Climatologies of subtropical mixing derived from 3D models V. Eyring1, M. Dameris1, V. Grewe1, I. Langbein2, and W. Kouker2 1DLR Institut für Physik der Atmosphäre, Oberpfaffenhofen, D-82234 Wessling, Germany 2Forschungszentrum Karlsruhe, Institut für Meteorologie und Klimaforschung, 76344 Eggenstein-Leopoldshafen, Germany
Abstract. Fingerlike structures reaching from lower into extra-tropical latitudes significantly
contribute to the tropical-extratropical exchange of air masses. This is also an
exchange of upper tropospheric and stratospheric air. Those so called streamers can,
on a horizontal plane, be detected in N2O or O3 since they are characterised by high
N2O or low O3 values compared to undisturbed mid-latitude values. A climatology of
streamer events has been established, employing the chemical-transport model
KASIMA, which is driven by ECMWF re-analyses (ERA) and operational analyses.
For the first time, the seasonal and geographical distribution of streamer frequencies has been determined on the basis of 9 years of
For the current investigation, a meridional gradient criterion has been newly
formulated and applied to the N2O distributions calculated with
KASIMA. A climatology has been derived by counting all streamer events between 21
and 25 km for the years 1990 to 1998. The results have been compared with a streamer climatology which has been established in the same
way employing data of a multi-year simulation with the coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM (E39/C). Both
climatologies are qualitatively in agreement, in particular in the northern hemisphere, where much higher streamer frequencies are found in
winter than in summer. In the southern hemisphere, the KASIMA analyses indicate
strongest streamer activity in September. E39/C streamer frequencies clearly
displays an offset from June to October, pointing to model deficiencies with
respect to tropospheric dynamics. KASIMA and E39/C results agree well from
November to May. Some of the findings give strong indications that the streamer events found
in the altitude region between 21 and 25 km are mainly forced from the troposphere and are
not directly related to the dynamics of the stratosphere, in particular not to the dynamics
of the polar vortex.
Sensitivity simulations with E39/C, which represent recent and possible future atmospheric conditions, have been employed to answer the question how
climate change would alter streamer frequencies. This shows that the seasonal cycle does not change but that significant changes occur in months
of minimum and maximum streamer frequencies. This could have an impact on the mid-latitude distribution of chemical tracers and compounds.
Citation: Eyring, V., Dameris, M., Grewe, V., Langbein, I., and Kouker, W.: Climatologies of subtropical mixing derived from 3D models, Atmos. Chem. Phys., 3, 1007-1021, doi:10.5194/acp-3-1007-2003, 2003.