Reducing Surface Wetness Leads to Tropical Hydrological Cycle Regime Transition

Abstract

AbstractEarth’s modern climate is characterized by wet, rainy deep tropics, however, paleoclimate and planetary science have revealed a wide range of hydrological cycle regimes connected to different external parameters. Here we investigate how surface wetness affects the tropical hydrological cycle. When surface wetness is decreased in an Earth‐like general circulation model, the tropics remain wet but the transition from a rainy to a rain‐free regime. The rain‐free regime occurs when surface precipitation is suppressed as negative evaporation (surface condensation) balances moisture flux convergence. The regime transition is dominated by near‐surface relative humidity changes in contrast to the hypothesis that relative humidity changes are small. We show near‐surface relative humidity changes responsible for the regime transition are controlled by re‐evaporation of stratiform precipitation near the lifting condensation level. Re‐evaporation impacts the near‐surface through vertical mixing. Our results reveal a new rain‐free tropical hydrological cycle regime that goes beyond the wet/dry paradigm.