The coupling of the stratosphere and troposphere in the tropical tropopause region is examined to understand the processes regulating stratospheric humidity and upper tropospheric humidity. Observations of temperatures, winds, water vapor and clouds are compared to simple models of transport and dehydration/sedimentation processes in the region. A simple model of cirrus clouds and dehydration along trajectories is able to reproduce the observed large-scale relationships between temperatures, clouds and water vapor. The model can also be used to predict distributions of the isotopic depletion of water vapor in the tropopause region. Variability on seasonal to inter-annual scales is also reproduced. The successes and failures of the model 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.