Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 3 2008 10.5194/acpd-8-10999-2008 http://www.atmos-chem-phys-discuss.net/8/10999/2008/ http://www.atmos-chem-phys-discuss.net/8/10999/2008/acpd-8-10999-2008.html http://www.atmos-chem-phys-discuss.net/8/10999/2008/acpd-8-10999-2008.pdf 10999 11037 2008-06-09 Validation of water vapour transport in the tropical tropopause region in coupled Chemistry Climate Models S. Kremser M. Rex markus.rex@awi.de U. Langematz M. Dameris I. Wohltmann Stiftung Alfred-Wegener Institute for Polar and Marine Research, Potsdam, Germany Institut für Meteorologie, Freie Universität Berlin, Berlin, Germany Institut für Physik der Atmosphäre, DLR-Oberpfaffenhofen, Wessling, Germany In this study backward trajectories from the tropical lower stratosphere were calculated for the Northern Hemisphere (NH) winters 1995–1996, 1997–1998 (El Niño) and 1998–1999 (La Niña) and summers 1996, 1997 and 1999 using both ERA-40 reanalysis data of the European Centre for Medium-Range Weather Forecast (ECMWF) and coupled chemistry climate model (CCM) data. The calculated trajectories were analyzed to determine the distribution of points where individual air masses encounter the minimum temperature and thus minimum water vapour mixing ratio during their ascent through the tropical tropopause layer (TTL) into the stratosphere. The geographical distribution of these dehydration points and the local conditions there determine the overall water vapour entry into the stratosphere. Results of two CCMs are presented: the ECHAM4.L39(DLR)/CHEM (hereafter: E39/C) from the German Aerospace Center (DLR) and the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (hereafter: FUB-CMAM-CHEM). In the FUB-CMAM-CHEM model the minimum temperatures are overestimated by about 7 K in Northern Hemisphere (NH) winter as well as in NH summer, resulting in too high water vapour entry values compared to ERA-40. However, the geographical distribution of dehydration points is fairly reproduced for NH winter 1995–1996 and 1998–1999 and in all boreal summers. The distribution of dehydration points suggests an influence of the Indian monsoon upon the water vapour transport. The E39/C model displays a temperature bias of about +3 K. 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