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Results are presented from an isopycnic coordinate model of ocean circulation in the Amundsen Sea, focusing on the delivery of Circumpolar Deep Water (CDW) to the inner continental shelf around Pine Island Bay. The warmest waters to reach this region are channeled through a submarine trough, accessed via bathymetric irregularities along the shelf break. Temporal variability in the influx of CDW is related to regional wind forcing. Easterly winds over the shelf edge change to westerlies when the Amundsen Sea Low migrates west and south in winter/spring. This drives seasonal on‐shelf flow, while inter‐annual changes in the wind forcing lead to inflow variability on a decadal timescale. A modelled period of warming following low CDW influx in the late 1980's and early 1990's coincides with a period of observed thinning and acceleration of Pine Island Glacier.

[1] Pine Island Glacier (PIG), Antarctica, is rapidly losing mass, supporting arguments that it may play a major role in 21st century sea-level rise. Yet this glacier's quantitative contribution to sea level based on theoretical and computational models is poorly known. We have developed a basinscale glaciological model to examine the sensitivity of PIG to a range of environmental forcings. While oceanic melt likely played the leading role in recent thinning and retreat, we find that the particular grounding-line geometry with an extended ice plain in the 1990s made it susceptible to such forcing. Our model further indicates that while the rate of grounding-line retreat should diminish soon, the glacier's mass loss may continue at rates similar to, or moderately elevated from, the present. While substantial, our modelderived maximum rate of 2.7 cm/century is considerably smaller than previous heuristically-derived bounds on the sea-level contribution. [2] The majority of Antarctica's current net ice loss occurs along the Amundsen Coast, where warm Circumpolar Deep Water (CDW) intrudes onto the continental shelf to cause strong (tens of meters per year) melting beneath the floating ice shelves that extend seaward from the grounded ice sheet [Rignot and Jacobs, 2002;Rignot et al., 2008;Thoma et al., 2008]. Previous studies have shown that this melt produces thinning near the grounding line (the transition from ice resting on bedrock to floating), producing large speedups that propagate inland rapidly [Payne et al., 2004;Shepherd et al., 2001;Thomas et al., 2004]. How the resulting thinning on grounded ice will evolve is one of the major uncertainties in 21st century sea-level projections, as acknowledged by the Intergovernmental Panel on Climate Change [2007].[3] Arguably, nowhere on the Antarctic Ice Sheet is change more apparent than at Pine Island Glacier, which has been described as the "weak underbelly" of Antarctica [Hughes, 1981]. Earlier studies documented speedup from about 2300 m yr −1 in 1974 to just over 3800 m yr −1 in 2007, concurrent with intervals of grounding-line retreat [Joughin et al., 2003;Rignot, 2008Rignot, , 1998]. Figure 1 shows subsequent speedup to nearly 4000 m yr −1 by late 2008, followed by little further change through early 2010. Using Terra-SAR-X satellite data, we also mapped the 2009 grounding-line position (Figure 1a). From 1996 to 2009, sections of the grounding line retreated by more than 20 km, substantially more than the 5-km retreat from 1992 to 1996 [Rignot, 1998]. Despite this retreat, an isolated, lightly grounded area developed forward of the main grounding line.[4] While an earlier model demonstrated that change near the grounding line rapidly diffuses inland [Payne et al., 2004], it predicted a response far more moderate than is observed, likely reflecting the model's use of a fixed grounding line. We implemented a similar depth-averaged, finite-element, numerical model for the majority of the PIG catchment basin (Figure 1a), but our implementation allowed the groundi...