Intercomparison of Antarctic ice shelf, ocean, and sea ice interactions simulated by two models

60 Pages·2017·8.29 MB·English

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Intercomparison of Antarctic ice shelf, ocean, and sea ice
interactions simulated by two models
KaitlinA.Naughten1,2,3,KatrinJ.Meissner1,2,BenjaminK.Galton-Fenzi4,3,MatthewH.England1,2,
RalphTimmermann5,HartmutH.Hellmer5,ToreHattermann6,5,andJensB.Debernard7
1ClimateChangeResearchCentre,Level4MathewsBuilding,UniversityofNewSouthWales,SydneyNSW2052,Australia
2ARCCentreofExcellenceforClimateSystemScience,Australia
3AntarcticClimate&EcosystemsCooperativeResearchCentre,PrivateBag80,HobartTAS7001,Australia
4AustralianAntarcticDivision,203ChannelHighway,KingstonTAS7050,Australia
5AlfredWegenerInstitut,Postfach120161,27515Bremerhaven,Germany
6Akvaplan-niva,P.O.Box6606,Langnes,9296Tromsø,Norway
7NorwegianMeteorologicalInstitute,P.O.Box43,Blindern,N-0313Oslo,Norway
Correspondenceto:KaitlinA.Naughten([email protected])
Abstract.AnincreasingnumberofSouthernOceanmodelsnowincludeAntarcticiceshelfcavities,andsimulatethermody-
namics at the ice-shelf/ocean interface. This adds another level of complexity to Southern Ocean simulations, as ice shelves
interact directly with the ocean and indirectly with sea ice. Here we present the first published model intercomparison and
evaluation of present-day ocean/sea-ice/ice-shelf interactions, as simulated by two models: a circumpolar Antarctic config-
5 uration of MetROMS (ROMS: Regional Ocean Modelling System coupled to CICE: Community Ice CodE) and the global
model FESOM (Finite Element Sea-ice/ice-shelf Ocean Model), where the latter is run at two different levels of horizontal
resolution.FromacircumpolarAntarcticperspective,wecompareandevaluatesimulatediceshelfbasalmeltingandsub-ice
shelf circulation, as well as sea ice properties and Southern Ocean water mass characteristics as they influence the sub-ice
shelfprocesses.Despitetheirdifferingnumericalmethods,thetwomodelsproducebroadlysimilarresults,andsharesimilar
10 biasesinmanycases.Bothmodelsreproducemanykeyfeaturesofobservations,butstruggletoreproduceothers,suchasthe
highmeltratesobservedinthesmallwarm-cavityiceshelvesoftheAmundsenandBellingshausenSeas.Severaldifferences
in model design show a particular influence on the simulations. For example, FESOM’s greater topographic smoothing can
alter the geometry of some ice shelf cavities enough to affect their melt rates; this improves at higher resolution, since less
smoothingisrequired.IntheinteriorSouthernOcean,theverticalcoordinatesystemaffectsthedegreeofwatermasserosion
15 due to spurious diapycnal mixing, with MetROMS’ terrain-following coordinates leading to more erosion than FESOM’s z-
coordinates.Finally,increasedhorizontalresolutioninFESOMleadstohigherbasalmeltratesforsmalliceshelves,through
acombinationofstrongercirculationandsmall-scaleintrusionsofwarmwaterfromoffshore.
1 Introduction
TheAntarcticIceSheet(AIS)hassignificantpotentialtodrivesealevelriseasclimatechangecontinues(DecontoandPollard,
20 2016; Golledge et al., 2015; Rignot et al., 2014; Mengel and Levermann, 2014). Paleo records indicate that the AIS was a
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majorcontributortosealevelchangeinpastclimateevents(Cooketal.,2013;Milleretal.,2012;RaymoandMitrovica,2012;
Duttonetal.,2015;O’Learyetal.,2013),andthemassbalanceofthemodern-dayAISisalreadynegative(Rignotetal.,2011;
ZwallyandGiovinetto,2011).TherateofretreatofmuchoftheAISwillbegovernedbytheocean(Golledgeetal.,2017),
as 40% of the ice sheet by area is grounded below sea level (Fretwell et al., 2013). This geometry provides the potential for
5 the ocean to melt large regions of the AIS from below. For example, the Amundsen sector of West Antarctica has bedrock
geometryfavourableforamarineicesheetinstability,andunstableretreatmayhavealreadybegun(Rignotetal.,2014).
TheoceandirectlyinteractswiththeAISthroughiceshelves,whicharethefloatingextensionsoftheland-basedicesheet.
Thepropertiesoficeshelfcavities,thepocketsofoceanbetweeniceshelvesandtheseafloor,determinethebasalmeltratesof
eachiceshelfwhichultimatelyaffectthemassbalanceoftheAISthroughdynamicalprocesses(Dupont,2005).Theseawater
10 iniceshelfcavitiescanbesourcedfromseveraldifferentwatermasses,whichaffectitstemperatureandsalinity.Manyofthese
sourcewatermassesareinfluencedbyseaiceprocesses(Jacobsetal.,1992;Nichollsetal.,2009).
Abetterunderstandingofocean/sea-ice/ice-shelfinteractionsinAntarcticaiscrucial,particularlygiventheirimportancefor
futuresealevelrise.However,theseinteractionstakeplaceinobservation-deficientregions.Inparticular,thereareveryfew
directmeasurementsinsideiceshelfcavities,andobservationsarealsoscarceintheseaicecoveredregionsoftheSouthern
15 Ocean(Rintouletal.,2010).Whileiceshelfbasalmeltratescanbeinferredusingremotesensingmethods(Rignotetal.,2013;
Depoorteretal.,2013),largeuncertaintiesremainregardingthecirculationpatternsdrivingthesemeltrates,andnopredictions
forthefuturecanbemadebasedonthesedata.
Consequently,muchofourunderstandingofocean/sea-ice/ice-shelfinteractionsisbasedonnumericalmodelling.Inrecent
yearsanincreasingnumberofoceanmodelshavebeguntoresolveiceshelfcavitiesandsimulatethermodynamicprocesses
20 at the ice shelf base (Dinniman et al., 2016). Given the variety of models involved, and the relative lack of observations
to constrain their tuning, it is desirable to conduct model intercomparison projects (MIPs, see e.g. Meehl et al. (2000)) by
whichseveralmodelsrunthesameexperimentandtheiroutputiscompared.Theresultinginsightsintomodelsimilaritiesand
differencescanideallybeattributedtomodeldesignchoices,withtheaimofguidingfuturedevelopment.
To date, the only MIPs considering ice shelf cavities are the ongoing ISOMIP experiments (Ice Shelf-Ocean Model In-
25 tercomparison Project) (Hunter, 2006; Asay-Davis et al., 2016) which use idealised domains and simplified forcing, and do
not include coupled sea ice models. The ISOMIP experiments are undoubtedly valuable, and are likely to provide particular
insights regarding the response of cavity circulation to warm versus cold forcing. However, idealised experiments such as
ISOMIPshouldbecomplementedbyintercomparisonsovermorerealisticdomains,withobservationallyderivedforcingand
coupledseaicemodels.Thesemodelconfigurationsarealreadybeingusedtobetterunderstandprocessesinobservedcavities
30 (Timmermann et al., 2012; Galton-Fenzi et al., 2012) and to provide future projections of ice shelf melt (Timmermann and
Hellmer,2013;Hellmeretal.,2012,2017),soanalysisofthesimilaritiesanddifferencesbetweensuchmodelsistimely.An-
otherimportantbenefitofrealisticdomainsistheopportunitytocomparemodeloutputtoavailableobservations,evenifthese
observationsarelimited.Therefore,anelementofmodelevaluation,aswellasmodelintercomparison,canbeincluded.
Inthispaperwepresentsuchanintercomparisonoftwooceanmodels,bothincludingiceshelfthermodynamicsandseaice
35 components,fromacircumpolarAntarcticperspective.Wefocusoniceshelfbasalmeltandsub-iceshelfcirculationacross
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eightregionsoftheAntarcticcoastline,butalsoconsiderinteriorSouthernOceanandseaiceprocessesastheyaffecticeshelf
cavities.Themodeloutputiscomparedtorelevantobservationswhereavailable.Finally,keyfindingsandtheirimplications,
aswellaspossibilitiesforfuturemodeldevelopment,arediscussed.
2 Modeldescriptions
5 Two coupled ocean/sea-ice/ice-shelf models are included in this intercomparison: MetROMS and FESOM. We run FESOM
at two different resolutions, for a total of three experiments (see Section 3). In this section we describe the two models and
comparetheirscientificdesign.
2.1 Overview
MetROMSconsistsoftheregionaloceanmodelROMS(RegionalOceanModellingSystem)(ShchepetkinandMcWilliams,
10 2005) including ice shelf thermodynamics (Galton-Fenzi et al., 2012), coupled to the sea ice model CICE (Community Ice
CodE) (Hunke et al., 2015) using the coupler MCT (Model Coupling Toolkit) (Larson et al., 2005; Jacob et al., 2005). The
couplingwasimplementedbytheNorwegianMeteorologicalInstitute(Debernardetal.,2017)andisdescribedinNaughten
etal.(2017).Weusethedevelopmentversion3.7oftheROMScode,version5.1.2ofCICE,andversion2.9ofMCT.
FESOM(FiniteElementSea-ice/ice-shelfOceanModel)isaglobaloceanmodelwithaninternallycoupledseaicemodel
15 (Danilovetal.,2015;Timmermannetal.,2009)andiceshelfthermodynamics(Timmermannetal.,2012).Ithasanunstruc-
tured mesh in the horizontal, consisting of triangular elements which allow for spatially varying resolution. The numerical
methodsassociatedwiththeunstructuredmesharedetailedbyWangetal.(2008)andWangetal.(2014),whiletheimplemen-
tationoftheiceshelfcomponentisdiscussedinTimmermannetal.(2012).
2.2 Domainandresolution
20 OurconfigurationofMetROMShasacircumpolarAntarcticdomainwithanorthernboundaryat30◦S.Horizontalresolution
isquarter-degreescaledbycosineoflatitude,andtheSouthPoleisrelocatedtoachieveapproximatelyequalresolutionaround
the Antarctic coastline. This leads to resolutions (defined as the square root of the area of each grid box) of approximately
15-20 km in the Antarctic Circumpolar Current (ACC), 8-10 km on the Antarctic continental shelf, and 5 km or finer at the
southernmostgroundinglinesoftheRoss,Filchner-Ronne,andAmeryIceShelves(Figure1a).
25 OurFESOMsetuphasaglobaldomainwithspatiallyvaryinghorizontalresolution.HerewedefineresolutioninFESOMas
thesquarerootoftheareaofeachtriangularelement,howeverthismetricmaynotbetrulycomparabletoMetROMS.When
discussingresolution,therealquestionishowmanyfeaturesoffluidflowcanberepresentedbyameshofacertainspacing.
In models with such different numerical methods as MetROMS (finite-volume) and FESOM (finite-element), the number of
featuresresolvedmayscaledifferentlywiththemeshspacing.Numericaldissipationandstabilisationbuiltintodifferenttime-
30 steppingroutinescanalsoinfluencethis“effectiveresolution”.Furthermore,MetROMSemploysastaggeredArakawaC-grid
for the ocean and B-grid for the sea ice (Arakawa and Lamb, 1977), by which different variables are calculated at different
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locationswithineachgridbox.InFESOM,allvariablesarecalculatedatthesamelocations(nodes),analogoustotheArakawa
A-grid.Thereissomeevidencethatthisdesigntendstoresolvefewerfeaturesoffluidflow(HaidvogelandBeckmann,1999),
andindeedFESOMappearstohavealowereffectiveresolutionthanfinite-differenceC-gridmodelswithcomparablenominal
grid spacing. These differences should be lessened in eddy-resolving simulations, where advection of momentum (which is
5 essentiallythesameinstaggeredandunstaggeredmeshes)dominates.However,thesimulationspresentedherearenoteddy-
resolving,sosomeinfluencewouldbeexpected.
Toaccountfortheseuncertainties,aswellastoinvestigatetheimportanceofresolutiononFESOM’sperformance,wehave
preparedtwomeshes:“low-resolution”(Figure1b)and“high-resolution”(Figure1c).Thehigh-resolutionmeshhasapproxi-
matelydoublethenumberof2Dnodesasthelow-resolutionmesh,buttheseextranodesarenotevenlyspacedthroughoutthe
10 domain. Outside the Southern Ocean, the two meshes have virtually identical resolution (not shown), ranging from 150-225
kmintheabyssalPacific,Atlantic,andIndianOceans,and50-75kmalongcoastlines.IntheopenSouthernOcean,resolution
rangesfrom20-100kmforthelow-resolutionmeshand15-50kmforthehigh-resolutionmesh.Bothmesheshavefinerres-
olution on the Antarctic continental shelf (approximately 8-10 km for low-resolution, 5-7 km for high-resolution) and in ice
shelfcavities(5-10kmforlow-resolution,3-7kmforhigh-resolution).Thegreatestdifferencebetweenthetwomeshesoccurs
15 in the Amundsen and Bellingshausen Seas, with approximate resolution of 11 km for the low-resolution mesh and 4 km for
high-resolution.
Inthevertical,MetROMSuses31terrain-followinglevelsusingthes-coordinatesystem,withincreasingverticalresolution
near the surface and bottom, and coarsest resolution in the interior. FESOM employs a hybrid z-sigma vertical coordinate
system, with the same discretisation for both the low- and high-resolution meshes. The region south of the 2500 m isobath
20 surroundingAntarctica,whichincludesalliceshelfcavitiesaswellasthecontinentalshelfandslope,hassigma-coordinates
with22levels.Theremainderofthedomainhasz-coordinates,comprisedof38levelsweightedtowardsthesurface.Bottom
nodesareallowedtodeviatefromthestandardz-levelsinordertomatchthegivenbathymetry.Bothmodelsarefree-surface,
whichleadstotime-varyingverticallevelsinMetROMS,butonlyaffectstheuppermostlayerinFESOM.
In both models, the use of terrain-following coordinates in the thin water columns of ice shelf cavities leads to extremely
25 high vertical resolution, often finer than 1 m in MetROMS, which limits the timestep. Our configuration of ROMS requires
a baroclinic timestep of 5 minutes for stability, with 30 barotropic timesteps for each baroclinic. In CICE, the timestep is
30minutesforbothdynamicandthermodynamicprocesses,andocean/sea-icecouplingisalsoperformedevery30minutes.
FESOMisrunwithatimestepof10minutesforthelow-resolutionmeshand9minutesforhigh-resolution.Theseaicemodel
operatesonthesametimestepastheoceancomponent.
30 2.3 Smoothingofbathymetryandiceshelfdraft
Steepbathymetrycanbeproblematicforterrain-followingcoordinateoceanmodels,asithasthepotentialtointroducepressure
gradient errors (Haney, 1991). Both ROMS (Shchepetkin, 2003) and FESOM (Wang et al., 2008) are designed to minimise
this issue with the splines density Jacobian method for the calculation of the pressure gradient force, which reduces errors
compared to the standard density Jacobian method. Nevertheless, a particular challenge arises at ice shelf fronts, which in
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reality are cliff faces that can reach several hundred metres in depth, but which models must represent as sloping surfaces.
Thissubstantialchangeinsurfacelayerdepthoveraslittleasonegridcellcreatessteeplyslopingverticallayerswithalarge
pressuregradient,andnumericalerrorsinthepressuregradientcalculationcoulddrivespuriouscirculationpatternsacrossthe
giveniceshelffront.
5 Inbothmodels,someamountofsmoothingofthebathymetryandiceshelfdraftisnecessaryfornumericalstabilityandto
reduce pressure gradient errors. On the other hand, excessive smoothing could alter the geometry of the ice shelf cavities to
the point where circulation is affected. An oversmoothed ice shelf front would be too shallow and gently sloping, providing
a pathway for warm surface waters to easily enter the cavity, where in reality a physical barrier exists. Near the grounding
linesatthebackoficeshelfcavities,oversmoothingwouldremovethedeepesticewhichmeltsmosteasily(LewisandPerkin,
10 1986). In this situation the water column thickness would be overestimated, allowing for greater transport of warm water to
the grounding line. In a coupled ice-sheet/ocean model, Timmermann and Goeller (2017) demonstrated that increased water
column thickness due to a thinning ice shelf can more than compensate for the reduced melting expected from the elevated
in-situfreezingpointattheiceshelfbase.Therefore,adelicatebalancemustbestruckwhensmoothingmodeltopographies,
inordertoachievethemostaccuratesimulation.
15 We prepared the MetROMS and FESOM domains using bathymetry, ice shelf draft, and land/sea masks from the RTopo-
1.05 dataset (Timmermann et al., 2010). MetROMS follows a 3-step smoothing procedure similar to that of Lemarié et al.
(2012). First, the “deep ocean filter” consists of a single pass of a Hanning filter (window size 3) over the bathymetry h,
with variable coefficients designed to remove isolated seamounts. Next, a selective Hanning filter is repeatedly applied to
bothlog(h)andlog(z ),wherez istheiceshelfdraft,untiltheslopeparameterr=(h h )/(h +h )satisfiesthe
ice ice i i+1 i i+1
−
20 conditionr<0.25everywhere(andsimilarlyforzice).Thisselectivefilterhascoefficientsscaledbythegradientofhorzice,
meaningthatregionswhicharealreadysmoothenoughwillnotbecomeoversmoothed.Finally,bothhandz undergoafinal
ice
twopassesofaregularHanningsmoothertoremove2Dnoise.Notethatthisseparatetreatmentofbathymetryandiceshelf
draftdoesnotdirectlyconsiderwatercolumnthickness,forwhichsomelargegradientsmayremain.
The smoothing procedure in FESOM is the same as described by Nakayama et al. (2014). The source bathymetry and
25 iceshelfdraftarefirstaveragedover4-minute( 1 ◦)intervals.ThenGaussianfiltersareappliedtobothfields,withspatially-
15
varyingradiiscaledbythedesiredfinalresolution.Forthisreason,high-resolutionregionsofthedomainreceivelesssmoothing
thanlower-resolutionregions.TheiceshelfdraftundergoesonepassoftheGaussianfilter,whilethebathymetryundergoesfour
passeswithlargerradii.Followinginterpolationtotheunstructuredmesh,theiceshelfdraftfieldreceivesselectivesmoothing
tosatisfythecriticalsteepnesslimitationofHaney(1991)atallpoints.Thisprocedurelimitstheslopeoftheiceshelfdraft,
30 andextremelyhighresolutionmaybenecessarytopreservesteepslopes.
Anotherregionofconcernisthegroundingline,wherewatercolumnthicknessapproacheszeroandverticallayersconverge.
Estimates of pressure gradient error, such as that of Haney (1991), scale inversely with the vertical layer thickness and can
thereforedivergenearthegroundingline.Toalleviatethisproblem,aminimumwatercolumnthicknessof50misenforced.
Inbothmodels,thebathymetryisartificiallydeepenedwherenecessarytosatisfythisconditionduringthesmoothingprocess.
5
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2.4 Oceanmixing
ROMSincludesseveraloptionsfortraceradvection(ShchepetkinandMcWilliams,2005),andthechoiceofadvectionscheme
isknowntoimpactthesimulation.ThecenteredandAkimafourth-ordertraceradvectionschemesaredominatedbydispersive
error, which can lead to undershoots of the freezing point and spurious sea ice formation in our MetROMS configuration
5 (Naughtenetal.,2017).Ontheotherhand,theupwindthird-ordertraceradvectionschemeisdominatedbydissipativeerror,
which can result in high levels of diapycnal mixing for some simulations (Lemarié et al., 2012; Marchesiello et al., 2009).
Indeed,problematicdiapycnalmixingrelatedtotheupwindthird-orderschemewasobservedindecadal-scalesimulationswith
our configuration of MetROMS (not shown). Therefore, the 25-year MetROMS simulation we present here uses the Akima
fourth-order tracer advection scheme, combined with explicitly parameterised Laplacian diffusion applied along isoneutral
10 surfaces,atalevelstrongenoughtosmoothoutmostdispersiveoscillations.Thisconfigurationshowsveryminimalspurious
seaiceformationcomparedtoasimulationwithflux-limited(i.e.locallymonotonic)upwindthird-orderadvection(Naughten
etal.,2017),andexhibitslessspuriousdiapycnalmixingthantheupwindscheme.Thediffusivitycoefficientis150m2/s,which
applies to the largest grid cell (approx. 24 km resolution) and is scaled linearly for smaller cells. Advection of momentum
uses the upwind third-order scheme in the horizontal and the centered fourth-order scheme in the vertical (Shchepetkin and
15 McWilliams,2005),andiscombinedwithparameterisedbiharmonicviscosityalonggeopotentialsurfaces,withacoefficient
of107m4/s(scaledbygridsizeaswithdiffusivity).
FESOM computes advection of momentum using the characteristic Galerkin method, and advection of tracers using the
explicitsecond-orderflux-corrected-transportscheme(Wangetal.,2014).TheLaplacianapproachisusedtoexplicitlyparam-
eterisebothdiffusivityandviscosity,withcoefficients600m2/sand6000m2/srespectively.Thesevaluesapplytoareference
20 areaof5800km2,andarescaledtotheareaofeachtriangularelement,scalingwiththesquarerootfordiffusivityandlinearly
forviscosity.At10kmresolution(elementareaof100km2),theresultingdiffusivityis78.8m2/s,comparedto62.4m2/sin
ROMS. The analogous viscosity terms cannot be directly compared between ROMS and FESOM, since they do not use the
sameparameterisation.
A flow-dependent Smagorinsky viscosity term is also applied in FESOM (Smagorinsky, 1963, 1993; Wang et al., 2014).
25 Inz-coordinateregions,tracerdiffusionisrotatedalongisoneutrals,andtheGent-McWilliamseddyparameterisationisused
(GentandMcWilliams,1990;Gentetal.,1995;Wangetal.,2014).Insigma-coordinateregions,diffusivityandviscosityare
bothappliedalongconstant-sigmasurfaces.
ForweaklystratifiedregionssuchastheSouthernOcean,thechoiceofverticalmixingparameterisationcanhaveasignifi-
cantimpactonsimulatedconvection(TimmermannandBeckmann,2004).MetROMSemploystheLarge-McWilliams-Doney
30 interiorclosurescheme(Largeetal.,1994)whichincludestheKPPboundarylayerparameterisation.Weimplementthesame
KPP modification as in Dinniman et al. (2011), which imposes a minimum surface boundary layer depth based on surface
stress, in the case of stabilising conditions. This modification is designed to address problems with excessive stratification
duringperiodsofrapidseaicemelt,andfollowsprinciplessimilartotheFESOMverticalmixingparameterisationdiscussed
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below. A shallow bias in mixed layer depths during the melt season is problematic for the accurate simulation of Southern
Oceanwatermasses,particularlyintheWeddellSea(TimmermannandBeckmann,2004).
TheverticalmixingschemeinourconfigurationofFESOM(Timmermannetal.,2009)consistsoftheRichardsonnumber
dependentparameterisationofPacanowskiandPhilander(1981),modifiedtohavemaximumverticaldiffusivitiesandviscosi-
5 tiesof0.05m2/s.OveradepthdefinedbytheMonin-Obukhovlength,calculatedasafunctionofsurfacestressandbuoyancy
forcing,anextra0.01m2/sisappliedtobothverticaldiffusivityandviscosity.ThiscombinationwasfoundbyTimmermann
andBeckmann(2004)toproducethemostrealisticrepresentationofwatermassesintheWeddellSea,avoidingtheexcessive
open-oceanconvectionwhichischaracteristicoftraditionalconvectiveadjustment.WealsotestedtheKPPparameterisation
(withoutthemodificationusedbyMetROMS)inshortsimulationswithourFESOMconfiguration(notshown).Comparedto
10 themodifiedPacanowski-Philanderscheme,thishadnosignificantimpactsonWeddellSeaconvection,atleastonthe5-year
timescale.
2.5 Iceshelfthermodynamics
Withterrain-followingcoordinates,itisrelativelystraightforwardtoincludeiceshelfcavitiesinanAntarcticdomain.Inboth
ROMSandFESOM,alloftheterrain-followingverticallayerssubductbeneaththeiceshelves.Thepressureoftheiceshelf
15 draftmustbeconsideredinthecalculationofthepressuregradient.ROMSverticallyintegratesthedensityofwaterdisplaced
byice,andassumesthedensityofthisdisplacedwaterisalinearfunctionofdepth,withcoefficient ∂ρ =4.78 10 3 kg/m4
∂z × −
and intercept given by the density in the first model layer. FESOM computes the pressure gradient force from the vertically
integratedhorizontaldensitygradientandassumesthatthehorizontalpressuregradientiszeroattheiceshelfbase.High-order
interpolationfordensityisdoneintheverticaltocomputehorizontaldensitygradientsasaccuratelyaspossible.
20 ROMSandFESOMsimulateiceshelfthermodynamics:theheatandsaltfluxesassociatedwithmeltingandrefreezingatthe
iceshelfbase.However,anynetmeltingorfreezingisnotactuallyappliedtotheiceshelfgeometry.Itisassumedthatglacial
flow of the ice shelf, surface accumulation, and basal melting are in dynamic equilibrium such that the geometry remains
constant. Removing this assumption necessitates coupling with an ice sheet model, which has recently been accomplished
forFESOM(TimmermannandGoeller,2017)andisunderdevelopmentforROMS(Gladstoneetal.,2017).Ice-sheet/ocean
25 couplingisanemergingfieldofclimatemodelling,andthefirstgenerationofmodelswillbecomparedandevaluatedaspart
oftheMISOMIPexperiments(MarineIceSheet-OceanModelIntercomparisonProject)(Asay-Davisetal.,2016).
Both ROMS and FESOM (Galton-Fenzi, 2009; Galton-Fenzi et al., 2012; Timmermann et al., 2012) implement the 3-
equation parameterisation of Hellmer and Olbers (1989) refined by Holland and Jenkins (1999). The heat and salt exchange
coefficientsγ andγ havetheform
T S
u u
∗ ∗
30 γT = κ+12.5Pr23 6 and γS = κ+12.5Sc32 6 (1)
− −
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wherePristhePrandtlnumberandScistheSchmidtnumber(bothdimensionlessconstants)andu isthefrictionvelocity,
∗
calculatedas
u∗=max Cd(u2+v2),u∗min (2)
(cid:16)p (cid:17)
where C is the drag coefficient (3 10 3 in ROMS, 2.5 10 3 in FESOM), u and v are the horizontal ocean velocity
d − −
× ×
5 componentsintheuppermostverticallayer,andu∗min isalowerboundforu∗ whichrepresentsmoleculardiffusion(10−3 in
ROMS,2.5 10 4inFESOM).Whiletheeffectofthedifferentdragcoefficientbetweenthemodelsislikelytobenegligible,
−
×
thelargerminimumu inROMSwillcausestrongermeltinginlocationswithveryweakflow,suchasatthegroundingline
∗
(Gwytheretal.,2016).
The turbulence term κ in Equation 1 has a different formulation between the two models. FESOM follows a very similar
10 approachtoJenkins(1991),bywhich
D
κ=2.12log u∗ 3 (3)
ν −
(cid:18) (cid:19)
whereD=10misareferenceboundarylayerdepth,andν=1.95 10 6 m2/sisthekinematicviscosity.ROMSinstead
−
×
usesasimplifiedversionofMcPheeetal.(1987)’sapproach,bywhich
2.5log 5300(u∗)2 +7.12 ifu >10 3and f >10 8
κ= |f| ∗ − | | − (4)
(cid:16) (cid:17)
0 otherwise
15 wheref istheCoriolisparameter.
While refreezing is implicit in the 3-equation formulation, none of our configurations include an explicit frazil ice model
suchasthatofSmedsrudandJenkins(2004)orGalton-Fenzietal.(2012).
2.6 Seaice
MetROMSincludestheseaicemodelCICE(Hunkeetal.,2015)whichisamulti-layer,multi-categorymodelwidelyusedin
20 globalcoupledmodelsaswellasregionalanduncoupledsetups.OurconfigurationofCICEhassevenicelayersplusonesnow
layer,andfiveicethicknesscategories.ItisexternallycoupledtoROMS,i.e.runsonseparateprocessors,withcommunication
drivenbythecouplerMCT(Larsonetal.,2005;Jacobetal.,2005).Therearesixbaroclinicoceantimesteps(5minutes)for
eachsea icetimestep(30minutes), andthecoupler exchangesfieldsevery seaicetimestep.Having longertimestepsfor the
seaicethanfortheoceaniscomputationallyfavourable,butitalsointroduceslagsinocean/sea-iceinteractions,becausethe
25 coupledfieldsaretime-averagedovertheprevious30minutes.
FESOM’s sea ice model is described by Danilov et al. (2015). It has a single ice layer (plus one snow layer) and a single
thickness category. It is internally coupled with the ocean, running on the same processors and the same timestep. While
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the FESOM sea ice model is generally less complex than CICE, it nonetheless has been shown to reproduce key features of
observedArcticandAntarcticseaice(Timmermannetal.,2009).
OurconfigurationofCICEusesthe“mushy”thermodynamicsschemeofTurneretal.(2013a).Italsoincludesthelevel-ice
meltpondparameterisationofHunkeetal.(2013),andtheDelta-Eddingtonradiationscheme(BrieglebandLight,2007).In
5 FESOM,seaicethermodynamicsfollowsParkinsonandWashington(1979)withthezero-layerapproachtoheatconduction
(Semtner,1976).
For sea ice dynamics, CICE uses elastic-anisotropic-plastic rheology (Tsamados et al., 2013) with the ridging-based ice
strength formulation of Rothrock (1975). FESOM has elastic-viscous-plastic rheology (Hunke and Dukowicz, 1997; Hunke,
2001) including a linear formulation of ice strength with coefficient P =15,000 N/m2. Sea ice transport follows an incre-
∗
10 mentalremappingapproachinCICE(LipscombandHunke,2004),withtheridgingparticipationandredistributionfunctions
ofLipscombetal.(2007).FESOMusesabackwardEulerimplicitadvectionschemeforseaicetransport.
3 Experimentaldesign
For this intercomparison, we simulated the 25-year period 1992-2016 using three model configurations: MetROMS, low-
resolutionFESOM,andhigh-resolutionFESOM(Figure1).
15 3.1 Initialconditions
All simulations are initialised using monthly-averaged observational or reanalysis products for January 1992. Initial ocean
temperatureandsalinityaretakenfromtheECCO2reanalysis(Menemenlisetal.,2008;Wunschetal.,2009),andextrapolated
intoiceshelfcavitiesusinganearest-neighbourmethodinCartesianspace.Initialoceanvelocityandseasurfaceheightareset
tozero.
20 Sea ice is initialised using the NOAA/NSIDC Climate Data Record for Passive Microwave Sea Ice Concentration (Meier
etal.,2013).WherevertheobservedAntarcticseaiceconcentrationexceeds0.15,themodelisinitialisedwithconcentration
1,icethicknessof1m,andsnowthicknessof0.2m.ThisisthesamemethodusedbyNaughtenetal.(2017)andissimilarto
thatofKjellssonetal.(2015).FESOM,havingaglobaldomain,alsorequiresinitialconditionsforArcticseaice.Wefollow
thesamemethodasfortheAntarctic,butsettheinitialicethicknessto2m,sinceArcticseaicetendstobethicker(Kwokand
25 Cunningham,2008;Worbyetal.,2008).Initialseaicevelocityissettozero.
Ourexperimentsdonotincludeaproperspinuptoaquasi-equilibriumstate.Forthepurposesofthisintercomparisonaround
theAntarcticmarginandcontinentalshelf,aswellasintheiceshelfcavities,weargueafullspinupisnotworththecompu-
tationalexpense.Thatisbecausetheprocesseswefocuson-onshoreflow,denseshelfwaterformation,andocean/ice-shelf
interaction-equilibratemuchmorequicklythantheinteriorocean.Indeed,area-averagedbasalmeltratesinourexperiments
30 stabilisewithin5-10yearsformosticeshelves.
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3.2 Atmosphericforcing
MetROMSandFESOMarebothforcedwiththeERA-Interimatmosphericreanalysis(Deeetal.,2011)using6-and12-hourly
fieldsovertheyears1992-2016.Duetodifferingimplementationsofmodelthermodynamics,thetwomodelsareforcedwith
differentcombinationsofatmosphericvariables.Bothmodelsutilise6-hourlyfieldsfornear-surfaceairtemperature,pressure,
5 and winds, which are linearly interpolated to each timestep. Near-surface humidity is derived from ERA-Interim’s 6-hourly
fields for dew point temperature; this conversion is performed in advance for MetROMS, but at run-time for FESOM. Both
modelsread12-hourlyfieldsforprecipitation(splitintorainandsnow)andevaporation,whicharenotinterpolatedintimebut
ratherappliedataconstantratewithastepchangeevery12hours,astheyrepresenttotalfluxesoverthegiven12-hourperiod.
MetROMSdiagnosesincomingshortwaveradiationfromERA-Interim’s6-hourlytotalcloudcover,whichisinterpolatedto
10 eachtimestep.Incominglongwaveradiationiscalculatedinternally.InFESOM,incomingshortwaveandlongwaveradiation
arereaddirectlyfromERA-Interim,as12-hourlyfieldswhichareappliedasstepchanges.
To account for the influence of iceberg calving on the Southern Ocean freshwater budget, both models are forced with
an additional surface freshwater flux representing iceberg melt. For this field we use the monthly climatology of Martin and
Adcroft(2010),interpolatedtoeachtimestep,andrepeatedannually.Riverrunofffromothercontinentsisnotconsidered.
15 3.3 Surfacesalinityrestoring
A persistent feature of many Southern Ocean models (Kjellsson et al., 2015; Heuzé et al., 2015; Sallée et al., 2013; Turner
et al., 2013b) is spuriously deep convection in the Weddell Sea, leading to an unrealistic open-ocean polynya as warm Cir-
cumpolarDeepWaterisbroughttothesurface.Thepossiblecausesofthiswidespreadmodelbiasincludeinsufficientsurface
freshwaterflux(Kjellssonetal.,2015)aswellasinsufficientsummermixedlayerdepths(TimmermannandBeckmann,2004).
20 Inbothcircumstances,salinityinthesubsurfaceWinterWaterlayerincreasesuntiltheweaklystratifiedwatercolumnbecomes
unstableandoverturns.
MetROMS is prone to deep convection in the Weddell Sea, and while tuning of the sea ice dynamics and ocean vertical
mixing helped to delay the onset of convection, the only permanent solution we found was surface salinity restoring. We
restore MetROMS to the World Ocean Atlas 2013 monthly climatology of surface salinity (Zweng et al., 2013), linearly
25 interpolated to each model timestep and repeated annually. Restoring has a timescale of 30 days and affects the uppermost
layer, whose thickness is time-varying but generally ranges from 1-3 m. We exclude the Antarctic continental shelf from
this restoring (defined as regions south of 60 S with bathymetry shallower than 1500 m, as well as all ice shelf cavities), as
◦
significantfresheningofAntarcticBottomWateroccursotherwise.Giventherelativelyscantobservationsonthecontinental
shelfmakinguptheWorldOceanAtlasproducts,restoringinthisregionmaynotbeappropriate.
30 FESOMdoesnotdevelopspuriousdeepconvectionintheWeddellSea,evenforlongsimulationswithoutrestoring.Possible
reasonsforthisdifferingbehaviourbetweenMetROMSandFESOMarediscussedinSection4.2.1.Nonetheless,weapplythe
samesurfacesalinityrestoringtoFESOMaswedotoMetROMS,sothattheexperimentsareassimilaraspossible.Restoring
10