ocean ice interactions: a cryospheric...
TRANSCRIPT
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iOcean‐Ice Interactions: A Cryospheric PerspectiveCryospheric Perspective
Tony [email protected]@bristol.ac.uk
Steph Cornford, Rupert Gladstone and Dan Martin (LLNL)Dan Martin (LLNL)
KISS short course Sept. 2013 Slide number 1/37
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Outline
• Evidence of cryospheric response to oceans• Evidence of cryospheric response to oceans
• The Marine Ice Sheet Instability
• Flowline modelling of Pine Island Glacier
D l t d t ti f d ti h• Development and testing of an adaptive mesh model and application to PIG
• Application to West Antarctica: initialization and climate forcing
KISS short course Sept. 2013 Slide number 2/37
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Components of an ice sheetComponents of an ice sheet
• slow‐flowing interior (~10 m/yr)
f t fl i i t ( 500 / )• fast‐flowing ice streams (>500 m/yr)
• floating ice shelves
• grounding line• grounding line
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Ice streams and outlet glaciers
• sections of fast flowing ice ~50 k idkm wide
• now thought to be crucial in dynamics of ice sheetsdynamics of ice sheets
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KISS short course Sept. 2013 Slide number 5/37
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Larsen ice shelves• collapse of ice shelf A in 1995
and B in 2002and B in 2002
• meltwater‐driven fracture understoodunderstood
MacAyeal and others 2003ac yea a d o e s 003
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Larsen ice shelves
• minimal direct effectminimal direct effect, however glaciers accelerated after llcollapse
• natural experiment i li k btesting link between
floating and grounded ice
2003/2005
1996/2000Rignot 2004Scambos and others 2004
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Grounding line retreat
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Links GL retreat to mass lossLinks GL retreat to mass loss
Suggests that mass loss is limited to ice streams
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limited to ice streams
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Thinning is caused by increased
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ice flow
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Accelerating responseg p1995Wingham and others (2009)
lib d ERS 2use cross‐calibrated ERS‐2 and ENVISAT radar altimetry to extend time series from
2006
to extend time series from 1995 to 2008
thinning rates increased fourfold
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Accelerating responseAccelerating response• Jenkins and others (2010)
identify a bedrock ridge underidentify a bedrock ridge under the ice shelf ~40 km from the current grounding line
• observed GL retreat rates consistent with GL occupying
Jenkins and others 2010
20002009ridge in mid 1990s
• Retreat has been consistent 1996
20002009
2011
since 1990s and accelerated through 2000s ‐ 0.95 ±0.09 km/yr with peak 2.8 ± 0.7
1992
km/yr with peak 2.8 ± 0.7 km/yr
KISS short course Sept. 2013 Slide number 12/37
Park and others (2013)
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Outline
• Evidence of cryospheric response to oceans• Evidence of cryospheric response to oceans
• The Marine Ice Sheet Instability
• Flowline modelling of Pine Island Glacier
D l t d t ti f d ti h• Development and testing of an adaptive mesh model and application to PIG
• Application to West Antarctica: initialization and climate forcing
KISS short course Sept. 2013 Slide number 13/37
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Marine ice‐sheet instability
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J. Glac. 1981
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flux across GLflux across GL sharply increasing function of
5q Hthickness –basic ingredient for marine ice sheet instability
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Genomics of bryozoans (sedentary organisms)
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Outline
• Evidence of cryospheric response to oceans• Evidence of cryospheric response to oceans
• The Marine Ice Sheet Instability
• Flowline modelling of Pine Island Glacier
D l t d t ti f d ti h• Development and testing of an adaptive mesh model and application to PIG
• Application to West Antarctica: initialization and climate forcing
KISS short course Sept. 2013 Slide number 19/37
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Flowline modelling of PIGg• Aim to use simple model of PIG to investigate behaviour from g1900 to 2200
• The model is cheap to run so pthat fine resolution is not an issue
• Also means 1000s experiments are possible so that can use ensembles to assess effects ofensembles to assess effects of parameter uncertainty
• Joughin et al (2010) use a 2‐dJoughin et al (2010) use a 2 d. version of the model and find limited GL retreat
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Melt model• Box model of sub‐shelf processes used to generate p gmean melt rates (Olbers and Hellmer 2010)T t d li it• Temperature and salinity conserved; 3‐equation melt model; fluxes found as a ;function of density differencesM i t b h lf• Means in two sub‐shelf boxes used to constrain empirical relation developed p pby Walker and others (2008)
• Generates high melt rates l t GL t d b
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close to GL as suggested by observations
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Varying inputs/parametersA rate factor (a measure of Varying inputs/parametersA = rate factor (a measure of how easily deformable the ice is, determined by temperature)
Minimax Latin Hypercube sampling was used to obtain 5000 combinations of these inputs (i bl f i l i )temperature).
Two parameters jointly
(i.e. we ran an ensemble of 5000 simulations)
p j ydetermine the “surface” mass balance profile (includes a contribution from tributaries).
Two parameters determine the profile of basal tractionprofile of basal traction coefficient.
A lateral drag parameterisation is used with channel width W
One parameter allows the initial (year 1900) thickness profile to vary
KISS short course Sept. 2013 Slide number 22/37
y
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Results• Likelihood proceedure to
accept or reject members b d fi b dbased on fit to observed thinning, grounding line positions and velocitypositions and velocity
• Grey are rejected; blue to red reduced discrepancyreduced discrepancy
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Outline
• Evidence of cryospheric response to oceans• Evidence of cryospheric response to oceans
• The Marine Ice Sheet Instability
• Flowline modelling of Pine Island Glacier
D l t d t ti f d ti h• Development and testing of an adaptive mesh model and application to PIG
• Application to West Antarctica: initialization and climate forcing
KISS short course Sept. 2013 Slide number 24/37
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Bisicles ice sheet model• Specifically designed for GL problemsp
• Based on CHOMBO adaptive‐mesh refinement developed by Lawrence Li N ti l L bLivermore National Lab.
• Uses a vertically‐integrated form of the stress equations proposed bythe stress equations proposed by Schoof and Hindmarsh (2010) known as L1L2
• Includes all stress terms but is vertically integrated
i• CHOMBO ensures conservation between grids and offers massive parallelization
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BISICLES
• Trial application to PineTrial application to Pine Island Glacier
• Simulation usingSimulation using reasonable melt increase of 50 m/yr
• Results dependent on resolution from single level (5k ) t i l l (~150 )(5km) to six levels (~150 m)
• Confirms need for sub‐km l tiresolution
KISS short course Sept. 2013 Slide number 26/37
Colours refer to velocity
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See moviesSee movies
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Outline
• Evidence of cryospheric response to oceans• Evidence of cryospheric response to oceans
• The Marine Ice Sheet Instability
• Flowline modelling of Pine Island Glacier
D l t d t ti f d ti h• Development and testing of an adaptive mesh model and application to PIG
• Application to West Antarctica: initialization and climate forcing
KISS short course Sept. 2013 Slide number 28/37
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Experimental designp g• Coupled problem but no such
coupled model existscoupled model exists
• Use a chain of models from global AOGCMs regionalglobal AOGCMs regional ocean and atmosphere models ice sheet model ice sheet model
• Connelly and Bracewell (2007) show HadCM3 and ECHAM5 toshow HadCM3 and ECHAM5 to do well for Antarctica
• Consider only West AntarcticaConsider only West Antarctica
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Experimental SRES scenarios A1B and E1 withp
design E1
and E1 with AOGCMS
HadCM3 ECHAM5HadCM3 ECHAM5
Snowfall ‐ regional atmospheric modelling
Melt ‐ regional ocean modellingp g
RACMO2 (Utrecht)
LMDZ4 (Grenoble)
modelling
BRIOS (AWI)
FESOM (AWI)( ) ( ) (AWI) (AWI)
Anomalies against 1980 to 1989
BISICLES ice
Anomalies against 1980 to 1989
KISS short course Sept. 2013 Slide number 30/37
sheet model
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Regional Southern Ocean model forcedOcean model forced using AOGCM output.p
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Ocean forcing – major ice shelvesOcean forcing major ice shelves• Warm water intrusion reported by
Hellmer and others (2012) forHellmer and others (2012) for
BRIOS also in FESOM for Ronne‐
Filchner
• Leads to 10‐20 fold increase in melt
Si il h f R i• Similar phenomenon for Ross ice
shelf after 2100 (FESOM only)
Ronne‐Filchner ice shelf Ross ice shelf
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Ocean forcing – smaller ice shelvesOcean forcing smaller ice shelves• FESOM and BRIOS do not represent
smaller shelves wellsmaller shelves well
• Use index of coastal warming and
t t lt l iconvert to melt anomaly using
empirical relation (e.g., Jacobs and
Rignot 2002)g )
• Warming of 1 to 2 C or 10‐20 m/yr
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Jacobs and Rignot 2002
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ResultsResults
B k d i thBackground is the initial velocity field
KEYKEY ‐
1980 ground line
Worst case by 2200
Control (no anomalies) shows some drift
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Amundsen Sea and Pine IslandAmundsen Sea and Pine Island• Deglaciation of Pine Island Glacier
and Smith Glaciersand Smith Glaciers
• Thwaites shows no retreat related to lack of buttressing?to lack of buttressing?
• Similar to recent GL observationsPine Island Glacier
• In general, increased accumulation dominatesdominates
Thwaites Glacier
Smith Glacier
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See moviesSee movies
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SummarySummary• Increased outflow is enough to compensate increased
snowfallsnowfall
• Sea level rise is predicted as ‐5 to 80 mm by 2200 depending on forcing (i.e., small)g ( , )
• Sea level rise is limited because
• GL retreat occurs late in the model run (c.f. ocean forcing)GL retreat occurs late in the model run (c.f. ocean forcing)
• Areas that retreat do not have much ice above buoyancy (so little effect or SLR) and/or( ) /
• Large retreat limited to narrow channels (e.g., Pine Island)
• Sea level rise appears to continue to increase beyond 2200 pp y
• Omits East Antarctica
• Fuller estimate requires coupling to regional ocean model
KISS short course Sept. 2013 Slide number 37/37
Fuller estimate requires coupling to regional ocean model
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Antarctic mass balance• interior thickening related to changes in snowfall• coastal thinning in WAIS (Pine Island, Smith and
Antarctic mass balance
Thwaites Glaciers) and EAIS (Cook and TottenGlaciers)
• close correspondence to ice velocity (ice streams)p y ( )
Shepherd and Wingham 2007
Pritchard and others 2009
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Demonstrated that GL retreat, flow acceleration and thinning all linked and caused byall linked and caused by increased ice shelf melt
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Initial conditionsDerived basal traction coeff.
Initial conditions• Observed ice sheet geometry
U h d b d L• Use methods based on Lagrange multipliers to find ice viscocity and basal traction consistent with observed velocities
• Evolve ice sheet for 50 years to allow noise to relax awaynoise to relax away
• Employ 3 levels of refinement from 5 km to 612 m
Derived rheology factor
km to 612 m
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