Winter thermal structure across the Laurentian Great Lakes

Abstract

The formation of winter stratification and thermal structure in general across the Great Lakes varies in character not just between lakes, but interannually within individual lakes. Three large datasets comprise all of the publicly available Great Lakes water temperature data that span both the winter and the entire water column. Multiple sites and multiple years of data are available for Lake Superior, as well as multiple years in Lake Huron and Lake Michigan, 2 years in Lake Ontario at multiple sites, and a single year at two sites in Lake Erie. The lakes show diverse manifestations of winter stratification, with Lake Superior reliably forming winter stratification, Lake Michigan rarely forming stratification, and Huron forming stratification in about half of the winters for which data are available (there is not enough data to evaluate this for Erie and Ontario). Whether a lake forms stratification or not in a given year is governed by how much heat a lake loses below the temperature of maximum density; a heat content of roughly −1 GJm⁻² relative to the temperature of maximum density appears to be a threshold for the formation of winter stratification. Minimum heat content in a given year is a strong function of average winter air temperature. When combined with a historical database of basin-wide air temperature, the winter stratification threshold can be used to hindcast stratification formation in Superior, Huron, and Michigan over the last century, showing that Michigan and Huron are currently undergoing a climate-driven shift in stratification status.