Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 1 2008 10.5194/acpd-8-489-2008 http://www.atmos-chem-phys-discuss.net/8/489/2008/ http://www.atmos-chem-phys-discuss.net/8/489/2008/acpd-8-489-2008.html http://www.atmos-chem-phys-discuss.net/8/489/2008/acpd-8-489-2008.pdf 489 520 2008-01-10 Seasonal and inter-annual variations in Troposphere-to-Stratosphere Transport from the Tropical Tropopause Layer J. G. Levine javi@bas.ac.uk P. Braesicke N. R. P. Harris J. A. Pyle Centre for Atmospheric Science, Dept. of Chemistry, Univ. of Cambridge, Cambridge, UK Atmospheric Chemistry Modelling Support Unit, National Centre for Atmospheric Science, University of Cambridge, Cambridge, UK European Ozone Research Coordinating Unit, University of Cambridge, Cambridge, UK now at: the British Antarctic Survey, Cambridge, UK In an earlier study of troposphere-to-stratosphere transport (TST) via the tropical tropopause layer (TTL), we found that the vast majority of air parcels undergoing TST from the base of the TTL enter the extratropical lowermost stratosphere quasi-horizontally and show little or no regional preference with regards to origin in the TTL or entry into the stratosphere. We have since repeated the trajectory calculations - originally limited to a single northern hemisphere winter period - in a variety of months and years to assess how robust our earlier findings are to change of timing. To first order, we find that the main conclusions hold, irrespective of the season, year and phase of the El Niño Southern Oscillation (ENSO). We also explore: the distribution of TST between the northern and southern hemispheres; the sensitivity of modelled TST to the definition of the tropopause; and the routes by which air parcels undergo transport exclusively to the stratospheric overworld. Subject to a dynamical definition of the tropopause, we identify a strong bias towards TST in the southern hemisphere, particularly during the northern hemisphere summer. The main difference on switching to the World Meteorological Organization's thermal tropopause definition is that much less TST is modelled in the subtropics and, relative to the dynamical definition, we calculate significantly less transport into the extratropical lowermost stratosphere (ELS) – an important region with regards to ozone chemistry. 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