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Australia: The Land Where Time Began |
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Antarctica – Larsen C Ice Shelf – Impact on Basal Melting of
Tide-Topography Interactions
Antarctic ice shelf basal melting contributes to the formation of
Antarctic Bottom Water, and by altering the offshore flow of grounded
ice streams and glaciers can affect global sea level. Basal melt rates (wb)
of ice shelves is influenced by tides, as tides contribute to ocean
mixing and mean circulation, as well as thermohaline exchanges with the
ice shelf. To investigate the relationship between topography, tides,
and wb for the
Larsen C Ice Shelf (LCIS) in the northwestern part of the Weddell Sea in
Antarctica Muller et al. used
a 3D model that was thermodynamically coupled to a nonevolving ice
shelf. Muller et al. found by
using their best estimates of the thickness of the ice and the
topography of the sea bed that the largest modelled LCIS melt rates
occurred in the northeast, and it was there that their model predicted
there were strong diurnal tidal currents (~0.4 m/sec). This distribution
differs significantly from models that do not include tidal forcing,
which predict that the largest melt rates occur along the deep grounding
lines. In order to explore melt rate sensitivity to geometry, initial
ocean potential temperature (θ0),
thermodynamic parameterisations of heat and freshwater
ice-ocean exchange, and tidal forcing, Muller et
al. compared several runs of
their model. The range of LCIS-averaged
wb is ~0.11 – 0.44
m/a. The spatial distribution of –wb
was found to be very sensitive to model geometry and thermodynamic
parameterisation while θ0 influenced the overall magnitude of
wb. A need for
high-resolution maps of ice draft and topography below the ice shelf,
together with measurements of ocean temperature at the ice shelf front
in order to improve the representation of ice shelves in coupled climate
system models is reinforced by these sensitivities in
wb
predictions.
Around Antarctica the oceans interact with the continental ice sheet at
the floating ice shelves which occupy about 50 % of the coastline of
Antarctica (Drewry et al.,
1982). Global ocean properties are influenced by melting at the base of
an ice shelf by producing cold melt-water plumes of low-salinity that
carry freshwater mass away from the continent and preconditioning the
surrounding waters of the continental shelf for formation of Antarctic
Bottom Water (e.g. Jacobs, 2004). An ice shelf can also be weakened by
basal melting, which increases the likelihood of calving events or
disintegration (Vieli et al.,
2007). The total mass balance of an ice shelf is determined by the
balance between ice gain, by advective input of grounded ice,
accumulation of snow, and the accretion of marine ice, and ice loss,
primarily basal melting and calving. Negative mass balance can
destabilise an ice shelf to the extent that it can be compromised. The
stresses that impede the offshore flow of the ice shelves can be reduced
by mass loss which leads to increased rapidity of the seaward movement
in the inflowing glaciers and ice streams (Rignot et
al., 2004; Scambos et
al., 2004). The ocean can
affect the overall mass balance of the Antarctic ice sheet, as well as
associated global sea level on decadal time scales, by these processes.
Though these is a general understanding of processes causing basal
melting of ice shelves (Lewis & Perkins, 1986; MacAyeal, 1984; Hellmer &
Olbers, 1989; Jacobs et al.,
1992; Holland & Jenkins, 1999), the ability to model accurately the
spatial distribution of basal melt rate (wb)
and the associated net loss of ice mass is limited by several factors
which include: ice shelf and seabed geometry is poorly known, a paucity
of hydrographic data which defines the nature of oceanic inflow to the
cavity beneath the ice shelf, and neglect of specific processes for the
purpose of making computation simpler and more efficient. Models which
attempt to project land ice contribution to the changes of sea level
over long time scales (e.g. Pollard & DeConto, 2009) will be subject to
errors which will potentially be very large until these deficiencies in
the model and data deficiencies are resolved.
The tides are a forcing that is usually excluded from numerical models
of basal melting of ice shelves. It has been postulated (MacAyeal, 1984)
that there is a relationship between tides and basal melting, noting
that the ice shelf isolates the cavity beneath the ice shelf from direct
forcing by the wind, and, therefore, increases the importance of tidal
currents as a source of oceanic kinetic energy for conversion to mixing.
It has been demonstrated by more recent studies that tides can be a
significant factor in interactions between ocean and ice shelves close
to the ice shelf boundaries (Makinson & Nicholls, 1999; Makinson, 2002;
Joughin & Padman, 2003; Holland, 2008; Robinson et
al., 2010). It was predicted
(Makinson et al., 2011) that
tides contribute about half of net loss of mass from the Filchner-Ronne
Ice Shelf (FRIS) in the southern Weddell Sea.
In this study Mueller et al.
focussed on understanding the sensitivity to errors in initial boundary
conditions for ice shelves where tidal forcing is significant. With this
aim they report on studies of sensitivity of the effects of adding tides
to an ocean model that is coupled thermodynamically to the Larsen C Ice
Shelf (LCIS) in the northwestern part of the Weddell Sea, Antarctica.
This ice shelf is subject to significant tidal variability (King et
al., 2011), and is ventilated
by cold, High Salinity Shelf Water (HSSW) (Nicholls et
al., 2004); in some respects
it is therefore similar to the much larger FRIS studied by Makinson et
al., 2011). When studying the
influence of tides under these conditions it is advantageous to use the
Larsen C Ice Shelf because of its smaller size as it allows a much finer
model grid resolution. As the Larsen C Ice Shelf is in a more northerly
location it is likely to respond to climate change earlier than the
Filchner-Ronne Ice Shelf. The view that the Larsen C Ice Shelf is
undergoing changes that may lead to weakening of the ice shelf is
supported by recent surface lowering of the Larsen C Ice Shelf (Shepard
et al., 2003; Fricker &
Padman, 2012). Important roles in ice shelf mass and elevation variability are played by atmospheric forcing and open ocean circulation (Fricker & Padman, 2012), though in this study their model is forced only with tides. The approach taken by Mueller et al. allowed them to focus on the factors that contributed to uncertainty in wb. Relevant sources of uncertainty with regard to the Larsen C Ice Shelf include the thickness of the water column (wct), the temperature of the ocean water that flowed into the cavity beneath the ice shelf, and the heat parameterisation and exchange of freshwater at the ice-ocean interface. They were particularly interested in the effect of errors in the water column thickness. On small spatial scales tidal currents can be very sensitive to water column thickness errors, and as a result there is significant uncertainty in the contribution of tides to wb. According to Mueller et al. this study complements the application of a plume model to the LCIS (Holland et al., 2009), which provides a valuable comparison for their simulations.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |