![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Totten Ice Shelf Melt and Acceleration caused
by Wind The Totten Glacier in
Eastern Antarctica could potentially raise the global sea level by 3.5
m, at least, but the sensitivity of the glacier to climate change has
not been well understood. The Totten Ice Shelf, which has exhibited
variable speed, thickness, and position of the grounding line in recent
years, couples the Totten Glacier to the ocean.
In
this study the velocity of the ice is compared to the oceanic wind
stress in order to understand the drivers of the interannual variability
and it was found that there is a consistent pattern of acceleration of
the ice shelf 19 months after upwelling anomalies occur at the
continental shelf break nearby. The sensitivity of climate forcing
observed by Greene et
al. is a
response to the redistribution of oceanic Heat that is wind-driven and
is independent of warming of the atmosphere and ocean on a large scale.
A link between the Totten Glacier and upwelling near the coast of East
Antarctica, where surface winds have been projected to intensify over
the next century as a result of greenhouse gas concentrations increasing
has been established by this study that was carried out by Greene et
al. An ice basin of 550,000
km2
area, with a base primarily below sea level, is drained by the Totten
Glacier, which indicates a potential vulnerability to rapid collapse
(Weertman, 1974; Schoof, 2007). The grounding line at which the Totten
Glacier begins to float to become the Totten Ice Shelf (TIS) has
retreated recently (Li et
al.,
2015) while the velocity of the ice shelf and terminus position have
been unstable (Li et
al.,
2016; Roberts et
al.,
2017). From 2002 to 2008 a marked lowering trend was shown by the Totten
Ice Shelf (Pritchard et
al.,
2012; Rignot, Jacobs, Mouginot & Scheuchl, 2013) which continued to at
least 2012 in the part of the glacier that was grounded (Young et
al.,
2015), though it is suggested by longer records of the Totten Ice Shelf
surface elevation that the subdecadal trends may represent only part of
a variability on a longer term (Roberts et
al.,
2017; Paolo, Fricker & Padman, 2015). A variable supply of
warm, salty, modified circumpolar deep water (mCDW) has been believed to
be a driver of changes in the TIS that have been observed (Li, Rignot,
Mouginot & Scheuchl, 2016; Miles, Stokes & Jamieson, 2016; Rignot &
Jacobs, 2002), which can
access the water cavity beneath the
Totten Ice Shelf by a network of bathymetric troughs (Greenbaum et
al., 2015). Along the outer
continental shelf it has been repeatedly shown by ship-based
observations that mCDW is present (Wakatsuchi et
al.,
1994; Bindoff, Rosenberg & Warner, 2000; Williams et
al.,
2011; Nitsche et
al.,
2017), and it was confirmed by a recent survey that the mCDW can
traverse the continental shelf and fill the troughs near the Totten Ice
Shelf ice front (Rintoul et
al.,
2016; Silvano et
al.,
2017). Interannual variability of the Totten Ice Shelf melt rate has
been linked by ocean models to the production of sea ice, which
generates water that is cold and dense with the potential to displace
mCDW and quench melt (Khazendar et
al.,
2013; Gwyther et
al.,
2014); though at the time of the only survey that was conducted on the
continental shelf no such cold, dense water was detected (Rintoul et
al., 2016; Silvano et
al.,
2017; Silvano, Rintoul & Herráiz-Borreguero, 2016). It is suggested by
both models and observations that the melt rate of the Totten Ice Shelf
is modulated by a supply of mCDW that is variable, though the mechanism
that drives the mCDW exchange across the continental shelf break has not
yet been explained, and no links between forcing mechanisms and the
response of the Totten Ice Shelf have been observed directly. Insights
into the drivers of the variability of the Totten Ice Shelf may be
present in West Antarctica, where it has been hypothesised that similar
behaviours that have been observed at the Pine Island Ice Shelf results
from a variable supply of CDW, which is forced onto the shelf by wind
processes at the shelf break (Wåhlin et al., 2013; Dutrieux et al.,
2014; Webber et
al.,
2017; Kim et al., 2017). In this paper Greene et
al. report the results of their
investigation into the causes of recent Totten Ice Shelf acceleration
and deceleration by comparing a 14-year time series of the velocity of
the ice shelf to oceanic wind stress. They used the velocity of the ice
surface as a proxy for changes in ice thickness that were driven by melt
as well as a direct measure of the response of the Totten Ice Shelf to
variable forcing. The velocity time series of the Totten Ice Shelf was
driven by a template-matching algorithm applied to 629 satellite image
pairs that were obtained between February 2001 and September 2014.
Reanalysis data of surface wind and sea ice data were used to calculate
zonal and meridional components of wind stress. They focused their study
on local regional upwellings, which develop at locations where surface
waters are caused to diverge by the wind. Surface water is transported
90o
to the left of the wind direction in the Southern Hemisphere as a result
of the rotation of the Earth, so divergence of surface water is given by
the mathematical curl of the wind stress. Greene et
al.
defined upwelling as the vertical water velocity at the bottom of the
surface layer, which they estimated from the wind stress curl. In order to assess the
response of the Totten Ice Shelf to interannual forcing from the ocean
Greene et
al.
limited analysis of velocity to a region where the ice shelf is bounded
laterally by shear margins, between 20 km and 40 km from the ice front,
where they expected minimum influence from the pinning points, processes
of calving, or velocity anomalies that are associated with the lateral
motion near the ice front. They report an increase of 5 % in surface
velocity from 2001-2006 which was followed by an immediate trend
reversal, slowing by 6 % by 2013. In 2005 and 2009 they observed minor
velocity minima, and in 2010 and 2014 they observed minor maxima. Totten Ice Shelf velocity
is correlated negatively, with a 19-month lag, with zonal wind through
the domain, which indicates that Totten Ice Shelf accelerates in
response to weakening of the eastward winds that drive the
Antarctic Circumpolar Current (ACC) or strengthening of the westward
winds that drive the Antarctic Coastal Current. Greene et
al.
assumed that the velocity of the Totten Ice Shelf is linked to a
variable supply of mCDW, with lag times primarily being due to the time
required for melt rate anomalies to integrate and cause sufficient
thinning to produce an observable response in surface velocity
(Christianso et al., 2016). To the north of the continental shelf break,
over the deep ocean, the negative correlation with the zonal wind
contrasts with the notion derived from classical Ekman dynamics, that
upwelling of warm deep water near 63oS
should be induced by positive zonal wind anomalies.
Over
the continental shelf, westward winds were expected to induce southward
transport of surface water, depress isotherms, and could therefore
prevent the surmounting of the continental shelf (mCDW) (Oshima et al.,
1996; Hayakawa et al., 2012), yet evidence is presented by Greene et
al. that competing processes
prevail. Prevailing westward winds over the continental
shelf serve as the southern component of the wind stress curl, which
causes upwelling along the continental slope. The negative correlation
between the velocity of the Totten Ice Shelf and the zonal wind over the
continental shelf is associated with a positive correlation between the
Totten Ice Shelf velocity and the upwelling along the continental slope.
The zonal winds over the deep ocean maintain their negative correlation
with Totten Ice Shelf velocity, in particular where downwelling occurs
in compensation for upwelling along the continental slope. It is
suggested by these observations that the meridional gradient of wind
stress contributes more to the variability of the Totten Ice Shelf
velocity than do zonal wind stress anomalies that are uniform. In the region the mean
coastal wind flow is oriented in such a way that its meridional
component is small or nil, except for the northward flow diversion
around Law Dome to the west of the Totten Ice Shelf front. Throughout
most of the region of the study, where the meridional component of the
mean velocity field in almost zero, Totten Ice Shelf shows a weak,
though slightly negative, relationship with meridional wind stress.
Totten Ice Shelf shows a weak (r2<0.2)
positive correlation with meridional wind where coastal wind directs
north around Law Dome. Totten Ice Shelf velocity is correlated
positively with the concentration of sea ice throughout much of the
domain, with small regions that have negative correlation over the
continental shelf and over the deep ocean. The relationship between the
concentration of sea ice and Totten Ice Shelf velocity is quite weak (r2<0.15)
everywhere in the region. It is revealed by linear
regression of upwelling and Totten Ice Shelf velocity that the Totten
Ice Shelf accelerates at times of strong upwelling along the continental
slope. Bathymetric contours are roughly followed by the relationship,
which indicates the role of the topography of the seafloor in blocking
mCDW intrusions at times of weak upwelling. Along the Antarctic Coastal
Current an upwelling zone lies upstream which exhibits a particularly
strong relationship with Totten Ice Shelf velocity (r2<0.85).
It is implied by this that Totten Ice Shelf accelerates in response to
increased melt following strong upwelling anomalies along the
continental slope. Correlation is maximised with a lag of 19 months,
which indicates the time required for upwelled mCDW to traverse the
continental shelf, enter the water cavity below Totten Ice Shelf, induce
melt, and lead to ice shelf acceleration by lateral shear stress that is
reduced.
Discussion Upwelling is implicated
further by oceanographic observations as a driver of mCDW variability on
the continental shelf, where temperature anomalies of 2oC
or more between 450 m and 650 m depth can result from shoaling of the
thermocline associated with upwelling along the nearby continental
slope. In the region of the study of the velocity time series
observations 550 m is the mean depth of the Totten Ice Shelf, and at
that depth a +2oC
temperature anomaly represents a 6-fold increase in thermal driving
potential relative to temperature minima of 0.4oC
above the in situ freezing point that has been observed. It has been
indicated by models that ice shelf melt rates scale superlinearly to
quadratically with thermal driving potential (Holland, Jenkins &
Holland, 2008; Little, Gnanadesikan & Oppenheimer, 2009; Gwyther et al.,
2015), which suggests that some areas of the base of the Totten Ice
Shelf can experience more than a 10-fold increase or decrease in melt
rate depending on the availability of mCDW that has been upwelled.
Widespread presences of
mCDW on the continental shelf is shown by profiling ship-based and float
observations, and the thickness of the mCDW layer is linked to upwelling
along the continental slope. Greene et
al.,
suggest that after mCDW surmounts the continental slope, the westward
winds driving the coastal current may enhance the delivery of mCDW to
the water cavity beneath the Totten Ice Shelf, where the base of the ice
shelf is highly sensitive to small changes in thermal forcing. Also,
cold meltwater from the cavity below the ice shelf may be flushed out or
cavity circulation may be intensified and increase the melt (Gwyther et
al., 2016). Surface velocity that is
averaged over the main trunk of Totten Ice Shelf reached a maximum in
early 2007 corresponding to a minimum (Roberts et al., 2017) ice
thickness that was reported, at the time of a minimised lateral shear
stress restraining the Totten Ice Shelf flow. The linear trend of
slowdown of Totten Ice Shelf amidst ongoing acceleration of surrounding
grounded ice is similar to a pattern seen at the Pine Island glacier,
where the ice shelf has shown a response to ocean forcing by
accelerating about 9 months after arrival at the ice front of thermal
anomalies (Christianson et al., 2016). The 19 month lag that was
observed by Greene et
al. from
the time of upwelling along the continental slope to acceleration of the
Totten Ice Shelf includes the time for the mCDW to traverse the
continental shelf. Totten Ice Shelf is much thicker than the Pine Island
Ice Shelf, on average, and is therefore expected to respond more slowly
to anomalies of basal melt (Christianson et al., 2016). The region along the
continental slope where upwelling is highly covariant with the velocity
of the Totten Ice Shelf is close to a persistent eddy feature where it
has repeatedly been revealed by dissolved silicate measurements that
upwelling (Wakatsuchi et al., 1994); warm, saline CDW has been detected
(Bindoff, Rosenberg & Warner, 2000; Williams et al., 2011); and it has
been shown that upwelling is correlated positively with the Southern
Annular Mode (SAM) (Hayakawa et al., 2012). The SAM is the leading mode
of climate variability in the Southern Hemisphere, it is influenced
seasonally by various natural and anthropogenic drivers (Fogt et al.,
2009), and its positive mode is associated with an intensification of
the eastward winds around Antarctica (Thompson et al., 2011). The SAM
has been tending towards its positive phase in summer in recent decades,
which is primarily due to effects of substances that deplete the ozone,
though an increasing influence of greenhouse gas in the atmosphere is
expected to dominate the SAM in the coming century and continue its
positive bias as the ozone hole recovers (Fyfe et al., 2007; Sigmond et
al., 2011). It is shown by projections that an intensification of the
Antarctic Circumpolar Current (ACC), that is driven by the wind, and an
upwelling increase, in particular along the Antarctica continental slope
(Fyfe et al., 2007; Wang, 2013; Spence et al., 2014). Greene et
al.
suggest it is possible that westward winds along the coast could weaken
in conjunction with a migration to the south of the divergence zone
(Spence
et al., 2014), In which case the delivery of the mCDW to the Totten Ice
Shelf could be weakened by a weakened coastal current; projections of
coastal westward winds near Totten Ice Shelf are few, however, and their
relationships to SAM or atmospheric greenhouse gas have not been
validated. According to Greene et
al. they have confirmed the role of
upwelling that is driven by the wind as a primary mechanism of delivery
of the mCDW on the continental shelf of East Antarctica and have shown
that upwelling of mCDW is directly correlated with the velocity of
Totten Ice Shelf that is melt driven. Over the Southern Ocean wind
patterns are expected to evolve throughout the 21st
century, and a shifting regime of upwelling could precipitate a response
that is marked in the Totten Glacier, unlocking the door to at least 3.5
m eustatic sea level potential (Greenbaum et al., 2015) in the vast ice
basin it drains.
Link
East Antarctica’s biggest glacier lost ice because of warm water and
strong winds
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |