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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.