Coherent Mechanistic Patterns of Tropical Land Hydroclimate Changes

Changes in tropical (30 S–30 N) land hydroclimate following CO₂-induced global warming are organized according to climatological aridity index (AI) and daily soil moisture (SM) percentiles. The transform from geographical space to this novel process-oriented phase space allows for interpretation of local, daily mechanistic relationships between key hydroclimatic variables in the context of time-mean and/or global-mean energetic constraints and the wet-get-wetter/dry-get-drier paradigm. Results from 16 CMIP models show coherent patterns of change in the AI/SM phase space that are aligned with the established soil-moisture/evapotranspiration regimes. We introduce an active-rain regime as a special case of the energy-limited regime. Rainfall shifts toward larger rain totals in this active-rain regime, with less rain on other days, resulting in an overall SM reduction. Consequently, the regimes where SM constrains evapotranspiration become more frequently occupied, and corresponding hydroclimatic changes align with the position of the critical SM value in the AI/SM phase space.