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Arthur Y. Hou and Malcolm K. W. Ko

Abstract

We have examined an idealized, zonally averaged, nonlinear, ageostrophic circulation forced by differential heating and parameterized eddy mixing for a range of mixing values and boundary conditions. Using a simple ƒ-plane channel model, we show that geostrophic and ageostrophic flows can have fundamentally different behaviors which may have important implications for the circulation and trace gas distributions in the stratosphere. Our main conclusions are: 1) As eddy forcing vanishes, an ageostrophic system is not constrained to approach radiative equilibrium (unlike a geostrophic system) and may tend to the limit of an inviscid diabatic circulation under a range of boundary conditions. 2) For stratospheric applications, we show that reduced eddy mixing in an ageostrophic model leads to a pronounced meridional contraction in the residual circulation (thereby limiting its poleward transport); as the eddy mixing vanishes, the circulation tends to an inviscid limit with a well-defined meridional width. This is characteristically different from the behavior of a geostrophic circulation, which vanishes in approximate proportions to the amount of eddy mixing. 3) Reduced eddy mixing in an ageostrophic model can ultimately lead to the steepening of the meridional slope of a long-lived tracer in the region between maximum rising and sinking motions. An implication of this study is that the comparatively weak wave driving in the Southern Hemisphere could produce a contracted residual circulation during the Southern winter, which may partially account for the asymmetry in the observed zonal-mean ozone column abundances between the Northern and Southern springs.

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Arthur Y. Hou, Hans R. Schneider, and Malcolm K. W. Ko

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.

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Robert M. MacKay, Malcolm K. W. Ko, Run-Lie Shia, Yajaing Yang, Shuntai Zhou, and Gyula Molnar

Abstract

In order to study the potential climatic effects of the ozone hole more directly and to assess the validity of previous lower resolution model results, the latest high spatial resolution version of the Atmospheric and Environmental Research, Inc., seasonal radiative dynamical climate model is used to simulate the climatic effects of ozone changes relative to the other greenhouse gases. The steady-state climatic effect of a sustained decrease in lower stratospheric ozone, similar in magnitude to the observed 1979–90 decrease, is estimated by comparing three steady-state climate simulations: I) 1979 greenhouse gas concentrations and 1979 ozone, II) 1990 greenhouse gas concentrations with 1979 ozone, and III) 1990 greenhouse gas concentrations with 1990 ozone. The simulated increase in surface air temperature resulting from nonozone greenhouse gases is 0.272 K. When changes in lower stratospheric ozone are included, the greenhouse warming is 0.165 K, which is approximately 39% lower than when ozone is fixed at the 1979 concentrations. Ozone perturbations at high latitudes result in a cooling of the surface–troposphere system that is greater (by a factor of 2.8) than that estimated from the change in radiative forcing resulting from ozone depletion and the model’s 2 × CO2 climate sensitivity. The results suggest that changes in meridional heat transport from low to high latitudes combined with the decrease in the infrared opacity of the lower stratosphere are very important in determining the steady-state response to high latitude ozone losses. The 39% compensation in greenhouse warming resulting from lower stratospheric ozone losses is also larger than the 28% compensation simulated previously by the lower resolution model. The higher resolution model is able to resolve the high latitude features of the assumed ozone perturbation, which are important in determining the overall climate sensitivity to these perturbations.

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Hans R. Schneider, Malcolm K. W. Ko, Nien Dak Sze, Guang-Yu Shi, and Wei-Chyung Wang

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 model are very sensitive to the choice of values for the friction and the eddy diffusion coefficients. The strength of the circulation increases with the mechanical damping and Kyy. At the same time, larger diffusion in the tracer transport equation reduces the equator to pole transport (Holton 1986). Depending on the amount of friction assumed in the stratosphere, increasing eddy diffusion can lead to an increase as well as a decrease in the net transport. It is shown that reasonable latitudinal gradients of ozone can be obtained by using small values for the mechanical damping [≈1/(100 days)] and Kyy (order 104 m2 s−1) for the mid- and high-latitude stratosphere.

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Malcolm K. W. Ko, Nien Dak Sze, Mikhail Livshits, Michael B. McElroy, and John A. Pyle

Abstract

A two-dimensional zonal-mean model with parameterized dynamics and an advanced photochemical scheme is used to simulate the stratospheric distributions of atmospheric trace gases including ozone. The model calculates the distributions of 37 species that are photochemically coupled via 140 reactions with rate data from WMO/NASA. A full diurnal treatment is used to calculate the diurnal variations of the short-lived species and the diurnal mean of the production/loss rates for the long-lived species. The calculated concentrations are compared with a wide range of observations with emphasis on the seasonal and latitudinal features. In this work, no post hoc adjustment of the dynamical parameters has been attempted to improve agreement with observations.

In general, the model results are in good agreement with observations, although several discrepancies are noted. Rather than focusing on any individual species, we look for systematic agreements and discrepancies between model and observations for a wide range of species. The model appears to successfully simulate the major features of the mixing ratio surfaces for the long-lived species. However, at the equatorial region, the model tends to underestimate the concentrations of upward diffusing species (e.g., CFCs, CH4, N2O) and overestimate the column abundances of the downward diffusing species (HNO3, HCl, O3). These discrepancies are systematically examined and their implications for transport parameterization assessed.

The model successfully simulates the general latitudinal and seasonal behavior of the local concentration and column abundance of O3. Apart from the overestimation of the column abundances at the equator, the model also underestimates its seasonal contrast at high latitudes. There are difficulties in explaining the observed low concentrations of NO2 in winter at high latitudes. It is shown that errors in the simulation of NO2 concentration in these regions can significantly affect the calculated seasonal and latitudinal behavior of the column abundance of ozone in the middle and high latitudes.

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