The ice streams flowing into the Amundsen Sea, West Antarctica, are losing mass due to changes in the oceanic basal melting of their floating ice shelves. Rapid ice-shelf melting is sustained by the delivery of warm Circumpolar Deep Water to the ice-shelf cavities, which is first supplied to the continental shelf by an undercurrent that flows eastward along the shelf break. Temporal variability of this undercurrent controls ice-shelf basal melt variability. Recent work shows that on decadal timescales the undercurrent variability opposes surface wind variability. Using a regional model, we show that undercurrent variability is driven by sea-ice freshwater fluxes, particularly those north of the shelf break, which affect the cross-shelf break density gradient. This sea-ice variability is caused by tropical Pacific variability impacting atmospheric conditions over the Amundsen Sea. Ice-shelf melting also feeds back onto the undercurrent by affecting the on-shelf density, thereby influencing shelf-break density gradient anomalies.

Little is known about Antarctic subglacial hydrology, but based on modeling, theory and indirect observations it is thought that subglacial runoff enhances submarine melt locally through buoyancy effects. However, no studies to date have examined effects of runoff on sea ice and circulation on the continental shelf. Here we use modeled and observational estimates of runoff to force a regional model of the Amundsen Sea Embayment. We find that runoff enhances melt locally (i.e. within the ice-shelf cavity), increasing melt at Thwaites ice shelf by up to 15 Gt/a given estimates of steady runoff, and up to 25 Gt/a if runoff is episodic as remote sensing measurements suggest. However runoff also has smaller nonlocal effects through freshwater influence on flow and stratification. We further find that runoff reduces summer sea-ice volume over the continental shelf (by up to 10\% with steady runoff but over 30\% with episodic runoff). Furthermore runoff is much more effective at reducing sea ice than an equivalent volume of ice-shelf meltwater – due in part to the latent heat loss associated with submarine melting. Results suggest that runoff may play an important role in continental shelf dynamics, despite runoff flux being small relative to ice-shelf melting – and that runoff-driven melt and circulation may be an important process missing from regional Antarctic ocean models.

South Georgia is a heavily glaciated sub-Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine-terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high-resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier. The addition of a postulated shallow inner sill in one fjord arm significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could accelerate both the subglacial plume-driven melt, and the melting of the wider submarine ice face, which is proposed as a possible explanation for the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub-Antarctic island fjord, highlighting the sensitivity of plume dynamics to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat.

The Amundsen Sea in West Antarctica features rapidly thinning ice shelves and large, seasonally recurring polynyas. Within these polynyas, sizable spring phytoplankton blooms occur. Although considerable effort has gone into characterising heat fluxes between the Amundsen Sea, its associated ice shelves, and the overlying atmosphere, the effect of the phytoplankton blooms on the distribution of heat remains poorly understood. In this modelling study, we implement a feedback from biogeochemistry onto physics into MITgcm-BLING and use it to show, for the first time, that high levels of chlorophyll – concentrated in the Amundsen Sea Polynya and the Pine Island Polynya – accelerate springtime surface warming in polynyas through enhanced absorption of solar radiation. The warm midsummer anomaly (on average between +0.2°C and +0.3C°) at the surface is quickly dissipated to the atmosphere, by small increases in latent and longwave heat loss as well as a substantial (17.5%) increase in sensible heat loss from open water areas. The summertime warm anomaly also reduces the summertime sea ice volume, and stimulates enhanced seasonal melting near the fronts of ice shelves. However larger effects derive from the accompanying cold anomaly, caused by shading of deeper waters, which persists throughout the year and affects a decrease in the volume of Circumpolar Deep Water on the continental shelf. This cooling ultimately leads to an increase in wintertime sea ice volume, and reduces basal melting of Amundsen Sea ice shelves by approximately 7% relative to the model scenario with no phytoplankton bloom.

A document by Alexander Thomas Bradley. Click on the document to view its contents.

South Georgia is a heavily glaciated sub-Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine-terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high-resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier, though this does not provide persuasive reasoning for the asymmetric glacier retreat. The addition of a postulated shallow inner sill in one fjord arm, however, significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could significantly increase submarine melting, which is proposed as a possible contribution to the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub-Antarctic island fjord, highlighting the sensitivity of fjord oceanography to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat.

We use the United Kingdom Earth System Model simulations from the Coupled Model Intercomparison Project 6 to analyse the dynamics of the Ross Gyre, West Antarctica, under historical and projected climate-change scenarios. During the historical period, the modelled Ross Gyre is relatively stable, with an extent and strength that are in reasonable agreement with observations. The projections exhibit an eastward gyre expansion into the Amundsen and Bellingshausen seas that starts during the 2040s. The associated cyclonic ocean circulation enhances the onshore transport of warm Circumpolar Deep Water into the eastern Amundsen Sea, a regime change that increases the subsurface shelf temperatures by up to 1.2◦C and is independent of future forcing scenario. The Ross Gyre expansion is generated by a regional surface stress curl intensification associated with anthropogenic forcing. If realised in reality, such a warming would strongly influence the future stability of the West Antarctic Ice Sheet.

Ice sheets such as Pine Island and Thwaites Glaciers which terminate at their ice shelves in the eastern Amundsen Sea, West Antarctica, are losing mass faster than most others about the continent. The mass loss is due to basal melting, this affected by a deep current thought to be guided by bottom bathymetry that transports warm Circumpolar Deep Water (CDW) from the continental shelf break towards the ice shelves. This current and associated heat transport are controlled by the near-surface winds that vary on a range of timescales due to both anthropogenic and natural effects. In this study we use idealised models to reproduce essential features of the Amundsen Sea circulation and heat transport. The aim is to elucidate the role of bathymetric features in shaping the circulation and in enabling heat transport from the deep ocean onto the continental shelf. Bathymetric variations along the continental slope enhance on-shelf heat transport by inducing breaks in the Antarctic Slope Front that separates off-shelf CDW from the colder, fresher shelf waters. The idealised model results imply that a ridge that blocks deep westward inflow from the Bellingshausen Sea leads to the existence of a deep cyclonic circulation on the shelf. Part of this circulation is an eastward undercurrent that flows along the continental shelf break. The broader cyclonic circulation transports heat that has been recently fluxed onto the shelf towards the south. These fundamental investigations will help refine the aims of future fieldwork and modelling.

The Southern Ocean is chronically under-sampled due to its remoteness, harsh environment and sea-ice cover. Ocean circulation models yield significant insight into key processes and to some extent obviate the dearth of data, however they often underestimate surface mixed layer depth (MLD), with consequences for water-column properties. In this study, a coupled circulation and sea-ice model was implemented for the region adjacent to the West Antarctic Peninsula (WAP), a climatically sensitive region which has exhibited decadal trends toward higher temperatures, a shorter sea-ice season and increasing glacial freshwater input, overlain by strong interannual variability. Hindcast simulations were conducted with different air-ice drag coefficients and Langmuir-circulation parameterizations to determine the impact of these factors on MLD. Including Langmuir circulation deepened the surface mixed layer, with the deepening being more pronounced in the shelf and slope regions. Optimal selection of an air-ice drag coefficient also increased the modeled MLD by similar amounts, and had a larger impact in improving the reliability of the simulated MLD interannual variability. This study highlights the importance of sea ice volume and redistribution to correctly reproduce the physics of the underlying ocean, and the potential of appropriately parameterizing Langmuir circulation to help correct for a bias towards shallow MLD in the Southern Ocean. The model also reproduces observed freshwater patterns in the WAP during late summer and suggests that areas of intense summertime sea-ice melt can still show net annual freezing due to high sea-ice formation during the winter.