Historic and projected changes in coupling between soil moisture and evapotranspiration (ET) in CMIP5 models confounded by the role of different ET components

Abstract

The coupling of soil moisture (SM) and evapotranspiration (ET) is a critical process of the terrestrial climate and water cycle, whose simulation in climate models exhibits substantial uncertainties. Here we investigate, across phase 5 of the Coupled Model Intercomparison Project models in present-day and future simulations, how this coupling manifests itself across the different components of ET: soil evaporation, transpiration, and canopy interception. We characterize summertime SM-ET coupling by (interannual) correlations, which we decompose into terms attributable to each ET component. The transpiration and soil evaporation terms share similar spatial patterns, but the contribution of transpiration, globally, is less positive. Canopy interception contributes a positive term to SM-ET coupling, reflecting the noncausal, rainfall-forced positive correlation between SM and canopy interception. Model differences are greatest for the transpiration term, which explains most of the model spread in SM-ET coupling. Models project a robust pattern of more positive SM-ET correlations in the future. In parts of the midlatitudes and Tropics, this increase reflects reduced precipitation and increased SM limitation on transpiration and soil evaporation. However, at higher latitudes (north of 50°N), increased SM-ET coupling is driven by the increased contribution of canopy interception induced by the increase in vegetation and precipitation. Analysis of ET partitioning is thus essential to the interpretation of simulated changes in ET and its drivers: While increased SM-ET correlations may suggest a widespread increase in SM limitation on ET in a warmer world, increases in actual SM control on land-atmosphere water fluxes are generally limited to regions of negative precipitation change.

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