A new interactive chemistry‐climate model: 2. Sensitivity of the middle atmosphere to ozone depletion and increase in greenhouse gases and implications for recent stratospheric cooling

Abstract

The sensitivity of the middle atmosphere circulation to ozone depletion and increase in greenhouse gases is assessed by performing multiyear simulations with a chemistry-climate model. Three simulations with fixed boundary conditions have been carried out: one simulation for the near-past ( 1960) and two simulations for the near-present ( 1990 and 2000) conditions, including changes in greenhouse gases, in total organic chlorine, and in average sea surface temperatures. Changes in ozone are simulated interactively by the coupled model. It is found that in the stratosphere, ozone decreases, and that in the Antarctic, the ozone hole develops in both the 1990 and the 2000 simulations but not in the 1960 simulation, as observed. The simulated temperature decreases in the stratosphere and mesosphere from the near past to the present, with the largest changes at the stratopause and at the South Pole in the lower stratosphere, in agreement with current knowledge of temperature trends. In the Arctic lower stratosphere, a cooling in March with respect to the 1960 simulation is found only for the 2000 simulation. Wave activity emerging from the troposphere is found to be comparable in the winters of the 1960 and 2000 simulations, suggesting that ozone depletion and greenhouse gases increase contribute to the 2000 - 1960 March cooling in the Arctic lower stratosphere. These results therefore provide support to the interpretation that the extreme low temperatures observed in March in the last decade can arise from radiative and chemical processes, although other factors cannot be ruled out. The comparison of the 1960 and 2000 simulations shows an increase in downwelling in the mesosphere at the time of cooling in the lower stratosphere ( in March in the Arctic; in October in the Antarctic). The mesospheric increase in downwelling can be explained as the response of the gravity waves to the stronger winds associated with the cooling in the lower stratosphere. Planetary waves appear to contribute to the downward shift of the increased downwelling, with a delay of about a month. The increase in dynamical heating associated with the increased downwelling may limit the cooling and the strengthening of the lower stratospheric polar vortex from above, facilitating ozone recovery and providing a negative dynamical feedback. In both the Arctic and Antarctic the cooling from ozone depletion is found to affect the area covered with polar stratospheric clouds in spring, which is substantially increased from the 1960 to the 2000 simulations. In turn, increased amounts of polar stratospheric clouds can facilitate further ozone depletion in the 2000 simulation

Interannual variation patterns of total ozone and lower stratospheric temperature in observations and model simulations

“…For both simulations, the horizontal resolution used in the GCMs was the T30 truncation, and the mesh size for CHEM was 3.75 • ×3.75 • . In E39/C, 39 vertical model levels reach from the surface up to 10 hPa (30 km) (Dameris et al, 2005), whereas in MAECHAM4-CHEM 39 model levels reach from the surface up to 0.01 hPa (80 km) Manzini et al, 2003). With more levels near the tropopause, E39/C is targeted more on processes around the tropopause Schnadt et al, 2002;Schnadt and Dameris, 2003).…”

“…With more levels near the tropopause, E39/C is targeted more on processes around the tropopause Schnadt et al, 2002;Schnadt and Dameris, 2003). MAECHAM4-CHEM, on the other hand, is a full troposphere-stratosphere-mesosphere model (Manzini et al, 1997;Manzini et al, 2003). It includes better radiation and dissipation processes, especially momentum deposition by dissipating gravity wave spectra, which is parameterized in MAECHAM4-CHEM (Manzini and McFarlane, 1998), but not in E39/C.…”

“…The purpose of this study is to extend this climatological validation to the comparison of interannual variation patterns. In this sense, we "validate" two recent 40 year transient simulations by ECHAM4.L39(DLR)/CHEM, called E39/C hereafter (Dameris et al, 2005), and MAECHAM4-CHEM Manzini et al, 2003). These simulations try to account for nearly all important sources of variance in the real atmosphere between 1960 and 2000.…”

“…Climatologies and trends from stratospheric "chemistryclimate models" are realistic and have been validated Austin et al, 2003;Manzini et al, 2003;Steil et al, 2003). The purpose of this study is to extend this climatological validation to the comparison of interannual variation patterns.…”

Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

“…Above, in the mesosphere, this situation facilitates the deceleration of the easterlies (westerlies) in the summer (winter) hemisphere leading to an increased circulation (upward in summer and downward in winter). It is not the purpose of the current work to detail this chain of effects, because this behavior of the gravity wave parameterization in the MAECHAM models has been documented and discussed in earlier works (Manzini and McFarlane, 1998;Manzini et al, 2003), although the changes in the background winds occurred for different reasons.…”