Keith W. Nicholls

Rapid erosion, drumlin formation, and changing hydrology beneath an Antarctic ice stream

What happens beneath a glacier affects the way it flows and the landforms left behind when it retreats. Direct observations from beneath glaciers are, however, rare and the subglacial environment remains poorly understood. We present new, repeat observations from West Antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record. We interpret erosion at a rate of 1 m a−1 beneath a fast-flowing ice stream, followed by cessation of erosion and the formation of a drumlin from mobilized sediment. We also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed. All these changes occurred on time scales of a few years or less. This variability suggests that an ice stream can reorganize its bed rapidly, and that present models of ice dynamics may not simulate all the relevant subglacial processes.

Iceberg trajectory modeling and meltwater injection in the Southern Ocean

This is the first large-scale modeling study of iceberg trajectories and melt rates in the Southern Ocean. An iceberg model was seeded with climatological iceberg calving rates based on a calculation of the net surface accumulation from each snow catchment area on the Antarctic continent. In most areas, modeled trajectories show good agreement with observed patterns of iceberg motion, though discrepencies in the Weddell Sea have highlighted problems in the ocean general circulation model output used to force the iceberg model. The Coriolis force is found to be important in keeping bergs entrained in the coastal current around Antarctica, and topographic features are important in causing bergs to depart from the coastal regions. The modeled geographic distribution of iceberg meltwater joining the ocean has been calculated and is found in many near-coastal regions to be comparable in magnitude to the excess of precipitation over evaporation (P-E).

Measurements beneath an Antarctic ice shelf using an autonomous underwater vehicle

The cavities beneath Antarctic ice shelves are among the least studied regions of the World Ocean, yet they are sites of globally important water mass transformations. Here we report results from a mission beneath Fimbul Ice Shelf of an autonomous underwater vehicle. The data reveal a spatially complex oceanographic environment, an ice base with widely varying roughness, and a cavity periodically exposed to water with a temperature significantly above the surface freezing point. The results of this, the briefest of glimpses of conditions in this extraordinary environment, are already reforming our view of the topographic and oceanographic conditions beneath ice shelves, holding out great promises for future missions from similar platforms.

Precise measurement of changes in ice-shelf thickness by phase-sensitive radar to determine basal melt rates

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Observation and Parameterization of Ablation at the Base of Ronne Ice Shelf, Antarctica

Parameterizations of turbulent transfer through the oceanic boundary layer beneath an ice shelf are tested using direct measurements of basal ablation. Observations were made in the southwestern part of Ronne Ice Shelf, about 500 km from open water. The mean basal ablation rate was measured over a month-long and a year-long period using phase-sensitive radar to record the thinning of the ice shelf. Ocean temperatures were observed within about 25 m of the ice shelf base over the period of the radar observations, while the tidally dominated ocean currents were estimated from tidal analysis of collocated current observations from an earlier period. Ablation rates derived using these ocean data and a number of bulk parameterizations of turbulent transfer within the boundary layer are compared with the direct measurements. The ablation rates derived using a parameterization that explicitly includes the impact of ocean currents on the turbulent transfer of heat and salt match the observations to within 40%; with suitable tuning of the drag coefficient,the mismatch can be reduced below the level of the observational errors. Equally good agreement can be obtained with two slightly simpler, current-dependent parameterizations that use constant turbulent transfer coefficients,andtheoptimalvaluesforthecoefficientsatthis particularlocationon RonneIce Shelfaregiven.

Estimates of the Southern Ocean general circulation improved by animal‐borne instruments

Over the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the existing Southern Ocean observing system is demonstrated herein by conducting two state estimation experiments, differing only in the use or not of seal data to constrain the system. Including seal-derived data substantially modifies the estimated surface mixed-layer properties and circulation patterns within and south of the Antarctic Circumpolar Current. Agreement with independent satellite observations of sea ice concentration is improved, especially along the East Antarctic shelf. Instrumented animals efficiently reduce a critical observational gap, and their contribution to monitoring polar climate variability will continue to grow as data accuracy and spatial coverage increase.

Modeling tidal currents beneath Filchner‐Ronne Ice Shelf and on the adjacent continental shelf: Their effect on mixing and transport

A depth-averaged tidal model has been applied to the southern Weddell Sea. The model domain covers the southern continental shelf, including the ocean cavity beneath Filchner-Ronne Ice Shelf. Reasonable agreement with the available current meter data has been achieved. Our results confirm that in areas with shallow water and large topographic gradients, tidal oscillations with peak velocities up to 1 m s−1 play a significant role in the vertical mixing and transport of water masses. The estimated energy dissipation beneath Filchner-Ronne Ice Shelf due to surface friction is 25 GW, approximately 1% of the worlds total tidal dissipation. Tidally induced Lagrangian residual currents converging at the ice front, an area of strong mixing, draw together water masses from the continental shelf and sub-ice shelf cavity. The model indicates that Lagrangian residual currents have fluxes of up to 250,000 m3 s−1, and speeds of over 5 cm s−1 along the ice front, with over 350,000 m3 s−1 being exchanged between the sub-ice shelf cavity and adjacent continental shelf. These currents are particularly efficient in ventilating the sub-ice shelf cavity within 150 km of Ronne Ice Front. Such strong tidal mixing will significantly modify the properties of water masses that flow through this region, particularly to the west of Berkner Island. The model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of Ronne Ice Front. Shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography.

Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica

An isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath Filchner-Ronne Ice Shelf, investigating the role of tides on sub-ice shelf circulation and ice shelf basal mass balance. Including tidal forcing causes a significant intensification in the sub-ice shelf circulation, with an increase in melting (3-fold) and refreezing (6-fold); the net melt rate and seawater flux through the cavity approximately doubles. With tidal forcing, the spatial pattern and magnitude of basal melting and freezing generally match observations. The 0.22 m a(-1) net melt rate is close to satellite-derived estimates and at the lower end of oceanographic values. The Ice Shelf Water outflow mixes with shelf waters, forming a cold (<-1.9 degrees C), dense overflow (0.83 Sv) that spills down the continental slope. These results demonstrate that tidal forcing is fundamental to both ice shelf-ocean interactions and deep-water formation in the southern Weddell Sea. Citation: Makinson, K., P. R. Holland, A. Jenkins, K. W. Nicholls, and D. M. Holland (2011), Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica, Geophys. Res. Lett., 38, L06601, doi: 10.1029/2010GL046462.

Predicted reduction in basal melt rates of an Antarctic ice shelf in a warmer climate

Floating ice shelves are vulnerable to climate change at both their upper and lower surfaces. The extent to which the apparently air-temperature-related retreat of some northerly Antarctic Peninsula ice shelves presages the demise of their much larger, more southerly, counterparts is not known, but air-temperature effects are unlikely to be important in the near future. Oceanographic measurements from beneath the most massive of these southerly ice shelves—the Filchner–Ronne Ice Shelf—have confirmed that dense sea water resulting from sea-ice formation north of the ice shelf flows into the sub-ice-shelf cavity. This relatively warm so-called High Salinity Shelf Water (HSSW) is responsible for the net melting at the ice shelfs base. Here I present temperature measurements, from the same sub-ice-shelf cavity, which show a strong seasonality in the inflow of HSSW. This seasonality results from intense wintertime production of sea ice, and I argue that the seasonal springtime warming can be used as an analogue for climate warming. For the present mode of oceanographic circulation, the implication is that warmer winters (a climate warming, leading to lower rates of sea-ice formation, would cause a reduction in the flux of HSSW beneath the ice shelf. The resultant cooling in the sub-ice cavity would lead, in turn, to a reduction in the total melting at the ice shelfs base. A moderate warming of the climate could thus lead to a basal thickening of the Filchner–Ronne Ice Shelf, perhaps increasing its longevity.

Oceanographic conditions south of Berkner Island, beneath Filchner-Ronne Ice Shelf, Antarctica

We have made oceanographic measurements at two sites beneath the southern Filchner-Ronne Ice Shelf. Hot-water drilled access holes were made during January 1999, allowing conductivity-temperature-depth (CTD) profiling and the deployment of instrument moorings. The CTD profiles show that the entire water column is below the surface freezing point. We estimate the (summer) flux of water between the two sites to be 2×106 m3 s−1. The summer potential temperature-salinity properties of the water column suggest that this flow is part of a recirculation in the deepest part of the subice shelf cavity and the Filchner Depression. The recirculation is driven by a combination of the melting of deep basal ice and the freezing that results from the depressurization of the cold buoyant water as it ascends the ice shelf base. The source of the water was high-salinity shelf water (HSSW) produced in the Ronne Depression. This is the water that provides the external heat necessary for the strong melting at the deep grounding lines in the vicinity of Foundation Ice Stream. Instruments moored at the drill sites show that during the winter HSSW formed on the Berkner Shelf flows beneath the ice shelf and largely displaces the recirculating water from the two sites. This provides an externally driven through flow that is warmer (nearer the surface freezing point) and slower than the internal recirculation and which is low enough in density to escape the Filchner Depression.