TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-10-2317-2016Impacts of marine instability across the East Antarctic Ice
Sheet on Southern Ocean dynamicsPhippsSteven J.Steven.Phipps@utas.edu.auhttps://orcid.org/0000-0001-5657-8782FogwillChristopher J.https://orcid.org/0000-0002-6471-1106TurneyChristian S. M.https://orcid.org/0000-0001-6733-0993Climate Change Research Centre, School of Biological, Earth
and Environmental Sciences, UNSW Australia, Sydney, NSW 2052,
AustraliaInstitute for Marine and Antarctic Studies, University of
Tasmania, Hobart, TAS 7001, AustraliaPANGEA Research Centre, UNSW Australia, Sydney, NSW 2052,
AustraliaSteven J. Phipps (Steven.Phipps@utas.edu.au)30September2016105231723286May201626May201619August20168September2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/10/2317/2016/tc-10-2317-2016.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/10/2317/2016/tc-10-2317-2016.pdf
Recent observations and modelling studies have demonstrated the potential for
rapid and substantial retreat of large sectors of the East Antarctic Ice
Sheet (EAIS). This has major implications for ocean circulation and global
sea level. Here we examine the effects of increasing meltwater from the
Wilkes Basin, one of the major marine-based sectors of the EAIS, on Southern
Ocean dynamics. Climate model simulations reveal that the meltwater flux
rapidly stratifies surface waters, leading to a dramatic decrease in the rate
of Antarctic Bottom Water (AABW) formation. The surface ocean cools but,
critically, the Southern Ocean warms by more than 1 ∘C at
depth. This warming is accompanied by a Southern Ocean-wide “domino
effect”, whereby the warming signal propagates westward with depth. Our
results suggest that melting of one sector of the EAIS could result in
accelerated warming across other sectors, including the Weddell Sea sector of
the West Antarctic Ice Sheet. Thus, localised melting of the EAIS could
potentially destabilise the wider Antarctic Ice Sheet.
Introduction
The Fifth Assessment Report of the Intergovernmental Panel on Climate
Change highlights the fact that current and future anthropogenic
greenhouse gas emissions are likely to affect the Earth's climate for
millennia to come . One major uncertainty
relates to how the marine-based sectors of the Antarctic Ice Sheet
will respond to future climate change, and particularly to changes in
Southern Ocean circulation . Given that large sectors of West and East
Antarctica have been demonstrated to be sensitive to Southern Ocean
circulation, there is an urgent need to understand potential ice
sheet–ocean feedbacks.
Of particular concern are the substantial sectors of the East Antarctic Ice
Sheet (EAIS) that are underlain by extensive marine-based subglacial basins.
These have an associated global sea level rise potential of
∼ 14 m and have been shown to be vulnerable to marine ice sheet
instability e.g.. The potential instability of
marine-based sectors of the EAIS adds considerable uncertainty to projections
of future global sea level rise, suggesting that current upper estimates of
an increase of ∼ 1 m over the coming century may be
conservative .
Recent studies have highlighted the sensitivity of the EAIS to changes in
ocean dynamics during climatically warm periods, including those that have
occurred in the past and those that are projected to occur in future.
Examining the climate of the Last Interglacial
(∼ 135 000–116 000 years ago), conclude that
the EAIS may have made a substantial contribution towards interglacial sea
levels. Climate modelling suggests that a southward migration of the Southern
Hemisphere westerly winds drove warm Circumpolar Deep Water onto the
continental shelves, causing pervasive ocean warming at depth along the
margins of the EAIS. This would have enhanced basal melting, potentially
triggering retreat of the EAIS through increased mass flux. A similar and
ongoing migration of the westerly winds, driven by anthropogenic forcings,
has taken place over recent decades and may continue into the future
e.g.. Examining both past and future
warm periods, and also demonstrate
the highly dynamic nature of the EAIS, and particularly its sensitivity to
warmer-than-present ocean temperatures. Using continental-scale ice sheet
models, they conclude that future anthropogenic warming has the potential to
cause partial or complete melting of the floating ice shelves around
Antarctica. Such a scenario would result in a retreat of the major subglacial
basins and an ongoing commitment towards global sea level rise.
While these previous studies have highlighted the potential sensitivity of
the EAIS to warmer-than-present conditions, and therefore to future
anthropogenic climate change, they have only considered one aspect of
ice–ocean coupling: the impact of changes in the oceanic circulation on the
ice sheet. consider instead the impact of changes in the
ice sheet on the ocean, and show that the dynamics of the Southern Ocean are
sensitive to increases in meltwater flux from the West Antarctic Ice Sheet
(WAIS). This raises the possibility of regional ice sheet–ocean feedback
loops that could accelerate any anthropogenically triggered melting of
sectors of the WAIS. also identify three distinct
feedback loops whereby basal melting of the Antarctic Ice Sheet influences
the circulation of the Southern Ocean. The dominant feedback loop involves
local cooling; this loop is negative. A secondary feedback loop involves
acceleration of the subpolar gyres; this loop is positive. A minor additional
feedback loop involves weakening of the Antarctic Circumpolar Current (ACC);
this loop is negative.
In this study, we explore such feedback loops from the perspective of the
EAIS. We present a suite of model simulations that consider idealised
scenarios corresponding to the collapse of the Wilkes Basin and analyse the
potential impact of the meltwater flux on the dynamics of the Southern Ocean
over the coming centuries and millennia. The Wilkes Basin is selected for the
current study as it is known to be particularly vulnerable to marine ice
sheet instability, with the potential to raise global sea levels by
∼ 3–4 m. It also lies alongside the Mawson
Gyre , which is connected to the other subpolar gyres
in the Southern Ocean and therefore provides a
mechanism by which meltwater may be transmitted around the continent.
To better understand both present and potential future interactions between
the EAIS and the ocean around Antarctica, we undertake a number of
simulations using a fully coupled climate system model. We use these
simulations to investigate the response of the Southern Ocean to meltwater
input from specific locations along the George V Coast, which fringes the
Wilkes Basin in East Antarctica (Fig. ). Previous climate
modelling studies have highlighted the sensitivity of the simulated regional
and global impacts to the precise location of the freshwater input
. By applying the freshwater fluxes at
different locations within our model, we are able to assess the sensitivity
of our results to the details of the experimental design.
Map of the Antarctic Ice Sheet, highlighting the location of the
Wilkes Basin.
The model simulations are described in Sect. . The response of
the Southern Ocean under a default scenario representing the collapse of the
Wilkes Basin is assessed in Sect. . The sensitivity of our
results to the precise location of the meltwater input is explored in
Sect. . The response to meltwater input is compared and
contrasted with the response to anthropogenic forcing in Sect. .
Finally, the results are discussed and conclusions presented in
Sect. .
Methods
We use the CSIRO Mk3L climate system model version 1.2 in this study. The
model comprises fully interacting atmosphere, ocean, land surface and sea ice
components, and is designed for long-term climate simulations
. The ocean model has a horizontal
resolution of 2.8∘× 1.6∘ and 21 vertical
levels, while the atmosphere model has a horizontal resolution of
5.6∘× 3.2∘ and 18 vertical levels. We use
CSIRO Mk3L here because it includes full dynamic complexity in its
sub-models, and yet is sufficiently fast to allow us to explore the long-term
evolution of the Southern Ocean to meltwater fluxes. Although the
computational efficiency is achieved at the cost of a relatively coarse
spatial resolution, the model has demonstrated utility for simulating past
and future changes in the climate system. This includes the millennial-scale
response to natural and anthropogenic forcings
and the long-term response of the Southern
Ocean to enhanced Antarctic meltwater input
.
Locations of freshwater input in experiments WILKES, WEST and EAST.
We employ an ensemble modelling approach. For each experiment, the model is
integrated three times. Each of these simulations is identical, except for
the fact that it is initialised from a different year of a pre-industrial
control simulation. Such an approach allows us to better distinguish between
natural internal variability and the forced response of the climate system to
the meltwater fluxes. The results reported here represent the ensemble mean
for each experiment, with any anomalies being calculated relative to the
pre-industrial control simulation. The statistical significance of the
anomalies is determined using a Student's t test, under the null hypothesis
that the climate of the hosing simulations is identical to the climate of the
control simulation.
We focus on three idealised scenarios, each of which represents a
hypothetical collapse of the Wilkes Basin: WILKES, WEST and EAST. The
locations of meltwater input within these experiments are shown in
Fig. . In all of the experiments, a freshwater flux of
0.048 Sv is applied for 900 years. This is based on the estimate of
, and is equivalent to an increase of ∼ 3.8 m
in global sea level. While the precise mechanisms that may trigger
instability across the extensive Wilkes Basin remain a subject of debate
, such a rapid and substantial
addition to global sea level in future is hypothetically possible. The
meltwater that is added to the ocean is assumed to have the same temperature
as the ambient seawater, and thus there is no heat flux associated with the
meltwater input. Once the freshwater flux has been applied, the experiments
are then integrated for a further 600 years to allow the climate system to
recover.
In its current configuration, the Wilkes Basin is drained through the Ninnis
and Cook ice streams along the George V Coast . The
meltwater input within the model simulations is therefore implemented as a
flux into the Southern Ocean in this region (Figs.
and ). However, the Wilkes ice sheet rests on two deep troughs
that lie below sea level . In the event of a
substantive collapse and retreat of the Wilkes Basin, discharge into the
Southern Ocean would therefore occur at a location up to
∼ 800 km south of the current position . We
do not take this into account in our experiments, maintaining a constant
location for the freshwater input throughout. Neither do we explicitly
account for calving of icebergs, assuming instead that meltwater is delivered
immediately and locally to the Southern Ocean.
Experiments WEST and EAST are identical to WILKES, except that the
hosing region is displaced to the west and east, respectively
(Fig. ). A critical difference between these two
additional experiments is that, in WEST, the hosing region is situated
in the open Southern Ocean, while in EAST it is located in an
embayment on the model grid. These experiments assess the sensitivity
of our results to the precise location of the freshwater input, and
therefore assess the sensitivity of our results to our assumptions
regarding the location of meltwater delivery from the Wilkes Basin.
In this study, we wish to isolate the effects of enhanced meltwater input
from the Wilkes Basin and to explore potential feedback loops. We therefore
intentionally neglect the meltwater that is likely to come from other sectors
of the Antarctic Ice Sheet under future climate change scenarios. We also use
a constant atmospheric CO2 concentration of 280 ppm in all of our
hosing experiments. However, elevated atmospheric CO2 concentrations can
have substantial implications for both sea ice production and Antarctic Bottom Water (AABW) formation
e.g..
We therefore run an additional experiment, 4CO2, in which the atmospheric
CO2 concentration is quadrupled. Starting from the pre-industrial level of
280 ppm, the CO2 concentration is increased at 1 % per year
until it reaches 1120 ppm after 140 years. It is then held constant
thereafter. There is no freshwater input into the Southern Ocean in this
experiment.
(a) The rate of Antarctic Bottom Water (AABW) formation (Sv), and
(b) the strength of the Antarctic Circumpolar Current (ACC) (Sv), in experiments WILKES
(red), WEST (green), EAST (dark blue) and 4CO2 (light blue). Thin lines
indicate individual ensemble members; thick lines indicate the ensemble
means. The values shown are 100-year running averages. Solid horizontal lines
indicate the mean values for the control simulation. Dashed vertical lines
indicate the years in which the freshwater hosing begins and ends.
Annual-mean climatology for the pre-industrial control simulation:
(a) sea surface temperature (∘C), (b) mean
temperature at a depth of 200–400 m (∘C),
(c) mean temperature at a depth of 400–700 m
(∘C), (d) mean temperature at a depth of
700–1000 m (∘C), (e) sea surface salinity
(psu), (f) convective depth (m), (g) sea ice
concentration (%) and (h) barotropic streamfunction
(Sv).
Past climate modelling studies have employed intermediate-complexity models
e.g. or have applied freshwater fluxes across large
sectors of the Southern Ocean e.g..
However, other recent studies have shown that Antarctic meltwater input can
be highly localised and yet still have
important implications for the Southern Ocean circulation, atmospheric
processes, sea ice regimes and potentially ice sheet dynamics
.
Thus, while our experiments represent idealised scenarios, they nonetheless
allow us to explore the response of the Southern Ocean to localised meltwater
input. This is critical for better understanding the evolution and response
of the Southern Ocean to future meltwater inputs that may be highly spatially
localised.
Marine instability across the Wilkes Basin
The rate of AABW formation in experiment WILKES is
shown in Fig. a. The mean rate of AABW formation in the control
simulation is 6.8 Sv, which is close to the observational estimate of
8.1–9.4 Sv. Upon the commencement of the freshwater
hosing, there is a rapid reduction of ∼ 20 % in the rate of AABW
formation. It then remains in a weakened state throughout the hosing phase.
There is a rapid recovery as soon as the freshwater hosing ceases, although
it takes several centuries before the rate of AABW formation returns to the
same level as in the control simulation. Considerable low-frequency
variability is apparent within each simulation, even after the application of
a 100-year running-mean filter. This highlights the merits of an ensemble
modelling approach, which allows us to better isolate the simulated response
of the climate system to the meltwater input.
For reference, the pre-industrial climatology of the model is presented in
Fig. . Consistent with observations e.g.,
simulated deep water formation occurs in the Weddell Sea, the western Ross
Sea and, to a lesser extent, in the region around the Amery Basin
(Fig. f). However, deep water is created through deep
convection in the open ocean, rather than forming over the continental
shelves. Other climate system models, including the higher-resolution models
that participated in the Coupled Model Intercomparison Project Phase 5
(CMIP5), display a similar bias . This reflects
the difficulty of capturing the fine-scale interactions that drive the
formation of AABW . The model simulates a
Weddell Gyre with a strength of 30 Sv but does not resolve a gyre in
the Ross Sea (Fig. h).
The response of the Southern Ocean to the freshwater input in experiment
WILKES is presented in Fig. . The changes are shown as the
ensemble-mean anomalies during the final 100 years of the hosing phase,
relative to the equivalent years of the pre-industrial control simulation.
Sea surface temperatures (SSTs) increase by ∼ 1 ∘C
off the coast of Wilkes Land (Fig. a). Weaker warming is also
apparent in other coastal regions, particularly in the Bellinghausen,
Amundsen and Ross seas. Cooling occurs over the open Southern Ocean, and is
strongest to the north of the Ross Sea. A similar temperature signal is
apparent at a depth of 200–400 m, with the greatest increase of
1.2 ∘C occurring at 122∘E along the coast
of Wilkes Land (Fig. b). However, the warming at this depth is
more evenly distributed around Antarctica. Intriguingly, the warming signal
is also found deeper within the water column, propagating westwards around
the coast of Antarctica with depth (Fig. c–d). At a depth of
400–700 m, the greatest temperature increase of
0.9 ∘C occurs at 75∘E, at the mouth of the
Amery Ice Shelf. The strongest warming at a depth of 700–1000 m is
also found in this region, but a temperature increase of a similar magnitude
is observed along the coast of Dronning Maud Land as well.
Thus, the response of the model is characterised by two key features:
firstly, there is a near-instantaneous reduction in AABW formation as
soon as the freshwater hosing is applied; secondly, there is a warming
signal along the coast of the EAIS that propagates westwards with
depth.
Mean anomalies for the final 100 years of the hosing phase (i.e.
years 801–900) of experiment WILKES, relative to the equivalent years of the
pre-industrial control simulation: (a) sea surface temperature
(∘C), (b) mean temperature at a depth of
200–400 m (∘C), (c) mean temperature at a
depth of 400–700 m (∘C), (d) mean
temperature at a depth of 700–1000 m (∘C),
(e) sea surface salinity (psu), (f) convective depth
(m), (g) sea ice concentration (%) and
(h) barotropic streamfunction (Sv). Only values that are
significant at the 5 % probability level are shown.
To explore the dynamical mechanisms that give rise to this behaviour,
the change in the sea surface salinity (SSS) is shown in
Fig. e. The meltwater input causes a strong reduction in
the SSS throughout the hosing region. However, the coastal currents
(Fig. ) also carry the fresher surface waters westward,
with negative SSS anomalies propagating as far west as the Weddell
Sea. This freshening reduces the density of the surface waters,
increasing the vertical stratification of the water column and
reducing convective depth (Fig. f; the convective depth
is defined here as the maximum depth over which the water column is
well mixed through convection of dense surface waters). The largest
changes occur in the regions of deep water formation within the model
(Fig. f), particularly in the Weddell Sea and, to a
lesser extent, in the western Ross Sea.
Remarkably, we therefore find that the largest changes in convective depth
occur on the opposite side of the continent from the region of freshwater
input. Furthermore, this outcome occurs despite the fact that the salinity
signal in the Weddell Sea is weak; the average decrease in SSS between 37 and
8∘W, where the greatest reduction in convective depth occurs,
is only 0.13 psu. This demonstrates that the Weddell Sea is extremely
sensitive to freshwater input within the model and can be significantly
impacted by melting on the other side of the continent, as a result of the
surface freshening being carried westwards by the coastal currents.
The precise mechanism whereby a surface freshening can give rise to warming
at depth is explored by . An enhanced meltwater
flux into the Southern Ocean stratifies the surface waters, reducing vertical
mixing and hence reducing the exchange of cold surface waters with the
underlying warmer waters. This leads to cooling at the surface and warming at
depth. We see the same mechanism operating in our simulations: as the fresher
surface waters propagate westwards, we find a reduction in convection,
accompanied by a reduction in temperature at the surface and an increase in
temperature at depth. However, an exception to this pattern is found along
the coast of Wilkes Land, where sea surface temperatures are warmer. This is
a region where there is no vertical mixing within the model
(Fig. f). Instead, the warming appears to be due to local sea
ice feedbacks, with a marked reduction in sea ice cover in this region
(Fig. g).
Climatological annual-mean ocean velocities (ms-1) for
the pre-industrial control simulation: (a) surface velocity,
(b) mean velocity at a depth of 200–400 m,
(c) mean velocity at a depth of 400–700 m and
(d) mean velocity at a depth of 700–1000 m.
The strength of the ACC in experiment WILKES is shown in
Fig. b. The mean strength of the ACC in the control simulation
is 175 Sv, which is stronger than the observational estimate of
136.7 ± 7.8 Sv. There is a small
reduction of ∼ 2 Sv upon the commencement of freshwater hosing,
although this reduction does not persist until the end of the hosing phase. A
weak reduction in the strength of the ACC in response to ice sheet melting is
consistent with the results of . The changes in the
barotropic streamfunction (Fig. h) reveal a reduction in the
strength of the Weddell Gyre but a positive gyre-like anomaly in the Ross
Sea. This contrasts with , who find a strengthening of
both the Weddell and Ross Sea Gyres in response to ice sheet melting.
Sensitivity to hosing region
The rates of AABW formation for experiments WEST and EAST are shown in
Fig. a. The behaviour of both experiments is broadly similar to
WILKES. There is a rapid and sustained reduction upon the application of
freshwater hosing, followed by a rapid recovery once the hosing ceases.
However, there is slightly stronger suppression of AABW formation in WEST
than in WILKES or EAST.
Consistent with the greater decrease in AABW formation, the surface warming
is strongest in WEST, with an increase of up to 1.9 ∘C in SST
(Fig. a). This is noticeably stronger than the maximum warming
of 1.2 ∘C in WILKES. In contrast, the surface temperature
response is noticeably weaker in EAST than in WILKES, with maximum warming of
only 0.9 ∘C (Fig. a). This pattern is mirrored
in the temperature response at depth (Figs. b–d and
b–d). Both WEST and EAST simulate a similar pattern of warming
to WILKES, including the westward propagation of the temperature changes.
However, the magnitude of the changes is largest in WEST and smallest in
EAST.
Examination of the surface freshening reveals the explanation for this
difference in behaviour between the experiments. In WEST, where the
freshwater is applied to the open ocean, the surface freshening propagates
further around the continent than in WILKES (Fig. e). In
contrast, in EAST, where the freshwater is applied to an embayment on the
model grid, the surface freshening remains much more localised
(Fig. e). Over the eastern part of the Weddell Sea, between 37
and 8∘W, the average decrease in SSS is 0.22 psu in
WEST but 0.13 psu in EAST. The resulting suppression of convection is
therefore stronger (Figs. f and f).
As Fig. , but for experiment WEST.
As Fig. , but for experiment EAST.
As Fig. , but for the first 100 years after the
atmospheric CO2 concentration has stabilised (i.e. years 141–240) in
experiment 4CO2.
Comparison with the response to anthropogenic forcing
Experiment 4CO2 allows us to compare and contrast the response to meltwater
input with the response to anthropogenic forcing. Figure a
reveals a strong and persistent collapse in the rate of AABW formation in
response to a quadrupling of the atmospheric CO2 concentration within
experiment 4CO2. This can be attributed to the large reduction in sea ice
cover Fig. g; see also. The
freshwater hosing experiments WILKES, WEST and EAST also feature a reduction
in the rate of AABW formation, indicating that melting of the EAIS might be
expected to amplify the anthropogenic signal.
There is a strong and persistent increase in the strength of the ACC
(Fig. b). This is in contrast to the small reduction in
strength in the freshwater hosing experiments, indicating that ice
sheet melting might act as a weak negative feedback in this case. This
result is consistent with the findings of . The
changes in the barotropic streamfunction indicate strengthening of the
Weddell Gyre and a strong positive gyre-like anomaly in the Ross
Sea. The change in the Weddell Gyre contrasts with the weakening in
the freshwater hosing experiments, while the changes in the Ross Sea
are consistent. Thus, ice sheet melting might be expected to act as a
negative feedback in the Weddell Gyre but a positive feedback in the
Ross Sea. This contrasts with the results of ,
where ice sheet melting acts as a positive feedback in both sectors.
The response of the Southern Ocean is presented in Fig. . The
changes shown are the ensemble-mean anomalies during the first 100 years
after the atmospheric CO2 concentration has stabilised (i.e.
141–240 years after the start of the experiment), relative to the equivalent
years of the pre-industrial control simulation. Although SST increases by up
to 5.4 ∘C along the coast of Antarctica
(Fig. a), the maximum warming adjacent to the coast decreases
with depth to 2.8 ∘C at 200–400 m
(Fig. b), 2.2 ∘C at 400–700 m
(Fig. c) and 2.3 ∘C at 700–1000 m
(Fig. d).
These figures compare with warming of up to ∼ 1 ∘C in
response to a collapse of the Wilkes Basin within experiment WILKES. In
specific locations, the responses to the two forcings can be similar in
magnitude. For example, at the mouth of the Amery Ice Shelf at
72∘E, the temperature increase at a depth of
700–1000 m is 1.0 ∘C in 4CO2 and
0.8 ∘C in WILKES. Ice sheet melting therefore acts as a
strong positive feedback, amplifying the warming adjacent to the grounding
lines of the Antarctic Ice Sheet.
Overall, this comparison reveals that melting of a single sector of
the EAIS has the potential to act as a positive feedback that can
amplify the response to anthropogenic forcing. Our experiments reveal
that ice sheet melting might be expected to act as a positive feedback
on the changes in AABW formation, the Ross Sea Gyre and, in
particular, on warming of the ocean at depth. Only in the case of the
ACC and the Weddell Gyre might ice sheet melting be expected to act as
a weak negative feedback.
Discussion and conclusions
Using a coupled climate system model, we have found that deep water
formation in the Weddell Sea is highly sensitive to freshwater input
from a key sector of the EAIS. A previous modelling study has also
found that overturning in the Weddell Sea is sensitive to changes in
the salinity of the surface ocean . However, we
establish here that remote freshwater input is sufficient to trigger a
reduction in Weddell Sea convection. Melting of the EAIS can therefore
lead to a reduction in AABW formation, as a result of fresher surface
waters being carried westwards by the coastal currents. This result
has significant implications for the response of the Southern Ocean to
the melting of sectors of the Antarctic Ice Sheet.
The warming at increasing depth around the coast can be attributed to the
coastal currents, giving rise to a “domino effect”. According to this
effect, the temperature changes propagate westwards around the coast of the
Antarctic continent with increasing depth, representing a positive feedback
mechanism that has the potential to amplify melting around the continent.
Indeed, we find that this mechanism occurs even without the freshwater melt
from other sectors of the Antarctic Ice Sheet. The rapid recovery in AABW
formation that we find within our experiments might therefore be considered
conservative.
We speculate that an initial melting, in this instance off the coast of
Wilkes Land, might set off a warming signal with increasing depth around the
coast. This would have the potential to enhance melting along vulnerable
grounding lines across the EAIS, which would then result in additional
warming and melting further to the west. Subsequent warming and melting might
be expected even if the melting of the Wilkes subglacial basin later reduces
in magnitude or ceases entirely. Thus, destabilisation of large sectors of the
EAIS could arise from warming and melting in just one area.
Furthermore, recent work has highlighted the sensitivity of the Weddell Sea
sector of the WAIS to changes in local ocean circulation .
This is exacerbated by the presence of steep reverse slope beds in regional
ice streams , making this sector particularly vulnerable to
warming . We have
established in this study that melting of the EAIS can lead to reduced
convection and warming at depth in the Weddell Sea. The Weddell Sea sector of
the WAIS is grounded at a depth of ∼ 1000–1200 m below sea
level . It is therefore vulnerable to the deep warming
that we find in our experiments, suggesting that localised melting of one
sector of the EAIS might be sufficient to destabilise at least one key sector
of the WAIS as well.
Through a comparison of our results with an experiment in which the
atmospheric CO2 concentration is quadrupled, we have shown that
anthropogenically induced melting of the EAIS has the potential to act
as a positive feedback on the CO2-forced changes in AABW formation,
the Ross Sea Gyre and, in particular, on the warming of the Southern
Ocean adjacent to the grounding lines of the Antarctic Ice Sheet. Only
in the case of the ACC and the Weddell Gyre might melting of the EAIS
be expected to act as a weak negative feedback. The response of the
Southern Ocean is sufficiently rapid that, on sub-centennial
timescales, it has the potential to exceed the magnitude of the
initial response to enhanced atmospheric CO2.
We have conducted a suite of experiments which reveal that, to first
order, our conclusions are robust with regard to the location of
freshwater input within the model. However, we also find that the
magnitude of the simulated changes is sensitive to the precise
location of the hosing region. This demonstrates that care must be
taken when designing experiments which simulate the effects of ice
sheet melt.
This study is based on a single climate system model, and therefore we cannot
rule out the possibility that the results are simply a model artefact. In
particular, one potential bias arises from the fact that CSIRO Mk3L forms
AABW through open-ocean deep convection rather than over the continental
shelves, consistent with other models . Given
the potential importance of the feedback loop that we have identified for
future changes in the Antarctic Ice Sheet and global sea level, similar
experiments should be conducted using additional models. Climate system
models with a high spatial resolution might be better able to resolve the
fine-scale interactions that control the formation of AABW
. Nonetheless, we have highlighted the
importance of incorporating ice sheet–ocean processes into the Earth system
models that are used to generate future climate projections.
Data availability
The Antarctic Ice Sheet is represented in Fig. 1 using the RADARSAT-1
Antarctic Mapping Project (RAMP) first Antarctic Mapping Mission (AMM-1)
synthetic aperture radar (SAR) image mosaic of Antarctica .
The image data can be accessed via
https://nsidc.org/data/docs/daac/nsidc0103_ramp_mosaic.gd.html. The
output from the CSIRO-Mk3L-1-2 pre-industrial control simulation can be
accessed via the Earth System Grid Federation (e.g.
https://esgf.nci.org.au/projects/esgf-nci/). The other data shown in
Figs. 2–9 is provided as a supplement to this manuscript.
The Supplement related to this article is available online at doi:10.5194/tc-10-2317-2016-supplement.
All the authors contributed towards the design of
the study. Steven J. Phipps conducted the model simulations and analysed the
output. Steven J. Phipps led the writing of the manuscript, with
contributions from all the authors.
Acknowledgements
This research was supported under the Australian Research Council's
Laureate Fellowships funding scheme (project ID FL10010019),
Future Fellowships funding scheme (project ID FT120100004) and
Special Research Initiative for the Antarctic Gateway Partnership (project ID
SR140300001). It was undertaken with the assistance of resources provided at
the Australian National University through the National Computational Merit
Allocation Scheme supported by the Australian Government. It also includes
computations using the Linux computational cluster Katana supported by the
Faculty of Science, UNSW Australia. The authors wish to acknowledge use of
the Ferret program for analysis and graphics; Ferret is a product of NOAA's
Pacific Marine Environmental Laboratory
(http://ferret.pmel.noaa.gov/Ferret/). This research forms a
contribution to the Australasian Antarctic Expedition 2013–2014
(www.spiritofmawson.com). Edited by:
C. Ritz Reviewed by: two anonymous referees
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