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GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L14205, doi:10.1029/2004GL019952, 2004

Causes of exceptional atmospheric circulation changes in the Southern Hemisphere

Gareth J. Marshall

British Antarctic Survey, Natural Environment Research Council, Cambridge, Cambridgeshire, UK


Peter A. Stott

Hadley Centre for Climate Prediction and Research, Met Office, University of Reading, Reading, UK


John Turner

British Antarctic Survey, Natural Environment Research Council, Cambridge, Cambridgeshire, UK


William M. Connolley

British Antarctic Survey, Natural Environment Research Council, Cambridge, Cambridgeshire, UK


John C. King

British Antarctic Survey, Natural Environment Research Council, Cambridge, Cambridgeshire, UK


Thomas A. Lachlan-Cope

British Antarctic Survey, Natural Environment Research Council, Cambridge, Cambridgeshire, UK


Abstract

We demonstrate that recent observed trends in the annual and austral summer Southern Hemisphere Annular Mode (SAM) are unlikely to be due to internal climate variability, since they exceed any equivalent-length trends in a millennial General Circulation Model (GCM) control run with constant forcings. In contrast we show that observed trends in the SAM are consistent with the combined effects of anthropogenic and natural forcings in GCM simulations. As these trends begin prior to stratospheric ozone depletion we challenge the assertion that this process is primarily responsible for changes in the SAM. Moreover, anthropogenic forcings have a larger effect on the austral summer SAM in combination with natural forcings than when acting in isolation.

Received 11 March 2004; accepted 7 July 2004; published 30 July 2004.

Index Terms: 1620 Global Change: Climate dynamics (3309); 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3349 Meteorology and Atmospheric Dynamics: Polar meteorology.

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Citation: Marshall, G. J., P. A. Stott, J. Turner, W. M. Connolley, J. C. King, and T. A. Lachlan-Cope (2004), Causes of exceptional atmospheric circulation changes in the Southern Hemisphere, Geophys. Res. Lett., 31, L14205, doi:10.1029/2004GL019952.