Climatic forcing for recent significant terrestrial drying and wetting

Terrestrial water storage (TWS) experienced a substantial change in the past few decades as detected by the Gravity Recovery and Climate Experiment (GRACE). However, the major causes of this change remain unclear, and none of the current state-of-the-art process-based hydrological models are able to reproduce the significantly drying/wetting trends in GRACE TWS. Here we investigate 12 terrestrial regions that show a significantly drying/wetting trend, using partial least-square regression (PLSR) to relate TWS anomalies to various climatic variables and leaf area index (LAI). Through PLSR modeling, we find changes in LAI, downward longwave radiation (DLW) and precipitation are most strongly associated with a wetting trend. Increases in precipitation appear to be responsible for the wetting trend in tropical/subtropical monsoon regions, while decreases in vegetation transpiration and atmospheric demand appear to be responsible for the wetting trend in extratropical regions. Enhanced atmospheric demand caused by increases in air temperature and the resulting enhanced DLW dominates the significant drying trend in the mid-latitude subtropical drylands, and in the cold and alpine regions. The PLSR modeling also suggests that, over global continents, the climatic forcing factors show a dominant impact on TWS over all the wetting regions, while only four out of the seven drying regions show climate-dominated drying, implying an additional impact of anthropogenic responses to the water stress on drying.