The modern “wet” tropics are dominated by the Intertropical
Convergence Zone, however “dry” tropics likely occurred in Earth’s
history. It is unclear how the tropics change between wet and dry
climates because recent progress has focused on modern and warmer
climates. We show the tropical hydrological cycle undergoes a wet-to-dry
regime transition when surface wetness is decreased in a general
circulation model. The dry regime occurs when precipitation is
suppressed by negative evaporation. The regime transition is dominated
by near-surface relative humidity, in contrast to our traditional
understanding which assumes changes in relative humidity are small. We
show near-surface relative humidity changes are controlled by
re-evaporation of stratiform precipitation. The moistening effect of
re-evaporation is non-local: re-evaporation happens near the lifting
condensation level and moisture diffuses downward to the near-surface.
Our results provide a first step toward understanding tropical
hydrological cycle changes between wet and dry climates.