Understanding the Feedback Between Stratification and Lake Surface Temperature in the Great Lakes

English Abstract:

During the last decades the Laurentian Great Lakes of North America have been experiencing a significant rise in temperature. Particularly relevant was the extraordinary warming that affected Lake Superior in summer 1998. This event can be taken as a useful example to understand the marked response of lakes to increasingly warmer atmospheric conditions.

In particular, the focus of this work is on investigating the existing feedbacks between lake surface temperature and stratification in lakes. The analysis has been carried out by using air2water, a simple model that has been recently proposed to predict lake surface temperature (LST) on the basis of air temperature solely. Model’s performance matches that typical of more complex process-based models, which however require a larger amount of input data: the RMSE between observed and simulated LST is in the order of 1°C. Besides LST, air2water also provides information about the seasonal stratification dynamics, thus representing a valid tool for the purposes of this work.

Results suggest that the unusual 1998 warming in Lake Superior is a consequence of two combined factors: extremely high air temperature in summer and increased strength of stratification resulting from a warm spring. In fact, a reduction of the volume that effectively concurs to the variation of surface water temperature causes a positive feedback between the thermal behavior and the stratification dynamics of the lake. In the light of the significant results obtained for Lake Superior, the analysis is extended to all the Great Lakes, providing some hints about the role of morphology (in particular the mean depth) in influencing their thermal behavior.