Simple and complex models are used to try to understand the processes controlling water vapor in the tropical tropopause region. Analytic models, chemical transport models and general circulation models are examined. Many models are able to reproduce the observed large-scale relationships between temperatures, clouds and water vapor. Variability on seasonal to inter-annual scales is also reproduced.

The successes and failures of various models allow several insights into important processes. Transport of water substance in 3 dimensions through the tropopause region is critical for understanding observed relationships between temperatures and clouds. Cloud microphysics may also affect the final distribution of water vapor entering the stratosphere.

These findings also shed some light on possible causes of the anomalous increase in stratospheric water vapor, and whether it will continue in the future. The impact of tropopause region water vapor on the radiative balance of the tropics and how it might change will also be discussed.