pathways and impact of southern ocean currents on antarctic icesheet melting in response to global...
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Pathways and impact of Southern Ocean Pathways and impact of Southern Ocean currents on Antarctic Icesheet melting in currents on Antarctic Icesheet melting in
response to global warmingresponse to global warming
Frank Colberg and Nathan BindoffFrank Colberg and Nathan BindoffFrank.colberg@csiro.auFrank.colberg@csiro.au
Thanks to: Petra Heil, Ben Galton-Fenzi, Helen Philips, TPAC team
TPAC
OutlineOutline
•Introduction and MotivationIntroduction and Motivation•Model setupModel setup
–Ocean, Seaice, IceshelvesOcean, Seaice, Iceshelves–Large scale circulation impacts on Large scale circulation impacts on
seaiceseaice•Climate change runs and resultsClimate change runs and results
–Focus on seaice, freshwater fluxes, Focus on seaice, freshwater fluxes, and bottom water propertiesand bottom water properties
•Summary and future workSummary and future work
IntroductionIntroduction
Global climate models used for the latest IPCC lack adequate representation of icesheet/ shelve physics
These add to uncertainties in future estimates of sea level rise
Velicogna and Wahr (2006) decreasing ice thickness follows
the reduction of ice shelves Thoma et al., 2008 modeled
variations in the influx of CDW on Amundsen Sea Cont Shelf may act as an indicator for the ice thickness variations
J.L. Chen et al, 2009
IntroductionIntroduction
Observations suggest changes in the hydrological cycle occurred and manifest themselves as a freshening signal in the upper ocean.
Mismatch between modelled hydrological cycle and observed (Figure, Helm et al, 2009)
Freshening signal apparent in the bottom water properties (Rintoul, 2007)
Hypothesis: is it plausible that this surplus of observed fresh water may arise from ice shelve melting
Gains support from fact that only 0.5% of additional iceshelf melting is needed (Helm et al, 2009)
Helm et al., 2009
Model Setup Model Setup
• ROMS (Shchepetkin et al, 2004) Shchepetkin et al, 2004)
• 1/8 degree horizontally, circumpolar, 20S-85S
• 25 vertical levels
• ~40.000.000 gridpoints -> largest model of ROMS to date
• No flux correction due to nature of experiments
• ~13Tb data, one model output: 1.2Gb, 5 per month
• Forcing: CORE (Large and Seager, 2006)
• Initialization: WOA (Conkright, 2002)
• Currently running on TPAC cluster: katabatic 256CPU
• 1 Model year simulated in 1 day
Model setup and componentsModel setup and components
AREA OF INTEREST
CDW
• After Budgell (2004)
• Ice dynamics based on Elastic-Viscous-Plastic (EVP) rheology (Hunke and Dukowicz, 1997 and Hunke, 2001)
• Ice thermodynamics based on Mellor and Kanta (1989) and Kakkinen and Mellor (1992)
• Two ice layers and a single snow layer are used in solving the heat conduction equation
• Molecular sub-layer separates bottom and ice-cover from upper ocean
Iceshelf dynamics as specified by Hunter (2006)
Hedstroem, 2009
Iceshelf mask, cavity thicknessIceshelf mask, cavity thickness
• Smith and Sandwell and BEDMAPSmith and Sandwell and BEDMAP (Lythe et al, 2001)(Lythe et al, 2001)
• Blended at 60SBlended at 60S
• Including all major iceshelves + fringesIncluding all major iceshelves + fringes
Cavity thickness
West
Schackleton
Depth of Iceshelf
Mertz
Ross
Filchner-RonneGeorge VI
Larsen
Rieser-Larsen
Abbot
Getz
Amery
Flimbul
Substantial seaice lossSubstantial seaice loss
•Faced with one major problem: loosing seaice•Important as sea ice is a good proxy for goodness
of ocean atmosphere state•2 major reasons: •Error in seaice code fluxes had wrong sign•Related to horizontal mixing in large scale context
Model results, ice concentration
Observations/ SSMI
(1) (2)
Salinity structure and Salinity structure and meridional heat transport meridional heat transport
across 50Sacross 50SYear 1
Year 3 Year 4
Year 2
Red : Ocean model without Smagorinsky type mixing
Blue : After applying Smagorinsky mixing
Eddy kinetic energy and Eddy kinetic energy and transport across Drake Passagetransport across Drake Passage
Maximum seaice concentrationMaximum seaice concentrationObservations/ SSMI Model - September mean over 10 years
•Maximum mean ice extent as observed by •SSMI:~18E7 km2
Climate change runsClimate change runs
•Base run: 12 years Base run: 12 years •Run 1: Whole suit of IPCC anomalies Run 1: Whole suit of IPCC anomalies •Run 2: Wind anomalies onlyRun 2: Wind anomalies only•Run 3: Run 3: All IPCC anomalies but windAll IPCC anomalies but wind•To distinguish between dynamic and buoyancy forcing To distinguish between dynamic and buoyancy forcing
and feedbackand feedback•Results from this/ last week !!Results from this/ last week !!•showing difference plots of: showing difference plots of:
– Ice concentration, thicknessIce concentration, thickness– Melting ratesMelting rates– Bottom water propertiesBottom water properties
Difference in ice concentration and Difference in ice concentration and thickness, timeseriesthickness, timeseries
Black: Base RunBlue: Run 1, IPCC allRed: Run 2, IPCC, windsGreen: Run 3, IPCC, all but winds
Response due to •enhanced upwelling of CDW•increased upper ocean warming due to changes in radiation•Transport of seaice to midlatitudes
Difference in ice concentration spatial Difference in ice concentration spatial patternpattern
BASE RUN
IPCC, all
IPCC, winds
IPCC, all but winds
Meltwater response to changes in Meltwater response to changes in surface forcingsurface forcing
Pos: 93 GT/a Neg: -216 GT/a
Black: Base RunBlue: IPCC (all)Red: IPCC (wind)Green: IPCC (all, but wind)
• (a) Enhanced melting only when buoyancy forcing terms are applied in - forcing-Additional freshwater flux into ocean: ~150Gt/a
• (b) Reduced melting only when anomalous wind forcing is applied Reduction: ~50Gt/a
Annual melting rates for scenarios Melt rates: Run 1 (IPCC, all) – Base RunSpatial pattern
No clear signal
Meltwater response for region
Black: base runBlue: all IPCC forcingRed: wind IPCC forcingGreen: buoyancy IPCC forcing
I
IV
IIIII
VI
V
VII
Temperature anomalies on 1000m depth contour – yearly means
Run 1, Run 2: similar response indicating CDW intrusion onto shelf
Year 1
Year 3
Year 5
Year 7
IPCC, all IPCC, all but winds
Only when we apply anomalous wind forcing are we getting temperature anomalies that put the focus on the WAIS
Melting decoupled from CDW intrusion ?
Bottom layer temperature Bottom layer temperature and salinity changes and salinity changes
0.5
-0.5
Temperature anomaly after 9 yearsof integration
Temperature bottom layer Temperature section along ~150E
0.5
-0.5
All IPCC forcing:
Buoyancy forcing:
Salt anomaly after 9 years of integration
Salinity bottom layer Salinity section along ~150E
All IPCC forcing:
Buoyancy forcing:Strong intrusion of CDW onto shelf
(1)
Bottom water properties and possible connection to iceshelf melting ?
Similar timing for enhanced melting in Mertz shelf and cooling plus freshening of BT
Accelerates after 4-6 years
Blue: IPCC all – Base RunRed: IPCC, winds - BRGreen: IPCC, all but winds - BRMelting rate Temperature
Melting as a function of depth
Buoyancy forcing•Enhanced melting for upper 500m •No change in deeper shelves evident
2 Years 8 Years
Wind forcing•Reduced melting for upper 500m for first 4 years•Enhanced melting for 500-1000m after 4 years
Suggests:• stronger contributions of deep iceshelves for longer integration times•These may act as to change bottom water properties
SummarySummaryDeveloped state of the art ocean – seaice – iceshelf model of ROMSLarge scale circulation greatly influences seaice production and likely affects shelf processes as it controls/ modifies the poleward heat/ salt transportIceshelves overestimate melt rates to some extent when compared with previous modelling studies, however, fluxes not adjusted yetIncluding full suit of IPCC forcing results in a net increased freshwater flux from iceshelvesIncluding anomalous wind forcing results in strong warm temperature anomalies in the WAIS areaBottom water property changes may be related to modifications in melt rates from selected iceshelvesBottom water is sensitive to changes in windforcing onlyInclusion of wind anomalies introduces a 4-5 time delay
Future workFuture workAnalysesExtending model runsModel improvement:
Refine iceshelf mask Include frazil ice dynamics (see GF, 2009) Model drift needs further exploration – note:
no flux corrections were appliedVertical mixing under seaice
Thanks ... !
Shelf MASK 1 Area Mkm2 MASK 2 Area MASK 3 Area Helmer, 2004 Area
Total 1400 12.8 3139 19.02 1725 14.7 906 12.33
Amery 27 0.292 56 0.4 42 0.34 17 0.55
Ross 235 4.4575 303 4.9 263 4.7 180
Filchner-Ronne 130 3.73 274 4 269 3.9 119
Getz 60 0.25 170 0.48 137 0.41 53
Rieser-Larsen 115 0.42 153 0.64 93 0.5 38
Schackleton 57 0.23 93 0.35 44 0.28 47
Abbot 50 0.26 105 0.5 72 0.3 18
Melt rates and sensitivity to shelf Melt rates and sensitivity to shelf maskmask
Iceberg calving amounts to ~2000Gt (Jacobs et al, 1992)No heat conduction into iceshelfNo frazil ice, No tidesMay all act as to enhance melting
Galton-Fenzi, 2009
Sea Ice componentSea Ice component• After Budgell (2004)
• Ice dynamics based on Elastic-Viscous-Plastic (EVP) rheology (Hunke and Dukowicz, 1997 and Hunke, 2001)
• Ice thermodynamics based on Mellor and Kanta (1989) and Kakkinen and Mellor (1992)
• Two ice layers and a single snow layer are used in solving the heat conduction equation
• Molecular sub-layer separates bottom and ice-cover from upper ocean
Wai: melt rate at the upper ice/ snow surfaceWao: freeze rate on the upper ice/ snow surfaceWfr: rate of frazil ice growthWio: freeze rate at the ice/ water interfaceWro: rate of run-off of surface melt water
Diagram of the different location where ice melting and freezing can occur
• Iceshelf dynamics as specified by Hunter Iceshelf dynamics as specified by Hunter (2006)(2006)
• Dont use full set of thermodynamical eqs Dont use full set of thermodynamical eqs (GF, 2009)(GF, 2009)
• Iceshelf in steady stateIceshelf in steady state
• No frazil ice No frazil ice
• No heat conduction into iceshelfNo heat conduction into iceshelf
IceshelvesIceshelves
Holland and Feltham, 2004Holland and Feltham, 2004
Descending plumes of HSSW are generated by sea ice Descending plumes of HSSW are generated by sea ice brine rejection (interact and modulate CDW)brine rejection (interact and modulate CDW) It will melt the base of the iceshelf. This fresh water It will melt the base of the iceshelf. This fresh water becomes supercooled as it rises (due to decrease of becomes supercooled as it rises (due to decrease of freezing temperature with depth)freezing temperature with depth) Resulting in direct basal freezing and production of frazil Resulting in direct basal freezing and production of frazil ice ice
CDW
Galton-Fenzi, 2009
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