ozone maximum at subpolar latitudes. The effect of increased wave driving is to intensify the residual circulation and extend it farther poleward, resulting in an ozone maximum at the pole. The role of the mesospheric drag is to further enhance these column ozone maxima. Model calculations show that the positions of the observed column ozone maxima are consistent with intensities of wave driving in the two hemispheres derived from data.

The impact of tropospheric planetary wave variability on stratospheric ozone

The goal of this project was to improve understanding of the role of the stratosphere in inducing long-term variations of the chemical composition of the troposphere. Changes in stratospheric transport occur on decadel timescales in response to changes in the structure of planetary wave patterns, forced in the troposphere. For many important tracers, such as column amounts of ozone, this variability of the transport leads to changes with signatures very similar to those induced by anthropogenic releases of chemicals into the atmosphere. During this project, a new interactive two-dimensional model of the dynamics, chemistry and radiation of the stratosphere was developed. The model was used to interpret available data of tracers. It was found that a fairly coherent picture of tracer distributions is obtained when a layer of reduced gravity wave drag is assumed for the lower stratosphere. The results suggest that the power of models to predict variability in tracer transport in the upper troposphere and lower stratosphere is limited until current theories of gravity wave breaking have been refined.