Uncertainties and assessments of chemistry-climate models of the stratosphere

Abstract

Abstract. In recent years a number of chemistry-climate models have been developed with an emphasis on the stratosphere. Such models cover a wide range of timescales of integration and vary considerably in complexity. The results of specific diagnostics are here analysed to examine the differences amongst individual models and observations, to assess the consistency of model predictions, with a particular focus on polar ozone. For example, many models indicate a significant cold bias in high latitudes, the 'cold pole problem', particularly in the southern hemisphere during winter and spring. This is related to wave propagation from the troposphere which can be improved by improving model horizontal resolution and with the use of non-orographic gravity wave drag. As a result of the widely differing modeled polar temperatures, different amounts of polar stratospheric clouds are simulated which in turn result in varying ozone values in the models. The results are also compared to determine the possible future behaviour of ozone, with an emphasis on the polar regions and mid-latitudes. All models predict eventual ozone recovery, but give a range of results concerning its timing and extent. Differences in the simulation of gravity waves and planetary waves as well as model resolution are likely major sources of uncertainty for this issue. In the Antarctic, the ozone hole has probably reached almost its deepest although the vertical and horizontal extent of depletion may increase slightly further over the next few years. According to the model results, Antarctic ozone recovery could begin any year within the range 2001 to 2008. For the Arctic, most models indicate that small ozone losses may continue for a few more years and that recovery could begin any year within the range 2004 to 2019. The start of ozone recovery in the Arctic is therefore expected to appear later than in the Antarctic in most models. Further, interannual variability will tend to mask the signal for longer in the Arctic than in the Antarctic, delaying still further the date at which ozone recovery may be said to have started. In the longer term, the model results suggest that full recovery of ozone to 1980 levels is not expected in the Antarctic until about the year 2050. Earlier recovery to 1980 levels may be possible in the Arctic, but model differences are too large compared with the simulated changes to obtain a reliable date.