Warm-season soil moisture-temperature coupling in historical and future climate projections

Monday, 15 December 2014
Ian N Williams, Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Anomalously dry soil moisture can lead to reduced evapotranspiration and increased surface temperatures. These higher temperatures have been hypothesized to further reduce soil moisture, enhance temperature variability, and prolong dry extremes, particularly in warmer climates. Historical and future climate projections from the Coupled Model Inter-comparison Project (CMIP5) were investigated in this study to identify climate trends attributable to soil moisture-temperature coupling and feedback processes. It was found that the probability of temperature extremes (departures from seasonal averages) increases under global warming, and the time taken to return toward average warm-season temperatures following temperature extremes (i.e. persistence) increases by more than 25% over many land regions. An analysis of changes in surface energy balance suggests that this increased persistence of warm temperature extremes results from decreased surface longwave radiative cooling at warm temperature extremes under global warming. This process was also found in radiative forcing experiments using a single-column climate model having a realistic radiation scheme and a simplified land surface model, where the sensitivity of extremes to the relationship between soil moisture and transpiration was further investigated. The persistence of warm extremes depends on the soil moisture content at stomatal closure (wilting point), where higher wilting points lead to larger reductions in surface longwave emission at warm extremes and increased persistence of extremes. These results highlight the importance of surface radiation regime and vegetation in soil moisture-temperature feedbacks.