Decadal Ocean Forcing and Antarctic Ice Sheet Response: Lessons from the Amundsen Sea

Adrian Jenkins,Stan Jacobs,James Smith,Pierre Dutrieux,Eric Steig,Hilmar Gudmundsson,Karen Heywood

doi:10.5670/oceanog.2016.103

Abstract

Mass loss from the Antarctic Ice Sheet is driven by changes at the marine margins. In the Amundsen Sea, thinning of the ice shelves has allowed the outlet glaciers to accelerate and thin, resulting in inland migration of their grounding lines. The ultimate driver is often assumed to be ocean warming, but the recent record of ocean temperature is dominated by decadal variability rather than a trend. The distribution of water masses on the Amundsen Sea continental shelf is particularly sensitive to atmospheric forcing, while the regional atmospheric circulation is highly variable, at least in part because of the impact of tropical variability. Changes in atmospheric circulation force changes in ice shelf melting, which drive step-wise movement of the grounding line between localized high points on the bed. When the grounding line is located on a high point, outlet glacier flow is sensitive to atmosphere-ocean variability, but once retreat or advance to the next high point has been triggered, ocean circulation and melt rate changes associated with the evolution in geometry of the sub-ice-shelf cavity dominate, and the sensitivity to atmospheric forcing is greatly reduced.

Highlights

Despite featuring prominently in debate over the potential instability of the marine-based West Antarctic Ice Sheet (Thomas et al, 1979; Hughes, 1981), the Amundsen Sea sector of Antarctica (Figure 1) received only limited attention until 1994, when Jacobs et al (1996) made the first observations of ocean conditions on the continental shelf in the eastern Amundsen Sea
The combination of warm ocean waters on the shelf and the deep draft of the glaciers draining from the heart of West Antarctica into the eastern Amundsen Sea was shown to give rise to exceptionally high melt rates beneath the floating ice shelves that form at the glacier termini (Jacobs et al, 1996; Rignot, 1998), raising the prospect of marine ice sheet retreat in response to loss of buttressing from thinning ice shelves
Ocean-driven thinning of the ice shelves led to acceleration of the inland ice flow (Rignot, 2008), and as grounded ice thinned in response, regions floated free of the bed and the grounding line retreated inland (Rignot, 1998; Joughin et al, 2010)

Summary

INTRODUCTION

Despite featuring prominently in debate over the potential instability of the marine-based West Antarctic Ice Sheet (Thomas et al, 1979; Hughes, 1981), the Amundsen Sea sector of Antarctica (Figure 1) received only limited attention until 1994, when Jacobs et al (1996) made the first observations of ocean conditions on the continental shelf in the eastern Amundsen Sea. The combination of warm ocean waters on the shelf and the deep draft of the glaciers draining from the heart of West Antarctica into the eastern Amundsen Sea was shown to give rise to exceptionally high melt rates beneath the floating ice shelves that form at the glacier termini (Jacobs et al, 1996; Rignot, 1998), raising the prospect of marine ice sheet retreat in response to loss of buttressing from thinning ice shelves. GLACIOLOGICAL CHANGE IN THE AMUNDSEN SEA SECTOR At the Last Glacial Maximum, grounded ice extended to, or near to, the continental shelf edge in the eastern Amundsen Sea, and well-dated marine geological data indicate that post-glacial retreat began around 20 kyr ago (Smith et al, 2014), taking the grounding line to within ~100 km of its current position by ~10 kyr ago (Hillenbrand et al, 2013).

Antarctic Bottom Water

Grounding line Grounding line

Findings

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