ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-9659-2011 Response of the Antarctic stratosphere to warm pool El Niño Events in the GEOS CCM Hurwitz M. M. 1 2 6 Song I.-S. 2 3 7 Oman L. D. 2 Newman P. A. 2 Molod A. M. 2 4 Frith S. M. 2 5 Nielsen J. E. 2 5 NASA Postdoctoral Program, NASA Goddard Space Flight Center, Greenbelt, MD, USA NASA Goddard Space Flight Center, Greenbelt, MD, USA Goddard Earth Sciences and Technology Center (GEST), University of Maryland, Baltimore County, Baltimore, MD, USA Earth System Science Interdisciplinary Center (ESSIC), University of Maryland, College Park, College Park, MD, USA Science Systems and Applications, Inc., Lanham, MD, USA now at: NASA Goddard Earth Sciences Technology and Research (GESTAR), Morgan State University, Baltimore, MD, USA now at: Next Generation Model Development Project, Seoul, South Korea 19 09 2011 11 18 9659 9669 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/9659/2011/acp-11-9659-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/9659/2011/acp-11-9659-2011.pdf

The Goddard Earth Observing System Chemistry-Climate Model, Version 2 (GEOS V2 CCM) is used to investigate the response of the Antarctic stratosphere to (1) warm pool El Niño (WPEN) events and (2) the sensitivity of this response to the phase of the QBO. A new formulation of the GEOS V2 CCM includes an improved general circulation model and an internally generated quasi-biennial oscillation (QBO). Two 50-yr time-slice simulations are forced by repeating annual cycles of sea surface temperatures and sea ice concentrations composited from observed WPEN and neutral ENSO (ENSON) events. In these simulations, greenhouse gas and ozone-depleting substance concentrations represent the present-day climate. The modelled responses to WPEN, and to the phase of the QBO during WPEN, are compared with NASA's Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis. <br><br> WPEN events enhance poleward tropospheric planetary wave activity in the central South Pacific region during austral spring, leading to relative warming of the Antarctic lower stratosphere in November/December. During the easterly phase of the QBO (QBO-E), the GEOS V2 CCM reproduces the observed 4–5 K warming of the polar region at 50 hPa, in the WPEN simulation relative to ENSON. <br><br> In the recent past, the response to WPEN events was sensitive to the phase of the QBO: the enhancement in planetary wave driving and the lower stratospheric warming signal were mainly associated with WPEN events coincident with QBO-E. In the GEOS V2 CCM, however, the Antarctic response to WPEN events is insensitive to the phase of the QBO: the modelled response is always easterly QBO-like. The QBO signal does not extend far enough into the lower stratosphere and upper troposphere to modulate convection and thus planetary wave activity in the south central Pacific.

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