Atmospheric Chemistry and Physics The impact of tropical recirculation on polar composition

• Main Authors: S. E. Strahan, M. R. Schoeberl, S. D. Steenrod
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2009
• Subjects: Antarctic; Simu; The Antarctic; Antarc*
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.608.3399; http://www.atmos-chem-phys.net/9/2471/2009/acp-9-2471-2009.pdf

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 eval-uation of the vertical transport rate and horizontal recircu-lation into the tropics between 16–32 km. These diagnos-tics are applied to two Global Modeling Initiative (GMI) chemistry and transport model (CTM) age tracer simulations to give new insights into the tropical transport characteris-tics 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 hav-ing 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 recircu-lation affects polar composition in both CTM experiments. Better tropical recirculation tends to improve the CH4 simu-lation in the Antarctic. However, mean age in the Antarctic lower stratosphere can be compromised by poor representa-tion of tropical ascent, tropical recirculation, or vortex barrier strength. The connection between polar and tropical compo-sition shown in this study demonstrates the importance of di-agnosing each of these processes separately in order to verify the adequate representation of the processes contributing to polar composition in models.