The Shift of Southern Hemisphere Storminess under Anthropogenic Climate Change around Antarctica and its Impacts - (SACAI)
Final Report Abstract
Extra-tropical cyclones (ETCs) are a dominating factor influencing mid-latitude and polar weather and climate. Especially on the Southern Hemisphere, ETCs are the major source of meridional energy transports. Antarctica, as one of the major area of net energy deficit and being thus one important driver of the earth climate system, is highly sensitive to potential changes in the characteristic and frequency occurrence of ETCs. Extra-tropical cyclone activity could possibly face substantial changes under anthropogenic influences. Their total number is discussed to reduce in both hemispheres, whereas intense ones potentially increase in frequency in specific regions. In the SH, also a shift of cyclone track density is analysed in future anthropogenic scenario simulations for specific climate models. In this project, SH extra-tropical cyclones are investigated systematically by a multi-model ensemble approach, allowing us to assess model-uncertainties. Thus, 9 integrations of 6 European coupled atmosphere-ocean general circulation models (AOGCMs) were analyzed for simulations of the recent climate and the 21st century according to the IPCC SRES A1B scenario. Model validation was done by means of ERA40 reanalysis products. All model integrations show decreasing total numbers of cyclone tracks with respect to the future scenario. Strong cyclones are defined by the 95th percentile of the Laplacian of mean sea level pressure (MSLP) in the 20C time period. These cyclones increase in 7 of 9 integrations (4 of 6 AOGCMs). In the ensemble mean, south of about 60°S increasing cyclone track densit y is found up to 20% in the Indic sector off the Antarctic coast. The decrease takes place north of about 50°S . Strong cyclones especially show increasing frequency in the Indic and Australian sector of the Southern Ocean with a maximum of 40% south of Tasman Sea. To investigate potential impacts on the Antarctic climate, moisture fluxes were investigated and related to cyclone activity, dividing the flux in its mean and transient components. Meridional moisture flux in total is increasing on the whole hemisphere. This increase is dominated by an increase of the specific humidity in a warmer climate. The meridional component of the transient part is orientated poleward on the whole hemisphere with maximum values around 40°S. A scali ng approach enables the possibility to distinguish between dynamical and thermodynamical changes. In regions of changing strong cyclone activity, the dynamical change of the meridional transient moisture flux is dominating. Net precipitation inside the Antarctic Circle (66°33’S), calculated by means of the divergence of the vertically integrated moisture flux, increases in all model integration by about 10%. On the other hand, the dynamical parts of moisture flux changes lead to a decrease of net precipitation by about 10%. This is mainly dominated by decreasing flux between Ross Sea and the Antarctic Peninsula, where the major part of the moisture inflow takes place. North of Amundsen Sea a slightly decreasing track density can be identified, whereas changes in these regions are not significant with respect to the multi-model ensemble mean. To clarify the role of interactions between the stratosphere and the troposphere a set of transient climate simulations has been performed with the chemistry-climate model (CCM) ECHAM Middle Atmosphere Chemistry (EMAC), accounting for different aspects of anthropogenic climate change. The results were analyzed with respect to general cyclone changes and the impact of stratospheric change on Antarctica. In addition, the coupled atmosphere-ocean CCM (AOCCM) EMAC-FUB-O has been implemented and further developed. EMAC is reproducing observed changes in the stratosphere due to ozone changes, as well as in the troposphere, compared to reanalyses and observations, so it is assumed to achieve plausible results for future changes. Regressing the surface trends on the SAM index, which is coherent with the strength of the polar vortex, and on the SST time series, it was found, that upper atmospheric changes have more impact on the Antarctic surface than the changes in the SSTs.
Publications
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Chemistry-climate model simulations of spring Antarctic ozone, J. Geophys. Res., 115, D00M11, 2010
Austin, J., H. Struthers, J. Scinocca, D.A. Plummer, H. Akiyoshi, A.J.G. Baumgaertner, S. Bekki, G.E. Bodeker, P. Braesicke, C. Brühl, N. Butchart, M.P. Chipperfield, D. Cugnet, M. Dameris, S. Dohmse, S. Frith, H. Garny, A. Gettelman, S.C. Hardiman, P. Jöckel, D. Kinnison, A. Kubin, J.F. Lamarque, U. Langematz, E. Mancini, M. Marchand, M. Michou, O. Morgenstern, T. Nakamura, J.E. Nielsen, G. Pitari, J. Pyle, E. Rozanov, T.G. Shepherd, K. Shibata, D. Smale, H. Teyssedre, Y. Yamashita
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Climate change projections and stratosphere-troposphere interaction, Climate Dynamics
Scaife, A. A., T. Spangehl, D. R. Fereday, U. Cubasch, U. Langematz, H. Akiyoshi, S. Bekki, P. Braesicke, N. Butchart, M. P. Chipperfield, A. Gettelmann, S. C. Hardiman, M. Michou, E. Rozanov und T. G. Sheperd
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The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales, Atmos. Chem. Phys., 11, 11221–11235, 2011
Weber, M., S. Dikty, J. P. Burrows, H. Garny, M. Dameris, A. Kubin, J. Abalichin, und U. Langematz
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IMILAST - a community effort to intercompare extratropical cyclone detection and tracking algorithms: assessing method-related uncertainties, Bull. Am. Met. Soc.
Neu, U.; Akperov, M.; Bellenbaum, N.; Benestad, R.; Blender, R.; Caballero, R.; Cocozza, A.; Dacre, H. F.; Feng, Y.; Fraedrich, K.; Grieger, J.; Gulev, S.; Hanley, J.; Hewson, T.; Inatsu, M.; Keay, K.; Kew, S. F.; Kindem, I.; Leckebusch, G. C.; Liberato, M. L.; Lionello, P.; Mokhov, I. I.; Pinto, J. G.; Raible, C. C.; Reale, M.; Rudeva, I.; Schuster, M.; Simmonds, I.; Sinclair, M.; Sprenger, M.; Tilinina, N. D.; Trigo, I. F.; Ulbrich, S.; Ulbrich, U.; Wang, X. L. & Wernli, H.
