604 research outputs found
Evaluation of basal melting parameterisations using in situ ocean and melting observations from the Amery Ice Shelf, East Antarctica
Ocean-driven melting of Antarctic ice shelves is causing accelerating loss of grounded ice from the Antarctic continent. However, the ocean
processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica's contribution to sea
level. Here, we analyse oceanographic data and in situ measurements of ice shelf melt collected from an instrumented mooring beneath the centre of
the Amery Ice Shelf, East Antarctica. This is the first direct measurement of basal melting from the Amery Ice Shelf and was made through the novel
application of an upward-facing acoustic Doppler current profiler (ADCP). ADCP data were also used to map a region of the ice base, revealing a
steep topographic feature or “scarp” in the ice with vertical and horizontal scales of ∼ 20 and ∼ 40 m, respectively. The
annually averaged ADCP-derived melt rate of 0.51 ± 0.18 m yr−1 is consistent with previous modelling results and glaciological
estimates. There is significant seasonal variation around the mean melt rate, with a 40 % increase in melting in May and a 60 % decrease in
September. Melting is driven by temperatures ∼ 0.2 ∘C above the local freezing point and background and tidal currents, which
have typical speeds of 3.0 and 10.0 cm s−1, respectively. We use the coincident measurements of ice shelf melt and oceanographic forcing
to evaluate parameterisations of ice–ocean interactions and find that parameterisations in which there is an explicit dependence of the melt rate
on current speed beneath the ice tend to overestimate the local melt rate at AM06 by between 200 % and 400 %, depending on the choice of
drag coefficient. A convective parameterisation in which melting is a function of the slope of the ice base is also evaluated and is shown to
underpredict melting by 20 % at this site. By combining these new estimates with available observations from other ice shelves, we show that
the commonly used current speed-dependent parameterisation overestimates melting at all but the coldest and most energetic cavity conditions.</p
Eddy and tidal driven basal melting of the Totten and Moscow University ice shelves
The mass loss from the neighboring Totten and Moscow University ice shelves is accelerating and may raise global sea levels in coming centuries. Totten Glacier is mostly based on bedrock below sea level, and so is vulnerable to warm water intrusion reducing its ice shelf buttressing. The mechanisms driving the ocean forced sub-ice-shelf melting remains to be further explored. In this study, we simulate oceanic-driven ice shelf melting of the Totten (TIS) and Moscow University ice shelves (MUIS) using a high spatiotemporal resolution model that resolves both eddy and tidal processes. We selected the year 2014 as representative of the period 1992 to 2017 to investigate how basal melting varies on spatial and temporal scales. We apply the wavelet coherence method to investigate the interactions between the two ice shelves in time-frequency space and hence estimate the contributions from tidal (<1.5 days) and eddy (2-35 days) components of the ocean heat transport to the basal melting of each ice shelf. In our simulation, the 2014 mean basal melt rate for TIS is 6.7 m yr-1 (42 Gt yr-1) and 9.7 m yr-1 (52 Gt yr-1) for MUIS. We find high wavelet coherence in the eddy dominated frequency band between the two ice shelves over almost the whole year. The wavelet coherence along five transects across the ice shelves suggests that TIS basal melting is dominated by eddy processes, while MUIS basal melting is dominated by tidal processes. The eddy-dominated basal melt for TIS is probably due to the large and convoluted bathymetric gradients beneath the ice shelf, weakening higher frequency tidal mode transport. This illustrates the key role of accurate bathymetric data plays in simulating on-going and future evolution of these important ice shelves
Future sea level change from Antarctica's Lambert-Amery glacial system
Future global mean sea level (GMSL) change is dependent on the complex response of the Antarctic ice sheet to ongoing changes and feedbacks in the climate system. The Lambert-Amery glacial system has been observed to be stable over the recent period yet is potentially at risk of rapid grounding line retreat and ice discharge given that a significant volume of its ice is grounded below sea level, making its future contribution to GMSL uncertain. Using a regional ice sheet model of the Lambert-Amery system, we find that under a range of future warming and extreme scenarios, the simulated grounding line remains stable and does not trigger rapid mass loss from grounding line retreat. This allows for increased future accumulation to exceed the mass loss from ice dynamical changes. We suggest that the Lambert-Amery glacial system will remain stable or gain ice mass and mitigate a portion of potential future sea level rise over the next 500 years, with a range of +3.6 to −117.5 mm GMSL equivalent
Seasonal dynamics of Totten Ice Shelf controlled by sea ice buttressing
Previous studies of Totten Ice Shelf have employed surface velocity
measurements to estimate its mass balance and understand its sensitivities to
interannual changes in climate forcing. However, displacement measurements
acquired over timescales of days to weeks may not accurately characterize
long-term flow rates wherein ice velocity fluctuates with the seasons.
Quantifying annual mass budgets or analyzing interannual changes in ice
velocity requires knowing when and where observations of glacier velocity
could be aliased by subannual variability. Here, we analyze 16 years of
velocity data for Totten Ice Shelf, which we generate at subannual resolution
by applying feature-tracking algorithms to several hundred satellite image
pairs. We identify a seasonal cycle characterized by a spring to autumn
speedup of more than 100 m yr−1 close to the ice front. The amplitude
of the seasonal cycle diminishes with distance from the open ocean,
suggesting the presence of a resistive back stress at the ice front that is
strongest in winter. Springtime acceleration precedes summer surface melt and
is not attributable to thinning from basal melt. We attribute the onset of
ice shelf acceleration each spring to the loss of buttressing from the
breakup of seasonal landfast sea ice.</p
A spectral line shape analysis of motional stark effect spectra
12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)Recent observations of MSE spectra carried out on Tore-Supra show discrepancies between experimental and theoretical intensities calculated at equilibrium. We present here a kinetic model, based on the selectivity of excitation cross sections of Stark states in the parabolic basis. Redistribution due to ion-atom collisions among Stark states of level n=3 allow to calculate the population of Stark states. This model permits to improve significantly the agreement between measured and calculated MSE spectra
Impact of ocean forcing on the Aurora Basin in the 21st and 22nd centuries
The grounded ice in the Totten and Dalton glaciers is an essential component of the buttressing for the marine-based Aurora basin, and hence their stability is important to the future rate of mass loss from East Antarctica. Totten and Vanderford glaciers are joined by a deep east-west running subglacial trench between the continental ice sheet and Law Dome, while a shallower trench links the Totten and Dalton glaciers. All three glaciers flow into the ocean close to the Antarctic circle and experience ocean-driven ice shelf melt rates comparable with the Amundsen Sea Embayment. We investigate this combination of trenches and ice shelves with the BISICLES adaptive mesh ice-sheet model and ocean-forcing melt rates derived from two global climate models. We find that ice shelf ablation at a rate comparable with the present day is sufficient to cause widespread grounding line retreat in an east-west direction across Totten and Dalton glaciers, with projected future warming causing faster retreat. Meanwhile, southward retreat is limited by the shallower ocean facing slopes between the coast and the bulk of the Aurora sub-glacial trench. However the two climate models produce completely different future ice shelf basal melt rates in this region: HadCM3 drives increasing sub-ice shelf melting to ~2150, while ECHAM5 shows little or no increase in sub-ice shelf melting under the two greenhouse gas forcing scenarios
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