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 ∼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. Plain Language Summary: In a number of locations along the Antarctic coastline, particularly in West Antarctica, warm waters which originate in the midlatitudes exist on the continental shelf, protected from cold air and freezing conditions at the surface. These warm waters are able to circulate underneath ice shelves—The vast floating extensions of fast‐flowing outlet glaciers—Leading to high levels of melt and thinning. The thinning, in turn, drives speedup and thinning of the glaciers, contributing to rising sea levels. To study this process, interactive models of ice dynamics and ocean circulation are used. As in other areas of Earth System modeling, such as weather prediction, the way in which these models are initialized impacts model predictions, and due to the complexity of interactions between ice and ocean, these impacts are poorly understood. Here, we use a novel model of ice‐ocean interaction and consider different ways of initializing the model using remotely‐observed data, applied to a very fast‐thinning system of West Antarctic glaciers. We find that 10–20 years predictions of ice‐ocean evolution depend strongly on initialization, but longer‐term predictions depend more on evolving climate. Key Points: Different initialization strategies, sliding laws, and climate forcings are applied to a coupled ice sheet‐ocean modelGrounded ice loss rates are determined by model initialization in the first 1–2 decades, but controlled by forcing over longer time scalesDespite the influence of forcing, overall grounding line retreat is affected as strongly by initialization as by forcing for 50‐year simulations [ABSTRACT FROM AUTHOR]