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Water vapour transport in the tropical tropopause region in coupled Chemistry-Climate Models and ERA-40 reanalysis data

by Stefanie Kremser, Ingo Wohltmann, Markus Rex, Ulrike Langematz, Martin Dameris, Markus Kunze
Atmos. Chem. Phys. ()
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In this study backward trajectories from the tropical lower stratosphere\nwere calculated for the Northern Hemisphere (NH) winters 1995-1996,\n1997-1998 (El Nino) and 1998-1999 (La Nina) and summers 1996, 1997 and\n1999 using both ERA-40 reanalysis data of the European Centre for\nMedium-Range Weather Forecast (ECMWF) and coupled Chemistry-Climate\nModel (CCM) data. The calculated trajectories were analysed to determine\nthe distribution of points where individual air masses encounter the\nminimum temperature and thus minimum water vapour mixing ratio during\ntheir ascent through the tropical tropopause layer (TTL) into the\nstratosphere. The geographical distribution of these dehydration points\nand the local conditions there determine the overall water vapour entry\ninto the stratosphere. Results of two CCMs are presented: the\nECHAM4.L39(DLR)/CHEM (hereafter: E39/C) from the German Aerospace Center\n(DLR) and the Freie Universitat Berlin Climate Middle Atmosphere Model\nwith interactive chemistry (hereafter: FUB-CMAM-CHEM). In the\nFUB-CMAM-CHEM model the minimum temperatures are overestimated by about\n9 K in NH winter and about 3 K in NH summer, resulting in too high water\nvapour entry values compared to ERA-40. However, the geographical\ndistribution of dehydration points is fairly similar to ERA-40 for NH\nwinter 1995-1996 and 1998-1999. The distribution of dehydration points\nin the boreal summer 1996 suggests an influence of the Indian monsoon\nupon the water vapour transport. The E39/C model displays a temperature\nbias of about +5 K. Hence, the minimum water vapour mixing ratios are\nhigher relative to ERA-40. The geographical distribution of dehydration\npoints is fairly well in NH winter 1995-1996 and 1997-1998 with respect\nto ERA-40. The distribution is not reproduced for the NH winter\n1998-1999 (La Nina event) compared to ERA-40. There is an excessive\nwater vapour flux through warm regions e. g. Africa in the NH winter and\nsummer. The possible influence of the Indian monsoon on the transport is\nnot seen in the boreal summer 1996. Further, the residence times of air\nparcels in the TTL were derived from the trajectory calculations. The\nanalysis of the residence times reveals that in both CCMs residence\ntimes in the TTL are lower compared to ERA-40 and the seasonal variation\nis hardly present.

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