Seasonal variation of methane, water vapor, and nitrogen oxides near the tropopause: Satellite observations and model simulations

Seasonal variations of several trace constituents near the tropopause are analyzed based on satellite measurements, and results are compared to a recent numerical model simulation. We examine methane, water vapor, and nitrogen oxides (NO_x) derived from Halogen Occultation Experiment (HALOE) satellite observations; these species have strong gradients near the tropopause, so that their seasonality is indicative of stratosphere-troposphere exchange (STE) and circulation in the near-tropopause region. Model results are from the Model for Ozone and Related Chemical Tracers (MOZART) stratosphere-troposphere chemical transport model (CTM). Results show overall good agreement between observations and model simulations for methane and water vapor, whereas nitrogen oxides near the tropopause are much lower in the model than suggested by HALOE data. The latter difference is probably related to the lightning and convective parameterizations incorporated in MOZART, which produce NO_x maxima not near the tropopause, but in the upper troposphere. Constituent seasonal variations highlight the imporatance of the Northern Hemisphere (NH) summer monsoons as regions for transport into the lowermost stratosphere. In MOZART, there is clear evidence that air from the monsoon region is transported into the tropics and entrained into the upward Brewer-Dobson circulation, bypassing the tropical tropopause.