Summer Dryness under a Warmer Climate over North America

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0450341EltahirOne of the most significant potential impacts of the global warming predictions by GCMs is the dryness of soil moisture in mid-latitudes during summer (e.g. Manabe et al., 1981; Wetherald and Manabe 1995). If this prediction proves to be true, it would have serious implications on water resources and agriculture in some of the most productive regions of the world. However, this prediction is also one of the least certain due to the complex set of interactions and feedbacks that shape the climatology of soil moisture in mid-latitudes (Seneviratne et al., 2002). These interactions involve soil hydrology, moist convection, and cloud dynamics. All are processes that need to be better represented in climate models.The role of soil hydrology in regulating the process of soil moisture drying will be investigated by studying the following hypothesis. If summer evaporation is limited by the availability of radiative energy to fuel the transformation of liquid water at the surface into water vapor, an atmospheric warming would lead to an increase in evaporation. Further, if this increase in evaporation is larger than the corresponding increase in rainfall, it would result in an initial drying of the soil moisture, which could be amplified by the soil moisture-rainfall feedback mechanism. However, if summer evaporation is limited by soil hydrology, or if the initial increase in evaporation is smaller than the corresponding increase in rainfall, any warming would result in little or no drying. We hypothesize that the latter set of conditions are more prevalent over North America. In the course of the proposed project, we will test this hypothesis to determine if, and under what conditions, would soil hydrology play any role in this global change process. Results of any study on summer dryness would depend to some extent on the skill of the land surface model. In the proposed project, the Global Soil Wetness Project Phase 2 simulations will be evaluated over North America by comparing to observations. Our analysis will determine the skill to which the land models reproduce the observed climate of surface hydrology. From these results, a more comprehensive analysis will look closely at the mechanisms by which hydrologic anomalies are captured by the models, and whether there is uniqueness in model features that results in high skill. This proposal has several intellectual merits. We propose to focus on the role of hydrologic processes in the predicted drying of soil moisture in mid-latitudes during summer. We will consider several potential global warming scenarios in a modeling approach that focuses on North America. We will test the significance of the drying process, and we will attempt to identify the physical mechanisms responsible for it. We propose to use a regional climate model that has been calibrated and tested against remote sensing data and other sources of observations (Pal and Eltahir, 2001). The broader impacts anticipated from the proposed activity include a better understanding and reduction in uncertainty about a key global change process, and education of graduate students in the general area of global climate change.