Transcript Antarctic Ice Sheet - Atmospheric Sciences at UNBC

Antarctic Ice Sheet
• The Antarctic and Greenland ice sheets
contribute directly to sea level and ocean
circulation, and hence potentially to climate
change.
• It is estimated that the Greenland and Antarctic
ice sheets together contain enough water to
raise sea level by almost 70 m.
• The continent itself makes up about 10% of the
land surface of the Earth with the combined area
of the ice sheets and ice shelves covering 14 ×
106 km2.
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• The ice sheet has 3 distinct morphological
zones: East Antarctica, West Antarctica, and the
Antarctic Peninsula, with its highest point being
Vinson Massif (at 5440 m), located in the
Ellsworth Mountains.
• The vast majority of the surface of Antarctica is
covered with ice, with the continent as a whole
containing around 30 × 106 km3 or 90% of the
world’s freshwater.
• The ice thickness varies across the continent,
with its maximum being 4776 m about 400 km
south of Dumont d’Urville.
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Antarctica
Source:
www.geology.com
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• The mass of ice conceals the details of the
land below, which is made up of subglacial mountains and lower elevation
topographic features.
• The ice that builds up in the interior of the
Antarctic flows down to the edge of the
continent in ice streams that move at
speeds of up to 500 m per year.
• There is a strong correlation between
surface air temperature (SAT) and
elevation over Antarctica.
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http://www.dailymail.co.uk/sciencetech/article-1154522/
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• There is a “dipole” of cold air temperature on the
East Antarctica plateau where mean annual
SATs are < -55oC, whereas the South Pole
experiences an annual SAT of -50oC.
• Only in the northern part of the Antarctic
Peninsula in summer do monthly mean SATs
exceed freezing; hence, over the greater part of
the Antarctic ice sheets, there is little or no direct
ablation of the snow surface.
• In the Antarctic Peninsula and around the coast
of East Antarctica, the annual cycle of
temperature takes a familiar form with a broad
summer maximum and a minimum in July or
August.
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Air temperature in Antarctica
Source: http://en.wikipedia.org/wiki/File:Era40t.png
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• However, moving southward onto the polar
plateau, the form of the cycle changes to a short,
peaked summer season and a “coreless” winter
during which the temperature varies relatively
little.
• Contributing to this cycle are the abrupt changes
in solar radiation at the beginning and end of
winter darkness, whereas during the dark
period, the surface radiation balance is
determined only by net longwave radiation.
• In addition, warm air advection onto Antarctica
has a strong semi-annual component, and the
snowpack acts as a heat reservoir, smoothing
out extremes of temperature variations.
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Source: King and
Turner (1997)
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• As the surface of the Antarctic continent
cools radiatively, the air close to the
surface also cools relative to the air aloft.
• The near-surface air is thus negatively
buoyant and, over a sloping surface, will
accelerate down the slope in response to
the buoyancy force.
• The resulting flow, known as a “katabatic
wind”, will be turned by the Coriolis force
and impeded by surface friction.
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• Even on some of the modest slopes of
Antarctica, the surface wind is primarily
determined by katabatic forcing.
• In Antarctica and Greenland, the
combination of extensive sloping surfaces
and uninterrupted surface cooling for
much of the year allows a large-scale
katabatic circulation to develop.
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Antarctic Winds
Source: van Lipzig et al. (2004)
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Source: Parish and Bromwich (1991)
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Source: King and Turner (1997)
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Source: Parish and Bromwich (1991)
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Source: van Lipzig et al. (2004)
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Katabatic Winds
http://en.wikipedia.org/wiki/File:Katabatic-wind_hg.png
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Katabatic Winds
http://en.wikipedia.org/wiki/File:Vent_catabatique_-_Catabatic_Wind.jpg
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http://scientistatwork.blogs.nytimes.com/2010/12/15/katabatic-winds-of-antarctica/
Source: King and
Turner (1997)
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Source: Parish and
Bromwich (1991)
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Blue Ice Areas
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Source: Bintanja (1999)
Source: Bintanja
(1999)
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Source: Bintanja
(1999)
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Precipitation
• Knowledge of precipitation formation
mechanisms, snowfall distribution over the
Antarctic continent, and the synoptic
origins of the precipitating air masses is
important for the investigation of whether
the ice sheets are growing or shrinking
(mass balance), how accumulation may
change in an environment of higher mean
SAT, etc.
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• It has been estimated that about 2300 km3 yr-1 of
snow falls on the continent each year, but it is
extremely difficult to make measurements of
precipitation in Antarctica, not least because of
the difficulty in distinguishing blowing snow from
snowfall.
• With the strong winds experienced over the
continent, conventional snow gauges give poor
results so that other means of assessing
accumulation must be found, including the use
of snow stakes, stratigraphy, a knowledge of
snowdrift density, etc.
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• Net accumulation of snow at the surface is often
taken as equivalent to precipitation in Antarctica.
• The largest accumulations are found slightly
inland of the coast, where the steepest slopes are
found and where air masses are lifted and cooled
as they move poleward.
• The largest accumulations are over 1000 mm yr-1,
found near southeastern Bellingshausen Sea,
whereas the lowest values of snowfall
accumulation (< 50 mm yr-1) occur over the
Antarctic Plateau; this minimum in precipitation is
strongly correlated to air temperature.
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Annual Precipitation (mm swe)
http://en.wikipedia.org/wiki/File:File-Dgv-surfbal-1.gif
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• The saturation vapour pressure, through
the Clausius-Clapeyron relation, is by far
the most important parameter in
determining the distribution of precipitation
in Antarctica.
• An ice sheet gains mass by accumulating
snow that is transformed into ice by
densification processes.
• It loses mass (ablation) mainly by melting
at the surface or base with subsequent
runoff or evaporation of the meltwater.
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• Meltwater may refreeze within the snow
instead of being lost, and some snow may
sublimate from the surface back to the
atmosphere.
• Ice may also be removed by flowing into a
floating ice shelf, from which it is lost by
basal melting and calving of icebergs.
• In general, net mass ablation at lower
altitudes are balanced by the downhill flow
of ice under internal deformation and by
the sliding and bed deformation at the
base.
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• This balance is expressed usually as the rate of
change of the equivalent volume of liquid water,
in cubic metres per year; for a steady state the
mass balance is zero.
• Mass balances are computed for both the whole
year and individual seasons (winter and
summer) with the specific mass balance being
the net summer and winter mass balances
averaged over the surface area, in metres per
year.
• Ice-covered regions are dynamic environments
that are characterized by forcing responses at
variable timescales, which may not always be
synchronous with external weather and climate
forcing factors.
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• While seasonal accumulation and ablation
processes might balance in approximate
terms, internal ice dynamical processes
occurring at both shorter and longer
timescales contribute to balance
inequalities.
• Antarctic temperatures are so low that
there is virtually no surface runoff; the ice
sheet mainly loses mass by ice discharge
into floating ice shelves, which experience
melting and freezing at their underside and
eventually break up to form icebergs.
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• The mass balance equation is:
• Bn = Ma – Mm – Mc ± Mb
• Where Bn is the mass balance at the end
of the balance year, Ma is annual surface
accumulation; Mm is annual loss by glacial
surface runoff, Mc is annual loss by calving
of icebergs, and Mb is the annual balance
at the bottom (melting or freezing-on of
ice).
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• This equation suggests that the mass
balance can be inferred from two methods:
a) by direct measurement of the change in
volume by monitoring surface elevation
change and b) by the budget method,
determining each term on the right-hand
side of the equation separately.
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Term
Accumulation
Grounded ice
Ice shelves
Total
Ablation
Calving
Sub-ice melting
Surface runoff
Total
Net mass
balance
Mass Rate
(Gt/yr)
Uncertainty (%)
1528
616
2144
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-2016
-554
-53
-2623
-479
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50
50
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• Mass loss by iceberg calving is clearly the
largest negative term in the budget, but
sub-ice-shelf melting cannot be neglected.
• Runoff of meltwater is largely controlled by
the small amount of melting which takes
place in coastal regions during the
summer, with a small additional
contribution from subglacial melting.
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• If the terms in the mass balance are summed,
we obtain an imbalance in the net mass budget
of the Antarctic ice sheet; however, given the
large uncertainties in some of the terms,
particularly in the amount of calving, the
significance of this imbalance is not very high.
• Blowing snow also plays a role in the mass
balance of Antarctic, both through the
divergence/convergence of mass and through
sublimation; however, it is a term that, on a
continental scale, is considerably less than the
others listed in Table 1.
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Blowing snow
frequency in July
2009 simulated
by a regional
climate model
(Source:
Lenaerts et al.
2012)
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Mean annual
blowing snow
transport
(Mg/m)
Source: Dery &
Yau (2002)
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• For long-term studies (up to 250,000 years) and
to reconstruct previous accumulation rates,
snow pit studies and ice core measurements
have been used widely.
• Snow pits are used to quantify vertical variations
in snow density and to identify accumulation
layers in the snow that can be used to calculate
densification rates.
• Ice core studies are used to reconstruct past
climate variations, particularly temperature
variations, using O18/O16 ratios from ice cores.
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• Lake Vostok, a lake as big as Lake
Ontario, lies in the heart of the Antarctic
continent hidden beneath 4 kilometers of
ice.
• It is believed that Lake Vostok has been
covered by the vast Antarctic ice sheet for
up to 25 million years.
• The lake was named for the Russian
research station that sits above its
southern tip - a place where in 1983 the
temperature fell to -89°C, the coldest ever
recorded temperature on Earth.
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• More than 145 lakes have been identified
beneath the thick Antarctic ice sheet, with
most covered by 3-4 kilometers of ice and
being several kilometers long.
• Lake Vostok, is an order of magnitude
larger than all other known subglacial
lakes, at 14,000 km2 in area, 900 m deep,
and a volume of approximately 5,400 km3
of water.
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• Antarctic ice effectively floats on the
surface of the lakes such that the ice sheet
exhibits slight depressions over the lakes
that appear in radar and laser elevations.
• The combination of heat from below and a
thick layer of insulating ice above keeps
the water temperature at the top of the
lakes at a relatively balmy -2oC.
• Since the lakes are bounded by faults, it is
likely the lakes receive flows of nutrients
that could support unique ecosystems,
perhaps an ancient and alien ecosystem
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adapted to life beneath the ice sheet.
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