Southern Ocean warming: Increase in basal melting and grounded ice loss

We apply a global finite element sea ice/ice shelf/ocean model (FESOM) to the Antarctic marginal seas to analyze projections of ice shelf basal melting in a warmer climate. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM. Results from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. While trends in sea ice coverage, ocean heat content, and ice shelf basal melting are small in simulations forced with ECHAM5 data, a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity of basal melting to increased ocean temperatures is found for the ice shelves in the Amundsen Sea. For the cold-water ice shelves in the Ross and Weddell Seas,decreasing convection on the continental shelf in the HadCM3 scenarios leads to an erosion of the continental slope front and to warm water of open ocean origin entering the continental shelf. As this water reaches deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. Highest melt rates at the deep FRIS grounding line causes a retreat of > 200km, equivalent to an land ice loss of 110 Gt/yr

Inland extent of the Weddell Sea Rift imaged by new aerogeophysical data

Peer reviewedPostprin

Tidal currents and vertical mixing processes beneath Filchner-Ronne ice shelf

Oceanographic measurements have been undertaken at sites on Filchner-Ronne Ice Shelf using hot-water drilled holes allowing conductivity-temperature-depth profiling and the deployment of instrument moorings. The data show that Western Shelf Water enters the sub-ice shelf cavity and occupies the lower portion of the water column. This is the water that provides the external heat necessary for melting within the sub-ice shelf cavity. A depth-averaged tidal model of the region has been used to show 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 resulting from surface friction is 25 GW, approximately 1% of the world’s total tidal dissipation. The model indicates that Lagrangian tidal 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. Furthermore, a one-dimensional turbulence closure ocean model has been applied to this sub-ice shelf environment which significantly lies near the critical latitude for the semi-diurnal tide. Here, the Coriolis frequency equals the tidal frequency, resulting in a strong depth dependent tidal current and thick boundary layers. Both the model and observations show that stratification significantly affects how the shape of the tidal current ellipse varies with depth. The model also shows that vertical mixing and basal melting are sensitive to tidal ellipse polarization with anticlockwise rotating tidal currents maintaining the highest melt rates. This sensitivity is due, in large part, to the proximity of the critical latitude

Tidal influence on Rutford Ice Stream, West Antarctica: observations of surface flow and basal processes from closely-spaced GPS and passive seismic stations

High-resolution surface velocity measurements and passive seismic observations from Rutford Ice Stream, West Antarctica, 40 km upstream from the grounding line are presented. These measurements indicate a complex relationship between the ocean tides and currents, basal conditions and ice-stream flow. Both the mean basal seismicity and the velocity of the ice stream are modulated by the tides. Seismic activity increases twice during each semi-diurnal tidal cycle. The tidal analysis shows the largest velocity variation is at the fortnightly period, with smaller variations superimposed at diurnal and semi-diurnal frequencies. The general pattern of the observed velocity is two velocity peaks during each semi-diurnal tidal cycle, but sometimes three peaks are observed. This pattern of two or three peaks is more regular during spring tides, when the largest-amplitude velocity variations are observed, than during neap tides. This is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream

West Antarctic Ice Sheet Initiative. Volume 2: Discipline Reviews

Seven discipline review papers are presented on the state of the knowledge of West Antarctica and opinions on how that knowledge must be increased to predict the future behavior of this ice sheet and to assess its potential to collapse, rapidly raising the global sea level. These are the goals of the West Antarctic Ice Sheet Initiative (WAIS)

Radio-echo sounding and waveform modeling reveal abundant marine ice in former rifts and basal crevasses within Crary Ice Rise, Antarctica

Crary Ice Rise formed after the Ross Ice Shelf re-grounded ~1 kyr BP. We present new ice-penetrating radar data from two systems operating at center frequencies of 7 and 750 MHz that confirm the ice rise is composed of a former ice shelf buried by subsequent accumulation. Stacks of englacial diffraction hyperbolas are present almost everywhere across the central ice rise and extend up to ~350 m above the bed. In many cases, bed reflections beneath the diffraction hyperbolas are obscured for distances up to 1 km. Waveform modeling indicates that the diffraction hyperbolas are likely caused by marine ice deposits in former basal crevasses and rifts. The in-filling of rifts and basal crevasses may have strengthened the connection between the ice rise and the surrounding ice shelf, which could have influenced local and regional ice dynamics. Three internal reflection horizons mark the upper limit of disturbed ice and diffraction hyperbolas in different sections of the ice rise, indicating at least three stages of flow stabilization across the ice rise. A surface lineation visible in MODIS imagery corresponds spatially to deepening and strong deformation of these layers, consistent with the characteristics of former grounding lines observed elsewhere in Antarctica

Radio-Echo Sounding Over Polar Ice Masses

Peer reviewedPublisher PD

Inversion of IceBridge gravity data for continental shelf bathymetry beneath the Larsen Ice Shelf, Antarctica

A possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from 1000 m. Localized overdeepenings, 20–30 km long and 900–1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550–700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68°S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers