Abstract
Abstract. Arctic stratospheric ozone depletion is closely linked to the occurrence of low stratospheric temperatures. There are indications that cold winters in the Arctic stratosphere have been getting colder, raising the question if and to what extent a cooling of the Arctic stratosphere may continue into the future. We use meteorological reanalyses from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim and NASA's Modern-Era Retrospective-Analysis for Research and Applications (MERRA) for the past 32 yr together with calculations of the chemistry-climate model (CCM) ECHAM/MESSy Atmospheric Chemistry (EMAC) and models from the Chemistry-Climate Model Validation (CCMVal) project to infer radiative and dynamical contributions to long-term Arctic stratospheric temperature changes. For the past three decades the reanalyses show a warming trend in winter and cooling trend in spring and summer, which agree well with trends from the Radiosonde Innovation Composite Homogenization (RICH) adjusted radiosonde data set. Changes in winter and spring are caused by a corresponding change of planetary wave activity with increases in winter and decreases in spring. During winter the increase of planetary wave activity is counteracted by a residual radiatively induced cooling. Stratospheric radiatively induced cooling is detected throughout all seasons, being highly significant in spring and summer. This means that for a given dynamical situation, according to ERA-Interim the annual mean temperature of the Arctic lower stratosphere has been cooling by −0.41 ± 0.11 K decade−1 at 50 hPa over the past 32 yr. Calculations with state-of-the-art models from CCMVal and the EMAC model qualitatively reproduce the radiatively induced cooling for the past decades, but underestimate the amount of radiatively induced cooling deduced from reanalyses. There are indications that this discrepancy could be partly related to a possible underestimation of past Arctic ozone trends in the models. The models project a continued cooling of the Arctic stratosphere over the coming decades (2001–2049) that is for the annual mean about 40% less than the modeled cooling for the past, due to the reduction of ozone depleting substances and the resulting ozone recovery. This projected cooling in turn could offset between 15 and 40% of the Arctic ozone recovery.
Highlights
Large losses of Arctic stratospheric ozone have been observed during cold winters over the past decades (WMO, 2011)
Lower-stratospheric temperature observations from the Microwave Sounding Unit (MSU) indicate a strengthening of the Brewer–Dobson circulation (BDC) from December to February and a weakening from March until May connected to the increase/decrease of planetary wave activity, which leads to a corresponding positive/negative temperature trend (Fu et al, 2010)
We focus on temperatures in the lower stratosphere at 50 hPa, as this is the region most critical for Arctic ozone depletion
Summary
Large losses of Arctic stratospheric ozone have been observed during cold winters over the past decades (WMO, 2011). Sinnhuber et al (2011) calculated that a temperature trend of −0.8 K decade−1 could enhance Arctic stratospheric ozone depletion enough to offset the recovery due to the expected future halogen decrease. Lower-stratospheric temperature observations from the Microwave Sounding Unit (MSU) indicate a strengthening of the Brewer–Dobson circulation (BDC) from December to February and a weakening from March until May connected to the increase/decrease of planetary wave activity, which leads to a corresponding positive/negative temperature trend (Fu et al, 2010). Trend calculations of an empirically derived BDCindex, which consists of the contrast between extra-tropical and tropical temperatures, suggest a significant strengthening of the Northern Hemisphere branch of the BDC during December throughout the depth of the stratosphere, and a significant weakening during March in the lower stratosphere (Young et al, 2012). Sensitivity simulations with EMAC using fixed mixing ratios of greenhouse gases (in particular CO2, N2O, CH4) and ozone depleting substances (ODS) are used to calculate the sensitivity of temperature changes to the respective radiatively active gases
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