Atmos. Chem. Phys. Discuss., 9, 1097-1121, 2009
www.atmos-chem-phys-discuss.net/9/1097/2009/
doi:10.5194/acpd-9-1097-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The impact of tropical recirculation on polar composition
S. E. Strahan1, M. R. Schoeberl2, and S. D. Steenrod1
1Goddard Earth Science and Technology Center, University of Maryland, Baltimore County, Baltimore, MD, USA
2NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. We derive the tropical modal age of air from an analysis of the water vapor tape recorder. We combine the observationally derived modal age with mean age of air from CO2 and SF6 to create diagnostics for the independent evaluation of the vertical transport rate and horizontal recirculation into the tropics between 16–32 km. These diagnostics are applied to two Global Modeling Initiative (GMI) chemistry and transport model (CTM) age tracer simulations to give new insights into the tropical transport characteristics of the meteorological fields from the GEOS4-GCM and the GEOS4-DAS. Both simulations are found to have modal ages that are in reasonable agreement with the empirically derived age (i.e., transit times) over the entire altitude range. Both simulations show too little horizontal recirculation into the tropics above 22 km, with the GEOS4-DAS fields having greater recirculation. Using CH4 as a proxy for mean age, comparisons between HALOE and model CH4 in the Antarctic demonstrate how the strength of tropical recirculation affects polar composition in both CTM experiments. The better the tropical recirculation is simulated, the better the CH4 simulation is in the Antarctic. Mean age in the Antarctic lower stratosphere can be compromised by poor representation of tropical ascent, tropical recirculation, or vortex barrier strength. The connection between polar and tropical composition shown in this study demonstrates the importance of diagnosing each of these processes separately in order to verify the adequate representation of the processes contributing to polar composition in models.

Citation: Strahan, S. E., Schoeberl, M. R., and Steenrod, S. D.: The impact of tropical recirculation on polar composition, Atmos. Chem. Phys. Discuss., 9, 1097-1121, doi:10.5194/acpd-9-1097-2009, 2009.
 
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