How is the Brewer-Dobson circulation affected by climate change, and which processes are relevant? (SHARP-BDC)
Final Report Abstract
This research project was successful in studying the importance of the residual mean circulation (the Brewer-Dobson circulation, BDC) for the Earth-climate system and how it may change in future. The understanding of basic processes affecting the BDC has been improved. The common use of observations and numerical model data helped a lot, on the one side it allowed the evaluation of model results, on the other side it supported a more robust assessment of future changes of the BDC. Based on new data sets the following major results have been found: - For the first time a systematic investigation of the impact of the model configuration by using different vertical resolutions and different vertical extents of the same model was carried out. All configurations of the model consistently showed a BDC strengthening from the preindustrial to the future climate state. - The hemispheric asymmetry in return dates of mid-latitude stratospheric ozone turned out to be a robust result across different CCMs. An attribution analysis performed with CCMs showed that chemically-induced changes in ozone are the major driver of the earlier return of ozone to 1980 levels in northern mid-latitudes. In this connection, transport changes are here of minor importance. - In boreal winter the tropical upward mass flux increases by about 1 %/decade in the upper and 2 %/decade in the lower stratosphere until the end of the 21st century. Mean age of air decreases by up to 60 and 30 days/decade, respectively. Changes in transient planetary and synoptic waves account for the strengthening of the BDC in the lower stratosphere, whereas upper stratospheric changes are due to improved propagation properties for gravity waves in future climate. - An additional aging of air by mixing throughout most of the lower stratosphere is found, except in the extratropical lowermost stratosphere where mixing reduces age of air. The mixing efficiency remains close to constant in a future climate, suggesting that the strength of two-way mixing is tightly coupled to the strength of the residual circulation in the lower stratosphere. This implies that mixing generally amplifies changes in age of air due to uniform changes in the residual circulation. - Model results indicated that in the lower stratosphere the tropical upward mass flux increases by about 2 %/decade. Rising ozone depleting substance (ODS) concentrations counteract the greenhouse gas (GHG) effect in the middle and upper stratosphere with a total decrease of about 0.5 %/decade. Changes in mean age of air showed a decrease of about 0.13 year/decade in the lower and middle stratosphere and a slight increase in the Arctic upper stratosphere. This investigation showed that non-additivity is not negligible to obtain the full change of the BDC in the past. - If changes in tropopause height are taken into account, the residual mean transport of mass between the troposphere and the stratosphere changes little in response to anthropogenic forcing. - A new formulation of the fractional release of halocarbons has been suggested which improves the calculation of the ozone depletion potential. Taking into account chemical loss in the calculation of stratospheric mixing ratios reduces significantly the time dependence in fractional release factors (FRFs). Based on this, a new transit time distribution was introduced. The refined formulation for EESC also leads to EESC levels in the year 1980 for the mid latitude lower stratosphere, which are significantly lower than previously calculated.
Publications
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(2013), Drivers of hemispheric differences in return dates of mid-latitude stratospheric ozone to historical levels, Atmos. Chem. Phys., 13, 7279-7300
Garny, H., G. E. Bodeker, D. Smale, M. Dameris, and V. Grewe
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(2013), The Brewer–Dobson circulation in a changing climate: Impact of the model configuration, J. Atmos. Sci., 70, 1437–1455
Bunzel, F., and H. Schmidt
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(2013), Unravelling impact factors for future changes in the Brewer-Dobson Circulation, J. Geophys. Res. Atmos., 118
Oberländer, S., U. Langematz, and S. Meul
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(2014), The effects of mixing on age of air, J. Geophys.Res. Atmos., 119, 7015–7034
Garny, H., T. Birner, H. Bönisch, and F. Bunzel
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(2015), A chemistryclimate model study of past changes in the Brewer-Dobson circulation, J. Geophys. Res. Atmos., 120, 6742–6757
Oberländer-Hayn, S., S. Meul, U. Langematz, J. Abalichin, and F. Haenel
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(2016), Earth System Chemistry Integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy, version 2.51), Geosci. Model Dev., 9, 1153-1200
Jöckel, P., H. Tost, A. Pozzer, M. Kunze, O. Kirner, C. A. M. Brenninkmeijer, S. Brinkop, D. S. Cai, C. Dyroff, J. Eckstein, F. Frank, H. Garny, K.-D. Gottschaldt, P. Graf, V. Grewe, A. Kerkweg, B. Kern, S. Matthes, M. Mertens, S. Meul, M. Neumaier, M. Nützel, S. Oberländer-Hayn, R. Ruhnke, T. Runde, R. Sander, D. Scharffe, and A. Zahn
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(2016), Is the Brewer-Dobson circulation increasing or moving upward?, Geophys. Res. Lett., 43, 1772–1779
Oberländer-Hayn, S., E. P. Gerber, J. Abalichin, H. Akiyoshi, A. Kerschbaumer, A. Kubin, M. Kunze, U. Langematz, S. Meul, M. Michou, O. Morgenstern, and L. D. Oman
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(2016), Transport pathways from the Asian monsoon anticyclone to the stratosphere, Atmos. Chem. Phys., 16, 2703-2718
Garny, H. and W. J. Randel
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(2017), A new time-independent formulation of fractional release, Atmos. Chem. Phys., 17, 3785-3797
Ostermöller, J., H. Bönisch, P. Jöckel, and A. Engel
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(2017), A refined method for calculating Equivalent Effective Stratospheric Chlorine, Atmos. Chem. Phys.
Engel, A., H. Bönisch, J. Ostermöller, S. Dhomse, M. P. Chipperfield, and P. Jöckel, P.
