Surface Expression and Apparent Timing of Subglacial Lake Oscillations Controlled by Viscous Ice Flow

Subglacial lakes are bodies of water that form at the base of ice sheets and glaciers. Ice‐surface elevation above these lakes responds to water volume changes, providing one of few ways to monitor subglacial hydrological systems. Here, we use numerical models to compare surface elevation‐derived estimates of subglacial‐lake length, water‐volume change, and highstand or lowstand timing with their true values. Because ice flow influences the surface expression of lake‐volume change, the correspondence between these estimates and their true values depends strongly on ice thickness, volume‐change rate, and basal drag coefficient. For many realistic combinations of these factors, viscous relaxation of the ice‐sheet surface can render lake volume‐changes unobservable with altimetry. Our results highlight the need for new estimation methods that account for the effects of ice flow, as well as improvements to current resolution limitations that render some events unobservable with altimetry. Plain Language Summary Hundreds of lakes lie beneath the Antarctic and Greenland ice sheets between the ice and the underlying bedrock or sediment. When the volume of water in these subglacial lakes changes, the upper surface of the ice sheet can lower or rise in response. Tracking these surface changes is one of the few ways that we can detect water flow beneath ice sheets, which is an important process that influences ice flow and subglacial ecosystems. However, the degree of correspondence between elevation changes and water volume changes is unclear and has not been directly investigated. To remedy this, we use theory and computer simulations to determine how subglacial‐lake water‐volume change influences surface‐elevation change. Our simulations show that slow volume‐change events can result in diminished elevation changes or in a lake being completely unobservable from the surface, especially if the overlying ice is thick. These results test the limits of what we can infer about water flow beneath the ice sheets from surface‐elevation observations above subglacial lakes. Key Points Altimetry‐derived estimates of subglacial lake size and volume change can deviate considerably from their true values due to ice flow Viscous relaxation of topography can force the apparent highstand and lowstand times to precede the true times by months to years Changes in subglacial lake volume are not always observable due to relaxation of the ice‐sheet surface and altimetry resolution lim