Role of Nonlinear Dynamics in Accelerated Warming of Great Lakes

In recent decades, the Laurentian Great Lakes have undergone rapid surface warming with the summertime trends substantially exceeding the warming rates of surrounding land. Warming of the deepest Lake Superior was the strongest, and that of the shallowest Lake Erie—the weakest of all lakes. We investigate the dynamics of accelerated lake warming in idealized coupled thermodynamic lake–ice–atmosphere models. These models are shown to exhibit, under identical seasonally varying forcing, multiple possible stable equilibrium cycles, or regimes, with different maximum summertime temperatures and varying degrees of wintertime ice cover. The simulated lake response to linear climate change in the presence of the atmospheric noise rationalizes the observed accelerated warming of the lakes, the correlation between wintertime ice cover and next summer’s lake-surface temperature, as well as higher warming trends of the (occasionally wintertime ice-covered) deep-lake vs. shallow-lake regions, in terms of the corresponding characteristics of the forced transitions between colder and warmer lake regimes. Since the regime behavior in the models considered arises due to nonlinear dynamics rooted in the ice–albedo feedback, this feedback is also the root cause of the accelerated lake warming simulated by these models.