Novel, direct observations of ice on Lake Superior during the high ice coverage of winter 2013–2014

During the 2013–2014 winter, the Lake Superior basin experienced record-low air temperatures and record-high ice cover. We present observations from three subsurface moorings which provided a novel view of the ice cover during this extreme winter. Each mooring carried a set of thermistors, and two sites had ADCPs, providing a rare glimpse into the subsurface characteristics and behavior of a large, partially ice-covered lake, including ice drift velocity, keel size and abundance, surface gravity wave suppression, and passive acoustics below the ice sheet. Ice drift velocities were well correlated with nearby wind velocities, with ice speeds approximately 4% of the wind speed and 20° to the right of the wind direction. Passing ice keels displaced the top subsurface float at the western mooring to depths of 6 m on a regular basis, and up to 11 m during one event. Anomalously large signal return strength at two ADCPs occurred sporadically at roughly the same time at two geographically separate locations, but only during periods of ice cover. The distribution of signal strength with depth suggests that these events are actually sounds from the ice cover itself, rather than reflectors in the water. Broadly speaking, ice cover on Lake Superior has more in common with oceanic ice cover than with ice cover on small lakes.