Simulated Effects of Idealized Laurentian Great Lakes onRegional and Large-Scale Climate*

Abstract

Comparison is made between general circulation model (GCM) cases with and without the inclusion of idealized Great Lakes, in the form of four rectangular bodies of water, each occupying a single grid cell of the GCM at R30 resolution. The presence of idealized Great Lakes, as opposed to land, results in a phase shift in the annual cycle of latent and sensible heat flux. Very high upward sensible heat flux occurs over these idealized Great Lakes during the early winter. On the average over a region encompassing these idealized Great Lakes, evaporation and precipitation increase during the autumn and winter and decrease during the late spring and summer due to the lakes. Annual average water vapor flux convergence increases. The Great Lakes also alter the meridional air temperature gradient. During the autumn and winter, the meridional temperature gradient is intensified to the north of the Great Lakes and diminished to the south. This intensifies the mean jet stream core and displaces it toward the north. This effect is reduced during the winter compared to the autumn because air temperature changes due to the lakes are unable to penetrate as deeply into the strongly stably stratified winter atmosphere. The increase in jet stream speed seems to increase synoptic wave activity to the northeast of the Great Lakes. As an additional experimental case, a swamp surface (saturated surface with no thermal capacity) is used to represent the Great Lakes. In this case there is little effect on the thermal state of the surface and atmosphere and on the fluxes between them. However, there is increased evaporation during the late summer and early autumn and increased precipitation throughout the summer and autumn. Annual water vapor flux convergence in this experimental case is greater than in the case with no lakes.