Evaluating the Contribution of Nearshore Offshore Exchange to Lake Superior Warming

Abstract:

Lake Superior is one of the largest lakes in the world and provides important benefits such as clean water, recreation, and waterway transportation between neighboring communities in the U.S. and Canada. Temperatures recorded at mid-lake buoys indicate the date of annual summer stratification is arriving earlier and surface temperatures are increasing. In this study, we use a realistically configured, 3D model of Lake Superior to characterize the exchange of nearshore waters with offshore waters and evaluate the hypothesis that warmer coastal waters, transported offshore, contribute to warming the surface waters of the central lake basin. There are relatively few studies on Superior’s circulation and it is unclear how long nearshore waters remain along the coast, an important timescale influencing the lake’s water quality and biogeochemistry. Preliminary results indicate that nearshore waters have months-long residence time in select locations along the southern coast where coastline and bottom slope seem to favor sluggish circulation. Nearshore-offshore exchange occurs rapidly when counterclockwise coastal circulation sets in during the summer stratified period. We assess the contribution of this exchange to lake-wide surface temperatures and biogeochemistry.