The relative impacts of initialisation and climate forcing in coupled ice sheet-ocean modelling: application to Pope, Smith and Kohler glaciers

Dynamically coupled ice sheet-ocean models are beginning to be used to study the response of the Antarctic Ice Sheet to fluctuations in ocean temperatures. However, initialising a coupled ice-ocean model for realistic settings is challenging and can introduce nonphysical transients. The extent to which such transients can affect model evolution and projection is unclear. Here, we use a synchronously-coupled model of ice-ocean dynamics to investigate the evolution of Pope, Smith and Kohler Glaciers, West Antarctica, over the next half-century. Two methods of coupled initialisation are used: in one, the ice-sheet model is constrained to fit observed velocities in its initial state; in the other, the model is constrained with both velocities and grounded thinning rates over a 4-year period while forced with simulated ocean melt rates. For each method, two climate scenarios are considered -- one where ocean conditions during this initialisation period persist indefinitely, and one where the ocean is in a permanent ``warm'' state -- as well as two ice-sheet basal sliding laws. At first, the model runs initialised with thinning rates exhibit volume loss much closer to observed values than those initialised with velocity only, but after 1-2 decades the forcing primarily determines rates of retreat. This ``crossover’’ timescale is expected to vary by glacier, however. Under the ``warm’’ scenario, grounding line retreat of ~30 km is simulated for Smith and Kohler, but it is questionable whether this will continue due to narrowing of submarine troughs and limiting of heat transport by controlling obstacles.