[1] The presence of warm Circumpolar Deep Water (CDW) intrusions on the Amundsen continental shelf has been linked to recent thinning of the outlet glaciers draining the West Antarctic ice sheet into the Amundsen Sea. Inflow of the CDW onto the shelf is thought to occur within a series of troughs that intersect the continental shelf break. We use observations between 1994 and 2011 and a numerical model to investigate the variability of CDW transport in a trough intersecting the shelf break at 113 W. The location of the main CDW inflow into the trough varies between its eastern flank and center, while the western part of the trough is filled by a recirculation that commonly entrains cooler water originating further south on the shelf. Thermocline depth decreases between the early and late 2000s with an indication that the depth of the 1994 thermocline was similar to the later years. Mooring results show that the CDW layer cools and thins in summer and thickens and warms in winter. In addition to a deeper thermocline in summer, we observe a stronger presence of Lower CDW in the bottom of the trough. 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.

Ice shelves in the Amundsen Sea Embayment have thinned, accelerating the seaward flow of ice sheets upstream over recent decades. This imbalance is caused by an increase in the ocean‐driven melting of the ice shelves. Observations and models show that the ocean heat content reaching the ice shelves is sensitive to the depth of thermocline, which separates the cool, fresh surface waters from warm, salty waters. Yet the processes controlling the variability of thermocline depth remain poorly constrained. Here we quantify the oceanic conditions and ocean‐driven melting of Cosgrove, Pine Island Glacier (PIG), Thwaites, Crosson, and Dotson ice shelves in the Amundsen Sea Embayment from 1991 to 2014 using a general circulation model. Ice‐shelf melting is coupled to variability in the wind field and the sea‐ice motions over the continental shelf break and associated onshore advection of warm waters in deep troughs. The layer of warm, salty waters at the calving front of PIG and Thwaites is thicker in austral spring (June–October) than in austral summer (December–March), whereas the seasonal cycle at the calving front of Dotson is reversed. Furthermore, the ocean‐driven melting in PIG is enhanced by an asymmetric response to changes in ocean heat transport anomalies at the continental shelf break: melting responds more rapidly to increases in ocean heat transport than to decreases. This asymmetry is caused by the inland deepening of bathymetry and the glacial meltwater circulation around the ice shelf.

[1] The glaciers draining into the Amundsen Sea Embayment are rapidly losing mass, making a significant contribution to current sea level rise. Studies of Pine Island Glacier (PIG) in this region indicate that the mass loss is associated with rapid melting of its floating ice shelf driven by warm Circumpolar Deep Water (CDW) that is able to penetrate all the way to its grounding line, and that recent intensification of the mass loss is associated with higher melt rates and stronger subice-shelf circulation. CDW is sourced from within the Antarctic Circumpolar Current (ACC) situated well north of the glacial ice fronts. To be able to access the Amundsen Sea glaciers, CDW must first cross the continental shelf break where the deep ocean meets the shallower waters of the continental shelf. Here, we present data that shows how CDW moves along the continental slope and across the shelf break into the Amundsen Sea. On-shelf flow of CDW is enhanced where a subsea trough bisects the shelf edge. A previously unreported undercurrent is observed 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 Lower CDW onto the shelf that would otherwise be blocked by the topography. These observations concur with the results of a theoretical modeling study of circulation in a similar topographic setting and also with the results of a regional ocean/ice modeling study of the Amundsen Sea specifically.

Changes in water mass distribution and horizontal circulation due to seasonal influences on the Ross Sea continental shelf are investigated using a circumpolar numerical model. An anticyclonic circulation cell that extends across the open shelf and into the ice shelf cavity is formed in the model. The increased east-west density gradient caused by the strong brine release in the Ross Sea polynya in winter results in an intensification of this anticyclonic cell from 1.5 Sv to 2.5 Sv. This supports the concept of a thermohaline-driven horizontal circulation on the Ross Sea continental shelf. In addition to a temporal change in the circulation strength, the changes in the density structure lead to complex temporal and spatial variability in the circulation around Ross Island. Due to seasonal variation in circulation strength and water temperatures, the area averaged basal melt rate of 25 cm a , with maxima in March and August.