• Introduction
• Model description
• Model results
• Summary
• References

FIGURES

• Figure 1
• Figure 2

Introduction

Ozone depletion and cooling are among the most significant perturbations of the middle atmosphere (MA) observed throughout the past several decades. These two phenomenon are known to be coupled via the existence of a negative feedback between ozone concentration and temperature [1]. CO₂-induced MA cooling occurs because above a certain height, the atmosphere becomes transparent to infrared radiation and the layers cool directly to space. In this region, colder temperatures modify the ozone distribution by reducing the efficiency of catalytic cycles that destroy ozone. The ozone increase associated with this mechanism enhances the amount of solar absorption and the associated heating counteracting the CO₂-induced cooling. The magnitude of this radiative feedback can be significant and should be taken into account in models. However ozone decreases, which have been observed over the past few decades, occur even though the MA atmosphere cools. This fact raises questions about the severity of the CFC-induced ozone depletion process and suggest that MA CO₂-induced cooling could mask some of the ozone depletion process.

In this study, the Canadian Middle Atmosphere Model (CMAM) has been run with and without interactive ozone and each case has been repeated with twice its CO₂ value. These 4 integrations have allowed us to address the impact of ozone feedback and its potential impact on the enhanced CO₂ cooling signal. The model is an extended version of the Canadian Climate model (CCCma GCM) with a lid at ~95km which is coupled with a comprehensive photochemical module to include a prognostic representation of ozone. The purpose of this set of experiments was to estimate the importance of incorporating the ozone radiative feedback on the enhance CO₂ cooling signal. This study quantifies the significance of this important feedback mechanism within a global model and shows the necessity of addressing CO₂ and ozone trends together with a new generation of models that use a prognostic representation of ozone.

Model description

The Canadian Middle Atmosphere Model (CMAM) [2] is a vertically extended and modified version of the general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCma) [3]. It is a spectral model with a hybrid vertical coordinate which is terrain following near the surface and changes to a pressure coordinate throughout the middle atmosphere. This version of the model is run with 50 levels and T32 spectral resolution in the horizontal and uses a 15-min time step. In the middle atmosphere the vertical resolution is ~3 km, and the model top is at p= 0.000637 hPa or about 95 km. This model version uses a parameterization scheme to describe the momentum deposition by a broad spectrum of subgrid-scale gravity waves [4].

The model includes a photochemical module to allow for the use of a prognostic and radiatively active representation of ozone [5]. The photochemistry module contains 44 species including odd-hydrogen, odd-nitrogen, odd-chlorine, and odd-bromine families, N₂O, CFC-11, and CFC-12, CH₃Br, CH₄, and its oxidation products including CO [6]. It has 127 photochemical reactions, including 34 photolysis reactions. Transport is active throughout the domain, while chemistry is solved on-line from ~6 km to the top of the model. For this model version a spectral advection scheme is used for the transport of moisture and other chemical constituents. A full diurnal cycle is simulated with photolysis rates provided by a lookup table. The chemistry solver is a mass-conserving fully implicit backward difference scheme.

Model results

The first two scenarios were done with prescribed ozone climatology and with CO₂ mixing ratios specified as 348 ppmv and 696 ppmv respectively. These experiments were then repeated with interactive ozone. All scenarios were performed with the same climatology for sea surface temperature. All runs have been spun-up for a period of several years.

Figure 1a below shows the cooling signal associated with CO₂ doubling obtained when ozone is fixed (black line) and this is compared with the interactive ozone case (green line) at the tropics for July conditions. It shows that throughout the middle atmosphere, the interactive ozone results contribute to reduce the amount of cooling compared with the prescribed case. Above 10 hPa, the importance of the radiative feedback (the difference between the two curves) increases with altitude and reaches a few degrees at the stratopause (Figure 1b). In this region the catalytic cycles, which are less efficient at colder temperature because of reduced O density, produces a significant ozone increase (~15%) shown in Figure 2. This effect enhances solar absorption thus heating of the atmosphere. The maximum radiative feedback appears to occur in the lower mesosphere (~4K at 0.3 hPa). In this region, the HO_x catalytic cycle which dominates the ozone loss terms, is not very temperature sensitive and the Chapman reactions that control ozone production will determine the nature of the photochemical feedback. The results show that the ozone response to MA cooling is roughly constant with height above the stratopause.

Figure 1. (a) July 10-year mean temperature profile at 3N between 2xCO₂ and 1xCO₂ for prescribed ozone scenarios (black line) and interactive ozone scenarios (green line). (b) Temperature Difference (K)

Figure 2. (a) July 10-year mean profile of ozone (ppmv) at 3N for the ozone interactive 1xCO₂ scenario (black line) and 2xCO₂ scenario (green line). (b) Ozone Difference (in %).

Figure 2 shows the impact of CO₂ doubling on the ozone profile. In the lower stratosphere, ozone decreases are obtained in response to its enhancement at higher altitudes. In-situ ozone decreases in this region (of ~2-3%) appears to be well correlated with the temperature difference. The two figures show that small changes in ozone have a noticeable radiative impact in the region. At the mesopause, the results suggest that the use of interactive ozone has little impact on the model heating budget. At 0.01 hPa, Figure 2 shows a large ozone increase in percentage which is misleading since it represents a very small amount in terms of actual change in concentration. The reason for this sharp ozone increase is not clear but its radiative impact is not likely significant as shown on the figures.

Summary

The results show that 2xCO₂-induced cooling reaches ~12K at the stratopause over the tropics for the prescribed ozone simulations. The cooling reaches 14-15 K in the polar stratopause regions where temperature maximum occurs. The incorporation of ozone radiative feedback enhances the solar heating in the summer hemisphere. In the winter hemisphere, enhanced infrared cooling due to ozone can be significant. These combined effects modify the model response to CO₂ doubling throughout the year. In the tropics, the incorporation of ozone radiative feedback reduces the cooling signal by several degrees at the stratopause and up to 4 K around 0.5 hPa. The ozone change is significant throughout the upper stratosphere/lower mesosphere showing a general increase between 10% and 15% in response to %CO₂ doubling. The magnitude of the associated radiative feedback varies with altitude and results in approximately 2K warming for a 10% increase in ozone at the stratopause. In the lower stratosphere, a small ozone reduction and associated cooling is obtained for CO₂ doubling with the ozone interactive model version.

References

[1] Akmaev, R. A. and V. I. Fomichev, Cooling of the mesosphere and lower thermosphere due to doubling of CO₂, Ann. Geophysicae , 16, 1501-1512, 1998.

[2] Beagley, S. R., J. de Grandpre^', J. N. Koshyk, N. A. McFarlane, and T. G. Shepherd, Radiative-dynamical climatology of the first-generation Canadian Middle Atmosphere Model, Atmos. Ocean, 35, 293-331, 1997.

[3] McFarlane, N. A., G. J. Boer, J.-P. Blanchet, and M. Lazare, The Canadian Climate Centre second generation GCM and its equilibrium climate, J. Clim., 5, 1013-1044, 1992.

[4] Medvedev, A. S., and G. P. Klaassen, Vertical evolution of gravity wave spectra and the parameterization of associated wave drag, J. Geophys. Res., 100, 25,841-25,853, 1995.

[5] de Grandpre^', J., S. R. Beagley, V. I. Fomichev, E. Griffioen, J. C. McConnell, A. S. Medvedev and T. G. Shepherd, Ozone climatology using interactive chemistry: Results from the Canadian Middle Atmosphere Model, J. Geophys. Res., 105, 26,475-26,491, 2000.

[6] de Grandpre^', J., J. W. Sandilands, J. C. McConnell, S. R. Beagley, P. C. Croteau, and M. Y. Danilin, Canadian Middle Atmosphere Model: Preliminary results from the Chemical Transport Module, Atmos. Ocean , 35, 385-431, 1997.