Impact of evolving subglacial hydrology on marine ice sheet dynamics

While basal friction of glaciers is strongly controlled by the state of the subglacial hydrologic system, most large-scale ice sheet models do not explicitly model subglacial drainage and its effects on basal traction. Instead, basal friction is either prescribed or calculated through simple drainage assumptions. Though it has been demonstrated that modeled ice sheet evolution is strongly affected by the choice of basal friction law employed, there has been little investigation into the impact of explicitly modeling subglacial hydrologic conditions. Here we use the coupled subglacial drainage and ice dynamics solvers in the MPAS-Albany Land Ice (MALI) model to explore how explicitly modeling subglacial drainage conditions affects the evolution of marine ice sheets. Using a simple test case following the MISMIP+ protocol, we show that coupling basal drainage to ice dynamics can lead to either steady or oscillatory equilibrium behavior under steady forcing. Including coupled subglacial hydrology leads to faster ice retreat under perturbations than assuming a fixed basal friction field, but less retreat than assuming the drainage system is always well-connected to the ocean. In conducting model parameter sensitivity tests, we find hydraulic conductivity to be a key parameter in affecting system behavior. Similar behavior is found when the model is then applied in a more realistic application to Thwaites Glacier, suggesting models that assume fixed basal friction fields could be underestimating mass loss in scenarios of glacier retreat.