Water Budget and Transfer Among Lake, Atmosphere, and Land in a Regional Climate Model of the North American Great Lakes Basin

We have applied the Coupled Hydrosphere-Atmosphere Research Model (CHARM) to climate change scenarios by using GCM output as boundary conditions. CHARM is a regional climate model with a domain covering the Laurentian Great Lakes, and features coupling of the atmosphere to a spatial array of 1-dimension lake column models. Increased CO₂ concentrations, which drive both the global and the embedded regional model, result in increased temperature of air and surface water. Seasonal effects include a higher proportion of winter precipitation falling as rain rather than snow; decreased length of snow season and year-round snowmelt in some parts of the domain; earlier onset and later termination of a thermally stratified water column; decreased lake ice cover; and increased evaporation from the lakes, in most places large enough to overbalance increased precipitation. In contrast to many previous simulations of the basin's water budget using offline hydrologic models, precipitation minus evapotranspiration is mostly increased over land, but the net change over the basin remains negative. Other phenomena include centers of minimum air temperature increase directly over the lakes during summer.