Pathways and impact of Southern Ocean currents on Antarctic Icesheet melting in response to global warming Frank Colberg and Nathan Bindoff Thanks to: Petra Heil, Ben Galton-Fenzi, Helen Philips, TPAC team TPAC

Outline Introduction and Motivation Introduction and Motivation Model setup Model setup –Ocean, Seaice, Iceshelves –Large scale circulation impacts on seaice Climate change runs and results Climate change runs and results –Focus on seaice, freshwater fluxes, and bottom water properties Summary and future work Summary and future work

Introduction 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

Introduction 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 Shchepetkin et al, 2004) ROMS (Shchepetkin et al, 2004) 1/8 degree horizontally, circumpolar, 20S-85S 25 vertical levels ~ 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 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 thickness Smith and Sandwell and BEDMAP (Lythe et al, 2001)‏ Smith and Sandwell and BEDMAP (Lythe et al, 2001)‏ Blended at 60S Blended at 60S Including all major iceshelves + fringes Including all major iceshelves + fringes Cavity thickness West Schackleton Depth of Iceshelf Mertz Ross Filchner-Ronne George VI Larsen Rieser-Larsen Abbot Getz Amery Flimbul

Substantial 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 meridional heat transport across 50S Year 1 Year 3Year 4 Year 2  Red : Ocean model without Smagorinsky type mixing  Blue : After applying Smagorinsky mixing

Eddy kinetic energy and transport across Drake Passage

Maximum seaice concentration Observations/ SSMIModel - September mean over 10 years Maximum mean ice extent as observed by SSMI:~18E7 km2

Climate 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 only Run 2: Wind anomalies only Run 3: ‏ All IPCC anomalies but wind Run 3: ‏ All IPCC anomalies but wind To distinguish between dynamic and buoyancy forcing and feedback To distinguish between dynamic and buoyancy forcing and feedback Results from this/ last week !! Results from this/ last week !! showing difference plots of: showing difference plots of: – Ice concentration, thickness – Melting rates – Bottom water properties

Difference in ice concentration and thickness, timeseries Black: Base Run Blue: Run 1, IPCC all Red: Run 2, IPCC, winds Green: 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 pattern BASE RUN IPCC, all IPCC, winds IPCC, all but winds

Meltwater response to changes in surface forcing Pos: 93 GT/a Neg: -216 GT/a Black: Base Run Blue: 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 scenariosMelt rates: Run 1 (IPCC, all) – Base Run Spatial pattern

No clear signal Meltwater response for region Black: base run Blue: all IPCC forcing Red: wind IPCC forcing Green: buoyancy IPCC forcing I IV III II 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 and salinity changes

Temperature anomaly after 9 years of integration Temperature bottom layer Temperature section along ~150E 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 Run Red: IPCC, winds - BR Green: IPCC, all but winds - BR Melting rateTemperature

Melting as a function of depth Buoyancy forcing Enhanced melting for upper 500m No change in deeper shelves evident 2 Years8 Years Wind forcing Reduced melting for upper 500m for first 4 years Enhanced melting for m after 4 years Suggests: stronger contributions of deep iceshelves for longer integration times These may act as to change bottom water properties

Summary Developed state of the art ocean – seaice – iceshelf model of ROMS Large scale circulation greatly influences seaice production and likely affects shelf processes as it controls/ modifies the poleward heat/ salt transport Iceshelves overestimate melt rates to some extent when compared with previous modelling studies, however, fluxes not adjusted yet Including full suit of IPCC forcing results in a net increased freshwater flux from iceshelves Including anomalous wind forcing results in strong warm temperature anomalies in the WAIS area Bottom water property changes may be related to modifications in melt rates from selected iceshelves Bottom water is sensitive to changes in windforcing only Inclusion of wind anomalies introduces a 4-5 time delay

Future work Analyses Extending model runs Model improvement: Refine iceshelf mask Include frazil ice dynamics (see GF, 2009)‏ Model drift needs further exploration – note: no flux corrections were applied Vertical mixing under seaice Thanks... !

Melt rates and sensitivity to shelf mask Iceberg calving amounts to ~2000Gt (Jacobs et al, 1992)‏ No heat conduction into iceshelf No frazil ice, No tides May all act as to enhance melting Galton-Fenzi, 2009

Sea 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 surface Wao: freeze rate on the upper ice/ snow surface Wfr: rate of frazil ice growth Wio: freeze rate at the ice/ water interface Wro: 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 (2006)‏ Iceshelf dynamics as specified by Hunter (2006)‏ Dont use full set of thermodynamical eqs (GF, 2009)‏ Dont use full set of thermodynamical eqs (GF, 2009)‏ Iceshelf in steady state Iceshelf in steady state No frazil ice No frazil ice No heat conduction into iceshelf No heat conduction into iceshelfIceshelves Holland and Feltham, 2004 Descending plumes of HSSW are generated by sea ice brine rejection (interact and modulate CDW)‏ Descending plumes of HSSW are generated by sea ice brine rejection (interact and modulate CDW)‏ It will melt the base of the iceshelf. This fresh water becomes supercooled as it rises (due to decrease of freezing temperature with depth)‏ It will melt the base of the iceshelf. This fresh water becomes supercooled as it rises (due to decrease of freezing temperature with depth)‏ Resulting in direct basal freezing and production of frazil ice Resulting in direct basal freezing and production of frazil ice CDW Galton-Fenzi, 2009

AACMODEL, ATM_PRESS, ANA_BSFLUX, ANA_BTFLUX, ASSUMED_SHAPE, AVERAGES, AVERAGES_AKT, AVERAGES_FLUXES, BULK_FLUXES, CURVGRID, DIFF_GRID, DIURNAL_SRFLUX, DJ_GRADPS, DOUBLE_PRECISION, EMINUSP, EW_PERIODIC, ICE_ADVECT, ICE_BULK_FLUXES, ICE_EVP, ICE_MK, ICE_MODEL, ICE_MOMENTUM, ICE_SMOLAR, ICE_SMOOTH, ICE_THERMO, ICESHELF, LMD_CONVEC, LMD_MIXING, LMD_RIMIX, LMD_SKPP, MASKING, MIX_S_TS, MIX_S_UV, MPI, NONLINEAR, NONLIN_EOS, NORTH_M3NUDGING, NORTH_M3RADIATION, NORTH_TNUDGING, NORTH_TRADIATION, POWER_LAW, PROFILE, RUNOFF, !RST_SINGLE, SALINITY, SOLAR_SOURCE, SOLVE3D, SOUTHERN_WALL, SPLINES, SPHERICAL, SPONGE, TS_A4HADVECTION, TS_A4VADVECTION, TS_DIF2, UV_ADV, UV_COR, UV_U3HADVECTION, UV_C4VADVECTION, UV_LDRAG, UV_VIS2, VAR_RHO_2D, VISC_GRID, VISC_3DCOEF

ntimes Number of timesteps for 3-D equations dt Timestep size (s) for 3-D equations. 45 ndtfast Number of timesteps for 2-D equations between each 3D timestep. 1 ERstr Starting ensemble/perturbation run number. 1 ERend Ending ensemble/perturbation run number. -1 nrrec Number of restart records to read from disk. T LcycleRST Switch to recycle time-records in restart file nRST Number of timesteps between the writing of data into restart fields. 1 ninfo Number of timesteps between print of information to standard output. T ldefout Switch to create a new output NetCDF file(s) nHIS Number of timesteps between the writing fields into history file ndefHIS Number of timesteps between creation of new history files. 1 ntsAVG Starting timestep for the accumulation of output time-averaged data nAVG Number of timesteps between the writing of time-averaged data into averages file ndefAVG Number of timesteps between creation of new time-averaged file E+01 tnu2(01) Horizontal, harmonic mixing coefficient (m2/s) for tracer 01: temp E+01 tnu2(02) Horizontal, harmonic mixing coefficient (m2/s) for tracer 02: salt E+02 visc2 Horizontal, harmonic mixing coefficient (m2/s) for momentum E-06 Akt_bak(01) Background vertical mixing coefficient (m2/s) for tracer 01: temp E-06 Akt_bak(02) Background vertical mixing coefficient (m2/s) for tracer 02: salt E-05 Akv_bak Background vertical mixing coefficient (m2/s) for momentum E-04 rdrg Linear bottom drag coefficient (m/s) E-03 rdrg2 Quadratic bottom drag coefficient E-02 Zob Bottom roughness (m) E+01 blk_ZQ Height (m) of surface air humidity measurement E+01 blk_ZT Height (m) of surface air temperature measurement E+01 blk_ZW Height (m) of surface winds measurement. 1 lmd_Jwt Jerlov water type. 2 Vtransform S-coordinate transformation equation. 2 Vstretching S-coordinate stretching function E+01 theta_s S-coordinate surface control parameter E+00 theta_b S-coordinate bottom control parameter Tcline S-coordinate surface/bottom layer width (m) used in vertical coordinate stretching rho0 Mean density (kg/m3) for Boussinesq approximation dstart Time-stamp assigned to model initialization (days) time_ref Reference time for units attribute (yyyymmdd.dd) E+01 Tnudg(01) Nudging/relaxation time scale (days) for tracer 01: temp E+01 Tnudg(02) Nudging/relaxation time scale (days) for tracer 02: salt E+01 Znudg Nudging/relaxation time scale (days) for free-surface E+01 M2nudg Nudging/relaxation time scale (days) for 2D momentum E+01 M3nudg Nudging/relaxation time scale (days) for 3D momentum E+00 obcfac Factor between passive and active open boundary conditions T0 Background potential temperature (C) constant S0 Background salinity (PSU) constant gamma2 Slipperiness variable: free-slip (1.0) or no-slip (-1.0). LMD_BOUND->lmd_nu0c=0.01