Characteristics, Influence, and Sensitivity of Ice Cover on the Great Lakes

Abstract

Three distinct aspects of ice cover on the Great Lakes are examined, including the influence of ice cover on water column processes, the characteristics of ice cover on Lake Superior during a high-ice year, and the sensitivity of ice on the Great Lakes to climate. Three subsurface moorings were deployed in Lakes Superior during the record-high ice year of 2013-2014, and datasets from these moorings are analyzed alongside remotely-sensed ice cover data and regional meteorological observations. Ice cover on Lake Superior is shown to be predominately free-drifting, behaving more like ice on the oceans than ice on smaller lakes. Pressure sensors on the moorings directly measured deep ice keels, which frequently occurred at depths of more than 6m and occasionally occurred at depths greater than 11m. Late-season ice on Lake Superior during the 2013-2014 winter delayed spring warming, which is a mechanism that has previously been identified, but not directly observed. Ice cover is shown to influence surface currents on Lake Superior, resulting in a large-scale redistribution of heat in the western basin of the lake. Ice cover on the Great Lakes shows a linear sensitivity to air temperature, both empirically and through three-dimensional hydrodynamic modeling. Small variations in seasonal air temperature, on the order of 1 to 2°C, can be the difference between a moderate to high ice year and a very low ice year. Air temperatures during the time of ice formation have a significant influence on the amount of seasonal-average ice cover, while air temperatures during the remainder of the year do not greatly affect the amount of ice that forms. Shallow regions of the Great Lakes (<10m) exhibit similar sensitivity to air temperature as do smaller inland lakes, but the absolute amount of ice that forms on each of these different systems varies.