Diagnosing the Drivers of the Recent Rapid Warming of the Great Lakes

The Great Lakes have been warming rapidly over recent decades, even more than surrounding land regions. This accelerated warming trend is most pronounced during summer, where the lakes are deepest. Analyses of in-situ observations, remote sensing, reanalysis data, and a regional climate model suggest that a combination of antecedent, internal lake processes (e.g. earlier stratification) and synchronous, large-scale meteorological forcing (e.g. increased insolation) is responsible for the amplified lacustrine warming trend during summer. This long-term trend (1979-2010) is characterized by an approximate step-change in lake temperature between 1997 and 1998, particularly for the deep Lake Superior. We present an attribution study involving the International Centre for Theoretical Physics Regional Climate Model Version Four (RegCM4) that aims to identify the key physical processes driving the accelerated lake warming and to assess the relative roles of antecedent lake processes during winter-spring and synchronous, large-scale meteorological forcing in summer. The model performs credibly in simulating the step-change in lake temperature between 1997 and 1998, including the enhancement of warming within the Great Lakes compared with air temperature increases over adjacent land. We find that earlier seasonal stratification explains much of the amplified lake warming, particularly at deep lakes points, whereas antecedent ice coverage exerts only a secondary influence. At shallow lake points, large-scale meteorological forcing during summer, especially reductions in cloudiness, appears to play a dominant role in the warming trend.