Reducing Surface Wetness Leads to Tropical Hydrological Cycle Regime Transition

Abstract:

The modern "wet" tropics are dominated by the InterTropical Convergence Zone (ITCZ), however "dry" tropics likely occurred in Earth's history, e.g. during Snowball Earth. Since recent progress in our understanding of the ITCZ has focused on modern and warmer wet climates it is unclear how the tropical hydrological cycle changes between wet and dry climate. Here we show that 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 the cancellation of negative evaporation and meridional moisture flux convergence. In contrast to our modern understanding, which assumes changes in relative humidity are small, the regime transition is dominated by changes in the near-surface relative humidity. We show the near-surface relative humidity changes are controlled by re-evaporation of stratiform rain using mechanism denial experiments. The moistening effect of re-evaporation is non-local: re-evaporation happens aloft near the lifting condensation level, and moisture diffuses downward to the near-surface. Overall our results provide a first step toward understanding tropical hydrological cycle changes between wet and dry climates.