The zonal mean transport of ozone and carbon monoxide (CO) near the tropicaltropopause is investigated using the Whole-Atmosphere Community ClimateModel version 4 (WACCM4). The variability in temperature, ozone and CO inthe model shows good agreement with satellite and balloon observations.Modeled temperature and tracers exhibit large and closely coupled annualcycles in the tropical lower stratosphere, as in the observations. Thethermodynamic and tracer budgets in the model are analyzed based on theTransformed Eulerian Mean (TEM) framework on log-pressure coordinates andalso using the isentropic formulation. Results show that the coupledseasonal cycles are mainly forced by tropical upwelling over altitudes withlarge vertical tracer gradients, in agreement with previous observationalstudies. The model also allows explicit calculation of eddy transport terms,which make an important contribution to ozone tendencies in the tropicallower stratosphere. The character of the eddy fluxes changes with altitude.At higher levels (~2 km above the cold point tropopause),isentropic eddy transport occurs during winter and spring in each hemispherein the sub-tropics, associated with transient Rossby waves acting on strongbackground latitudinal gradients. At lower altitudes, close to the tropicaltropopause, there is a maximum in horizontal eddy transport during borealsummer associated with the Asian monsoon anticyclone. Sub-seasonalvariability in ozone and CO, tied to fluctuations in temperature, isprimarily driven by transient tropical upwelling. In isentropic coordinates,the overall tracer budgets are similar to the log-pressure results,highlighting cross-isentropic advection as the main term in the time-meanbalance, with large seasonality above the tropopause. However, in isentropiccoordinates the tracer variability is largely reduced on both seasonal andsub-seasonal timescales, because tracer fluctuations are highly correlatedwith temperature (as a response to upwelling).
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