We use a trajectory model to diagnose mechanisms that produce the observed and modeled tropical lower stratospheric water vapor seasonal cycle. We confirm that the seasonal cycle of water vapor is primarily determined by the seasonal cycle of tropical tropopause layer (TTL) temperatures. However, between 10° N and 40° N, we find that evaporation of convective ice in the TTL plays a key role contributing to the water vapor seasonal cycle there. The Asian monsoon region is the most important region.