Evaluation of transport in the lower tropical stratosphere in a global chemistry and transport model

[1] Off-line chemistry and transport models (CTMs) use meteorological information from a general circulation model (GCM) or from a data assimilation system (DAS) to calculate the evolution of stratospheric constituents. Here constituent fields from two CTM simulations are compared with each other and with observations from satellite, aircraft, and sondes to judge the realism of the tropical transport. One simulation uses winds from a GCM and the second uses winds from a DAS that has the same GCM at its core. A simulation using the GCM fields reproduces many observed features for O3, CH4, and the age of air. The same comparisons for a simulation using DAS fields show rapid upward tropical transport and excessive mixing between the tropics and middle latitudes. The assimilation system changes the temperature and wind fields to produce consistency between a GCM forecast and observations, behaving like an additional forcing has been added to the equations of motion and possibly leading to the unrealistic transport produced by the DAS fields. These comparisons highlight aspects of the transport in the lower tropical stratosphere, and suggest that while a CTM driven by DAS fields provides good short-term simulations when event-by-event comparisons with observations are desired, a CTM driven by GCM fields may be more appropriate for long-term calculations such as required to assess the impact of changes in stratospheric composition.