THE ROLE OF SOIL MOISTURE TRANSITION ZONES IN PREDICTING CLIMATE IN NORTH AMERICA

Soil moisture can influence climate through its impact on evaporation and other surface energy fluxes. Work by Koster at al (2004) identified hotspots, or regions of strong coupling between soil moisture and precipitation, one of which is located in the central Great Plains of North America. It is hypothesized that such hotspots occur in transition zones between wet and dry climates where the atmosphere is responsive to precipitation generation, but is also still sensitive to soil moisture and therefore soil moisture can be expected to influence precipitation.

The current work was done using the RegCM3 regional climate model to analyze the key mechanisms driving high sensitivity of soil moisture in these transition zones. Two 30-year simulations were performed over the period 1980 through 2009. The first simulation was performed with soil moisture fully interactive and the second simulation held soil moisture fixed at the climatology of the first simulation allowing for only one-way interaction. The differences between the simulations provide a measure of the strength of the soil moisture – rainfall feedback. The model results show an enhanced sensitivity to soil moisture in the transition zone from the dry western United to the wet eastern United States. This enhanced sensitivity occurs due to shift in soil moisture between a drier soil-controlled regime and a wetter atmosphere-controlled regime. Furthermore, the results demonstrate that vegetation type within the transitions zone plays a critical role in the sensitivity, with crop and grasses being the most sensitive and forests being less sensitive. Knowledge of soil moisture and vegetation states during spring and summer months can improve the predictability of rainfall.