Uncertainties and assessments of chemistry-climate models of the stratosphere

[摘要] In recent years a number of chemistry-climate models have been developed withan emphasis on the stratosphere. Such models cover a wide range of time scales of integration and vary considerably in complexity. The results ofspecific diagnostics are here analysed to examine the differences amongst individual models and observations, to assess the consistency of modelpredictions, with a particular focus on polar ozone. For example, many modelsindicate 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 byimproving model horizontal resolution and with the use of non-orographic gravity wave drag. As a result of the widely differing modelled polartemperatures, different amounts of polar stratospheric clouds are simulatedwhich in turn result in varying ozone values in the models.

The results are also compared to determine the possible future behaviour ofozone, with an emphasis on the polar regions and mid-latitudes. All modelspredict eventual ozone recovery, but give a range of results concerning itstiming and extent. Differences in the simulation of gravity waves and planetary waves as well as model resolution are likely major sources ofuncertainty for this issue. In the Antarctic, the ozone hole has probably reached almost its deepest although the vertical and horizontal extent ofdepletion may increase slightly further over the next few years. According tothe model results, Antarctic ozone recovery could begin any year within therange 2001 to 2008.

The limited number of models which have been integrated sufficiently far indicate that full recovery of ozone to 1980 levels may not occur in theAntarctic until about the year 2050. For the Arctic, most models indicate that small ozone losses may continue for a few more years and that recoverycould 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 theAntarctic.

Further, interannual variability will tend to mask the signal for longer thanin the Antarctic, delaying still further the date at which ozone recovery maybe said to have started. Because of this inherent variability of the system,the decadal evolution of Arctic ozone will not necessarily be a direct response to external forcing.

[学科分类] 大气科学