Linking Soil Moisture and Summertime Surface Temperature Variability

Abstract

Summertime temperature variability in the late 20th century simulations of the CMIP3 and CMIP5 climate models used in the IPCC assessments is much greater than the observed temperature variability -- particularly in the continental mid-latitudes. In recent years, interactions between landand atmosphere have been shown through climate model simulations to contribute significantly to temperature variability in summer, mainly via processes involving soil moisture. However, theoretical understanding of the mechanisms linking soil moisture and surface temperature variability is incomplete. In this study, we investigate these links using a simple model of land-atmosphere interactions. We derive a simplified expression for the surface temperature variance by coupling the surface energy and the soil moisture budgets via parameterizations of the dominant processes acting at the land surface interface. We identify two regimes of surface temperature variability, distinguishing whether soil moisture-related processes at the land-surface interface amplify or damp surface temperature variability caused by fluctuations in radiative forcing. These two regimes are distinguished by differences in the processes that determine evapotranspiration which, in turn, are governed by the summertime mean soil moisture content. We found that temperature variability errors in two CMIP3 GCMs are mostly due to an incorrect geographical placement of the boundary separating the two temperature variability regimes in the GCMs and to errors in the variance of the radiation forcing and precipitation forcing. The changes in temperature variability that are projected for the 21st century are consistent with our findings: regions that display the largest increase in temperature variability are those that experience a sufficient reduction of soil moisture to shift the soil-moisture related processes at the land-surface interface from the damped regime to the amplified regime.