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Processes and variability of warm water inflow to the Amundsen Sea continental shelf

Walker, Dziga; Jenkins, Adrian; Assmann, Karen; Shoosmith, Deb; Brandon, Mark; Jacobs, Stan; Nicholls, Keith. Processes and variability of warm water inflow to the Amundsen Sea continental shelf. [Key Findings]

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Abstract/Summary

The glaciers draining into the Amundsen Sea Embayment are rapidly losing mass, making a significant contribution to current sea level rise. The mass loss is associated with rapid melting of the floating ice shelves that from the marine termini of the glaciers and is driven by warm Circumpolar Deep Water (CDW) intrusions on the Amundsen Sea continental shelf that are able to penetrate beneath the ice shelves. Recent intensification of the mass loss appears to be associated with higher melt rates and stronger sub-ice-shelf circulation, and it has been suggested that the cause is a change in the supply of CDW to the continental shelf. The CDW is sourced from within the Antarctic Circumpolar Current, situated well north of the glacial ice fronts, and the critical question to be addressed is how it crosses the shelf break from deep ocean to shallow continental shelf. The inflows appear to be concentrated within a series of troughs that intersect the continental shelf break, and new analyses of data and model results from one of these troughs has significantly advanced our understanding of the processes that control the inflows. A previously unreported undercurrent has been identified flowing eastward along the shelf edge, and when this current encounters the trough mouth it circulates southward into the trough and toward the glaciers. Upwelling associated with this trough circulation appears to allow a cooler form of CDW onto the shelf that would otherwise be blocked by the topography. Observations show that the location of the main CDW inflow into the trough varies between its eastern flank and centre, while the western part of the trough is filled by a recirculation that entrains cooler water originating further south on the shelf. The thickness of the CDW inflow decreased from the early 1990s to the early 2000s, then increased again to the late 2000s. Seasonally, the CDW layer cools and thins in summer and thickens and warms in winter. The summertime cooling appears to be associated with stronger upwelling of cooler CDW from greater depth. Critically, the heat flux onto the shelf is controlled by current velocities rather than CDW temperature and the majority of the heat is carried onto the shelf by background flow rather than episodic events. These observations concur with the results of a theoretical modelling study of circulation in a similar, idealised topographic setting and also with the results of a regional ocean/ice modelling study of the Amundsen Sea. The new observations lend support to the conclusion of that earlier study that seasonal and inter-annual variability in the CDW inflow is controlled by changes in the wind forcing of the currents that flow along the shelf edge.

Item Type: Key Findings
Date made live: 28 Mar 2014 14:52
URI: http://nora.nerc.ac.uk/id/eprint/506900

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