Reducing Surface Wetness Leads to Tropical Hydrological Cycle Regime Transition

TL;DR

This study demonstrates that reducing surface wetness in an Earth-like climate model can cause a transition from a rainy to a rain-free tropical regime, driven by near-surface humidity changes and re-evaporation processes.

Contribution

It uncovers a new rain-free tropical hydrological cycle regime and highlights the role of near-surface humidity and re-evaporation in regime transitions.

Findings

Decreased surface wetness leads to rain-free tropical regimes.

Near-surface relative humidity changes are key to the transition.

Re-evaporation of stratiform precipitation influences humidity and regime change.

Abstract

Earth'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 transition from a rainy to 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.