The relative impacts of initialisation and climate forcing in coupled ice sheet-ocean modelling: 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. Initialising 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 initialisation 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 initialisation, two climate scenarios are considered: one where ocean conditions during the initialisation period persist indefinitely, and one where the ocean is in a permanent ``warm" state. At first, model runs initialised with thinning data exhibit volume loss rates much closer to observed values than those initialised with velocity only, but after 1-2 decades the forcing primarily determines rates of volume loss and grounding line retreat. Such behaviour is seen for both basal sliding laws, although volume loss rates differ quantitatively. Under the ``warm" scenario, a grounding line retreat of ~30 km is simulated for Smith and Kohler, although variation in total retreat due to initialisation 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.