The Relative Impacts of Initialization and Climate Forcing in Coupled Ice Sheet-Ocean Modeling: Application to Pope, Smith, and Kohler Glaciers

Coupled ice sheet-ocean models are beginning to be used to study the response of ice sheets to ocean warming. Initializing an ice-ocean model is challenging and can introduce nonphysical transients, and the extent to which such transients can affect model projections is unclear. We use a synchronously-coupled ice-ocean model to investigate evolution of Pope, Smith and Kohler Glaciers, West Antarctica, over the next half-century. Two methods of initialization are used: In one, the ice-sheet model is constrained with observed velocities in its initial state; in another, the model is constrained with both velocities and grounded thinning rates over a 4-year period. Each method is applied to two basal sliding laws. For each resulting initialization, two climate scenarios are considered: one where ocean conditions during the initialization period persist indefinitely, and one where the ocean is in a permanent warm state. At first, model runs initialized with thinning data exhibit volume loss rates much closer to observed values than those initialized with velocity only, but after 1-2 decades, the forcing primarily determines rates of volume loss and grounding line retreat. Such behavior is seen for both basal sliding laws, although volume loss rates differ quantitatively. Under the warm scenario, a grounding line retreat of similar to 30 km is simulated for Smith and Kohler, although variation in total retreat due to initialization is nearly as large as that due to forcing. Furthermore it is questionable whether retreat will continue due to narrowing of submarine troughs and limiting of heat transport by bathymetric obstacles.

Automated summary

Coupled ice sheet-ocean models are being used to study the response of ice sheets to ocean warming. Initializing such models is challenging and can introduce nonphysical transients, and the impact of these transients on model projections is uncertain.