Summer water temperature variability of the Great Lakes: Seasonal footprinting versus synchronous summertime atmospheric forcing

Water bodies often exhibit less temperature variability than air over proximate land at time scales ranging from diurnal cycle to secular trend, due to the large heat capacity of water. One exception to this is the Laurentian Great Lakes. Observations from recent decades suggest that the surface of the Great Lakes warmed more rapidly than the air over surrounding land and showed greater year-to-year variability in summer. The pioneering study of Austin and Colman (2007) shed light on this paradox by empirically relating summer water temperatures to springtime stratification, and further, to ice cover in the preceding winter. However, a clear physical picture of this 'seasonal footprinting' and its role in the summer water temperature variability are still elusive. Here, we employ a regional climate model with interactive lake processes to demonstrate the feasibility of the seasonal footprinting mechanism, and to investigate its significance for the summer water temperature variability compared with synchronous summertime atmospheric forcing. We explore the relative roles of these two influences in the context of the amplified warming trend of the Great Lakes in recent decades, characterized by the presence of an approximate step-change in lake temperature and ice cover between 1997 and 1998.