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Investigating the impact of ocean warming on Antarctic ice shelves

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Gwyther, DE (2016) Investigating the impact of ocean warming on Antarctic ice shelves. PhD thesis, University of Tasmania.

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Abstract

The Antarctic ice sheet contains enough ice to raise global sea levels by over 50
metres. The stability of the Antarctic ice sheet and subsequently, projections of
future sea level rise, depends strongly on interactions with the Southern Ocean.
The ice sheet begins to oat around the coastal margins of Antarctica, forming
floating ice shelves. These ice shelves buttress the flow of ice sheets into the ocean,
and control contributions to sea level rise. Changes to the thickness of ice shelves,
such as through melting at the base (basal melting), is thus a critically important
factor controlling Antarctic ice discharge, and future sea level. Scientists and field
technicians are limited logistically in their ability to observe basal melting in situ
and so numerical modelling, such as with the Regional Ocean Modelling System
(ROMS), forms an important tool for investigating these critical environments.
In order to produce credible modelling results, key uncertainties in modelling must
first be addressed. One such uncertainty is the effect of basal roughness on modulating
turbulence within the ice{ocean boundary layer, and how the parameterisation
of this affects melting and freezing. Using ROMS with an idealised ice shelf geometry,
the link between basal roughness and melting and freezing was explored.
A rougher interface led to more turbulent exchange and stronger melting or freezing.
This showed that the scheme employed to simulate turbulent mixing across the
boundary layer was important for simulating refreezing. Lastly and most importantly,
employing a spatially-varying drag coefficient, which was rougher for areas
of refreezing and smoother for areas of melting, led to variations in the melt/freeze
rate across the entire ice shelf. This is strong motivation for geophysical estimation
of this parameter and improving ice shelf-ocean models to account for spatial
variability in basal roughness.
Idealised models were also used to investigate how melting responds to changes in
the thermal environment of the ocean cavity. The simulations showed that while
circulation is weak in a cold ocean cavity environment, it is the strongest driver of
melting. This is opposed to a hot ocean cavity environment, where buoyancy driven
circulation is strong but melting is mostly strongly driven by heat availability. These
simulations further reinforce that melting is not necessarily primarily driven by heat
availability and strongest at the deep, grounding line. Instead, melting can be driven
by strong flow, such as tides.
The understanding that was further developed through idealised modelling was applied
to a realistic scenario: basal melting beneath the Totten Glacier ice shelf, East
Antarctica. The Totten Glacier drains a significant portion of the East Antarctic
Ice Sheet. The modelled area average basal melt rate was 9.1 m yr-¹, which varies
on seasonal and interannual (≈ 6-year) time scales. Furthermore, a causal link
was demonstrated between cold, dense water production in the nearby Dalton ice
tongue polynya blocking access of warmer off-shelf waters to the Totten ice shelf
cavity environment.
Several models of the Totten Glacier ice shelf were run, where the forcing was
identical but the shape and geometry of the cavity was varied. Comparing the melt
rate of different cavity geometries gives us insight into the sensitivity of melting
to geometry and internal variability. The melt rate is relatively insensitive to the
shape of the cavity, and instead is controlled by the heat flux into the cavity. These
model runs were climatologically forced, and yet displayed significant variability.
As a result, any glaciological and oceanographic observations of the Totten Glacier
ice shelf should be long enough in order to filter this internal variability. Lastly,
a simple linear relationship between ocean water on the adjacent continental shelf
to melt rate is developed, which suggests that the current contribution to sea level
rise from the Totten Glacier outflow is ≈ 0:20 mm yr-¹, which under the RCP8.5
scenario, will rise by 80% by 2100.
This thesis improves understanding of the Totten Glacier ice shelf, a possible future
cause of significant sea level rise, through explaining the main drivers of melt.
Furthermore, these results improve understanding of the uncertainties within ice
shelf-ocean numerical models, and help to further constrain projections of sea level
change in the future.

Item Type: Thesis (PhD)
Keywords: Antarctica, oceanography, glaciology, computer modelling, basal melting
Copyright Information:

Copyright 2015 the Author

Additional Information:

Chapter 3 appears to be the equivalent of the post-print version of an article published as: D. E. Gwyther, B. K. Galton-Fenzi, M. S. Dinniman, J. L. Roberts, J. R. Hunter. 2015, The effect of basal friction on melting and freezing in ice shelf-ocean models, Ocean modelling, 95, 38-52

Chapter 4 appears to be the equivalent of a pre or post-print version of an article published as: D. E. Gwyther, E. A. Cougnon, B. K. Galton-Fenzi, J. l. Roberts, J. R. Hunter, M. S. Dinniman, 2016. Modelling ice shelf melting in a changing ocean cavity environment. Annals of Glaciology, 57(73), 131-141.

Chapter 5 appears to be the equivalent of a pre or post-print version of an article published as: D. E. Gwyther, B. K. Galton-Fenzi, J. R. Hunter, J. L. Roberts. 2014. Simulated melt rates for the Totten and Dalton ice shelves. Ocean science, 10(3) 267-279.

Date Deposited: 28 Oct 2016 04:36
Last Modified: 12 Dec 2016 00:32
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