Determining Land-Atmosphere Coupling Trends in a Changing Climate from Multi-Model Ensembles

Several of the metrics and indices of land-atmosphere coupling, developed substantially through modeling and observational research projects in the Global Energy and Water Cycle Experiment (GEWEX) over the last 20 years, are applied to Coupled Model Intercomparison Project Phase 5 (CMIP5) results. Pre-industrial, historical, and the extreme RCP8.5 future climate projections from 15 GCMs are examined in three categories: links between soil moisture and surface fluxes, links between surface fluxes and near-surface meteorology, and the impact of the land surface on the boundary layer. Because specific models exhibit non-systematic extreme outlier behavior for calculated changes in different quantities, we cannot objectively exclude specific models from a multi-model average of climate change signals. Thus, we instead examine only the agreement among models at each location and season for a positive/negative change in each quantity. This provides a straightforward means for assessing local probability (given the null hypothesis of an equal chance each model would produce a positive or negative change), as well as field significance based on assumptions of the global or regional number of spatial degrees of freedom. Spatio-temporal analysis suggests changes in land-atmosphere coupling over the last 150 years may be primarily associated with either changes in aerosol loading, greenhouse gas increases, or land use changes, depending on location and season. Future projections suggest generally increasing degrees of feedback by the land surface on the atmosphere, and a weakening role of entrainment at the top of the atmospheric boundary layer over continents, driven progressively by the changing background climate. Projected trends toward more hydrologic extremes may result from the stronger positive feedbacks between land and atmosphere.