Mechanistic patterns of land hydroclimate changes in a changing climate

Climate model predictions of land hydroclimate changes show large geographic heterogeneity, and differences between models are large. We introduce a new process-oriented phase space that reduces the dimensionality of the problem but preserves (and emphasizes) the mechanistic relations between variables. This transform from geographical space to climatological aridity index (AI) and daily soil moisture (SM) percentiles allows for interpretation of local, daily mechanistic relations between the key hydroclimatic variables in the context of time-mean and/or global-mean energetic constraints and the wet-get-wetter/dry-get-drier paradigm. Focusing on the tropics (30S-30N), we show that simulations from 16 different CMIP models exhibit coherent patterns of change in the AI/SM phase space that are aligned with the established soil-moisture/evapotranspiration regimes. Results indicate the need to introduce an active-rain regime as a special case of the energy-limited regime. In response to CO₂-induced warming, rainfall only increases in this regime, and this temporal rainfall repartitioning is reflected in an overall decrease in soil moisture. Consequently, the regimes where SM constrains evapotranspiration become more frequently occupied, and hydroclimatic changes align with the position of the critical soil moisture value in the AI/SM phase space. Analysis of land hydroclimate changes in CMIP6 historical simulations in the AI/SM phase space reveal the very different impact of CO₂ forcing and aerosol forcing. CESM2 Single Forcing Large Ensemble Experiments are used to understand their roles.