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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.