Intercomparison of Antarctic ice-shelf, ocean, and sea-ice interactions simulated by MetROMS ...

36 Pages·2017·20.64 MB·English

Geosci.ModelDev.,11,1257–1292,2018
https://doi.org/10.5194/gmd-11-1257-2018
©Author(s)2018.Thisworkisdistributedunder
theCreativeCommonsAttribution4.0License.
Intercomparison of Antarctic ice-shelf, ocean, and sea-ice
interactions simulated by MetROMS-iceshelf and FESOM 1.4
KaitlinA.Naughten1,2,3,KatrinJ.Meissner1,2,BenjaminK.Galton-Fenzi4,3,MatthewH.England1,2,
RalphTimmermann5,HartmutH.Hellmer5,ToreHattermann6,5,andJensB.Debernard7
1ClimateChangeResearchCentre,Level4MathewsBuilding,UNSWSydney,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,0313Oslo,Norway
Correspondence:KaitlinA.Naughten([email protected])
Received:27October2017–Discussionstarted:1November2017
Revised:20February2018–Accepted:9March2018–Published:10April2018
Abstract.AnincreasingnumberofSouthernOceanmodels thisimprovesathigherresolution,sincelesssmoothingisre-
now include Antarctic ice-shelf cavities, and simulate ther- quired.IntheinteriorSouthernOcean,theverticalcoordinate
modynamics at the ice-shelf/ocean interface. This adds an- systemaffectsthedegreeofwatermasserosionduetospuri-
other level of complexity to Southern Ocean simulations, ousdiapycnalmixing,withMetROMS’terrain-followingco-
as ice shelves interact directly with the ocean and indi- ordinateleadingtomoreerosionthanFESOM’szcoordinate.
rectly with sea ice. Here, we present the first model inter- Finally, increased horizontal resolution in FESOM leads to
comparisonandevaluationofpresent-dayocean/sea-ice/ice- higherbasalmeltratesforsmalliceshelves,throughacom-
shelfinteractions,assimulatedbytwomodels:acircumpo- binationofstrongercirculationandsmall-scaleintrusionsof
lar Antarctic configuration of MetROMS (ROMS: Regional warmwaterfromoffshore.
OceanModellingSystemcoupledtoCICE:CommunityIce
CodE)andtheglobalmodelFESOM(FiniteElementSea-ice
OceanModel),wherethelatterisrunattwodifferentlevels
of horizontal resolution. From a circumpolar Antarctic per- 1 Introduction
spective,wecompareandevaluatesimulatedice-shelfbasal
meltingandsub-ice-shelfcirculation,aswellassea-iceprop- The Antarctic Ice Sheet (AIS) has significant potential to
ertiesandSouthernOceanwatermasscharacteristicsasthey drive sea level rise as climate change continues (Deconto
influencethesub-ice-shelfprocesses.Despitetheirdiffering andPollard,2016;Golledgeetal.,2015;Rignotetal.,2014;
numericalmethods,thetwomodelsproducebroadlysimilar MengelandLevermann,2014).Palaeorecordsindicatethat
resultsandsharesimilarbiasesinmanycases.Bothmodels the AIS was a major contributor to sea level change in past
reproducemanykeyfeaturesofobservationsbutstruggleto climateevents(Cooketal.,2013;Milleretal.,2012;Raymo
reproduceothers,suchasthehighmeltratesobservedinthe and Mitrovica, 2012; Dutton et al., 2015; O’Leary et al.,
smallwarm-cavityiceshelvesoftheAmundsenandBelling- 2013), and the mass balance of the modern-day AIS is al-
shausen seas. Several differences in model design show a ready negative (Rignot et al., 2011; Zwally and Giovinetto,
particular influence on the simulations. For example, FE- 2011; Shepherd et al., 2012). The ocean is an important
SOM’s greater topographic smoothing can alter the geome- driverofAISretreat(Golledgeetal.,2017;JoughinandAl-
tryofsomeice-shelfcavitiesenoughtoaffecttheirmeltrates; ley,2011),as40%oftheicesheetbyareaisgroundedbelow
sealevel(Fretwelletal.,2013).Thisgeometryprovidesthe
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
1258 K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions
potentialfortheoceantomeltlargeregionsoftheAISfrom derstandprocessesinobservedcavities(Timmermannetal.,
below. For example, the Amundsen sector of West Antarc- 2012; Galton-Fenzi et al., 2012) and to provide future pro-
ticahasbedrockgeometryfavourableforamarineice-sheet jectionsofice-shelfmelt(TimmermannandHellmer,2013;
instability,andunstableretreatmayhavealreadybegun(Rig- Hellmeretal.,2012,2017),soanalysisofthesimilaritiesand
notetal.,2014). differences between such models is timely. Another impor-
The ocean directly interacts with the AIS through ice tantbenefitofrealisticdomainsistheopportunitytocompare
shelves, which are the floating extensions of the land-based modeloutputtoavailableobservations,eveniftheseobserva-
ice sheet. The properties of ice-shelf cavities, the pockets tionsarelimited.Therefore,anelementofmodelevaluation,
of ocean between ice shelves and the seafloor, determine aswellasmodelintercomparison,canbeincluded.
the basal melt rates of each ice shelf which ultimately af- In this paper, we present such an intercomparison of two
fect the mass balance of the AIS through dynamical pro- oceanmodels,bothincludingice-shelfthermodynamicsand
cesses(Dupont,2005).Theseawaterinice-shelfcavitiescan sea-ice components, from a circumpolar Antarctic perspec-
be sourced from several different water masses, which af- tive. We focus on ice-shelf basal melt and sub-ice-shelf cir-
fect its temperature and salinity. Many of these source wa- culation across eight regions of the Antarctic coastline but
termassesareinfluencedbysea-iceprocesses(Jacobsetal., alsoconsiderinteriorSouthernOceanandsea-iceprocesses
1992;Nichollsetal.,2009). as they affect ice-shelf cavities. The model output is com-
A better understanding of ocean/sea-ice/ice-shelf interac- pared to relevant observations where available. Finally, key
tions in Antarctica is crucial, particularly given their im- findingsandtheirimplications,aswellaspossibilitiesforfu-
portance for future sea level rise. However, these interac- turemodeldevelopment,arediscussed.
tions take place in observation-deficient regions. In particu-
lar, there are very few direct measurements inside ice-shelf
2 Modeldescriptions
cavities, and observations are also scarce in the sea-ice-
coveredregionsoftheSouthernOcean(Rintouletal.,2010).
Two coupled ocean/sea-ice/ice-shelf models are included
Nonetheless, some measurements have been made at great
in this intercomparison: MetROMS-iceshelf (hereafter
expense(e.g.Nichollsetal.,2006;McPhailetal.,2009;Ven-
MetROMS) and Finite Element Sea-ice Ocean Model 1.4
ables and Meredith, 2014). While ice-shelf basal melt rates
(hereafter FESOM). We run FESOM at two different reso-
canbeinferredusingremotesensingmethods(Rignotetal.,
lutions for a total of three experiments (see Sect. 3). In this
2013;Depoorteretal.,2013),largeuncertaintiesremainre-
section, we describe the two models and compare their sci-
gardingthecirculationpatternsdrivingthesemeltrates,and
entificdesign.
nopredictionsforthefuturecanbemadebasedonthesedata.
Consequently, much of our understanding of ocean/sea-
2.1 Overview
ice/ice-shelfinteractionsisbasedonnumericalmodelling.In
recentyears,anincreasingnumberofoceanmodelshavebe- MetROMS consists of the regional ocean model ROMS
guntoresolveice-shelfcavitiesandsimulatethermodynamic (Regional Ocean Modelling System) (Shchepetkin and
processes at the ice-shelf base (Dinniman et al., 2016, and McWilliams, 2005) including ice-shelf thermodynamics
referencestherein;Mathiotetal.,2017).Giventhevarietyof (Galton-Fenzi et al., 2012), coupled to the sea-ice model
modelsinvolved,andtherelativelackofobservationstocon- CICE(CommunityIceCodE)(Hunkeetal.,2015)usingthe
straintheirtuning,itisdesirabletoconductmodelintercom- couplerMCT(ModelCouplingToolkit)(Larsonetal.,2005;
parisonprojects(MIPs;see,e.g.Meehletal.,2000)bywhich Jacob et al., 2005). The coupling was implemented by the
several models run the same experiment and their output is NorwegianMeteorologicalInstitute(Debernardetal.,2017)
compared.Theresultinginsightsintomodelsimilaritiesand and is described in Naughten et al. (2017). We use the de-
differencescanideallybeattributedtomodeldesignchoices, velopment version 3.7 of the ROMS code, version 5.1.2 of
withtheaimofguidingfuturedevelopment. CICE,andversion2.9ofMCT.
To date, the only MIPs considering ice-shelf cavities are FESOMisaglobaloceanmodel(Wangetal.,2014)with
the ongoing ISOMIP experiments (Ice Shelf-Ocean Model an internally coupled sea-ice model (Danilov et al., 2015;
Intercomparison Project) (Hunter, 2006; Asay-Davis et al., Timmermann et al., 2009) and ice-shelf thermodynamics
2016) which use idealised domains and simplified forcing, (Timmermann et al., 2012). It has an unstructured mesh in
anddonotincludecoupledsea-icemodels.TheISOMIPex- thehorizontal,consistingoftriangularelementswhichallow
periments are undoubtedly valuable and are likely to pro- forspatiallyvaryingresolution.Thenumericalmethodsasso-
videparticularinsightsregardingtheresponseofcavitycir- ciatedwiththeunstructuredmesharedetailedbyWangetal.
culationtowarmversuscoldforcing.However,idealisedex- (2008)andWangetal.(2014),whiletheimplementationof
periments such as ISOMIP should be complemented by in- the ice-shelf component is discussed in Timmermann et al.
tercomparisons over more realistic domains, with observa- (2012).
tionally derived forcing and coupled sea-ice models. These
model configurations are already being used to better un-
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K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions 1259
Figure1.Horizontalresolution(km)oftheMetROMSgridandbothFESOMmeshesaroundAntarctica.Resolutionisdefinedasthesquare
rootoftheareaofeachgridbox(MetROMS)ortriangularelement(FESOM).Notethatvaluesabove20kmarenotdifferentiated.
2.2 Domainandresolution resolutionmesh,buttheseextranodesarenotevenlyspaced
throughoutthedomain.OutsidetheSouthernOcean,thetwo
mesheshavevirtuallyidenticalresolution(notshown),rang-
OurconfigurationofMetROMShasacircumpolarAntarctic
domain with a northern boundary at 30◦S. Horizontal res- ingfrom150to225kmintheabyssalPacific,Atlantic,and
Indian oceans, and 50–75km along coastlines. In the open
olution is quarter-degree scaled by cosine of latitude, and
SouthernOcean,resolutionrangesfrom20to100kmforthe
the South Pole is relocated to achieve approximately equal
low-resolution mesh and 15–50km for the high-resolution
resolution around the Antarctic coastline. This leads to res-
mesh. Both meshes have finer resolution on the Antarctic
olutions (defined as the square root of the area of each grid
continental shelf (approximately 8–10km for low resolu-
box) of approximately 15–20km in the Antarctic Circum-
tion,5–7kmforhighresolution)andinice-shelfcavities(5–
polar Current (ACC), 8–10km on the Antarctic continen-
10km for low resolution, 3–7km for high resolution). The
tal shelf, and 5km or finer at the southernmost grounding
greatest difference between the two meshes occurs in the
lines of the Ross, Filchner–Ronne, and Amery ice shelves
Amundsen and Bellingshausen seas, with approximate res-
(Fig.1a).
olution of 11km for the low-resolution mesh and 4km for
Our FESOM setup has a global domain with spatially
thehigh-resolutionmesh.
varying horizontal resolution. Here, we define resolution in
Inthevertical,MetROMShas31terrain-followinglevels
FESOMasthesquarerootoftheareaofeachtriangularel-
using the s-coordinate system, with increasing vertical res-
ement;however,thismetricmaynotbetrulycomparableto
olutionnearthesurfaceandbottom,andcoarsestresolution
MetROMS.Whendiscussingresolution,therealquestionis
in the interior. FESOM employs a hybrid z-σ vertical coor-
the smallest flow features that are captured by a mesh of
dinatesystem,withthesamediscretisationforboththelow-
a certain spacing. In models with such different numerical
andhigh-resolutionmeshes.Theregionsouthofthe2500m
methods as MetROMS (finite volume) and FESOM (finite
isobath surrounding Antarctica, which includes all ice-shelf
element),thesmallestresolvedfeaturemayscaledifferently
cavitiesaswellasthecontinentalshelfandslope,hasσ co-
with the mesh spacing. Numerical dissipation and stabilisa-
ordinates with 22 levels. The remainder of the domain has
tionbuiltintodifferenttime-steppingroutinescanalsoinflu-
z coordinates, comprised of 38 levels weighted towards the
ence this effective resolution. Furthermore, MetROMS em-
surface. Both models are free-surface, which leads to time-
ploys a staggered Arakawa C grid for the ocean and B grid
varyingverticallevelsinMetROMSbutonlyaffectstheup-
for the sea ice (Arakawa and Lamb, 1977) by which differ-
permostlayerinFESOM.
entvariablesarecalculatedatdifferentlocationswithineach
Inbothmodels,theuseofterrain-followingcoordinatesin
gridbox.InFESOM,allvariablesarecalculatedatthesame
thethinwatercolumnsofice-shelfcavitiesleadstoenhanced
locations (nodes), analogous to the Arakawa A grid. There
verticalresolution,oftenfinerthan1minMetROMS,which
issomeevidencethatthisdesigntendstoresolvefewerfea-
limits the time step. Our configuration of ROMS requires a
turesoffluidflow(HaidvogelandBeckmann,1999),andin-
baroclinictimestepof5minforstability,with30barotropic
deed FESOM appears to have a lower effective resolution
time steps for each baroclinic. In CICE, the time step is
thanfinite-differenceC-gridmodelswithcomparablenomi-
30minforbothdynamicandthermodynamicprocesses,and
nalgridspacing.
ocean/sea-ice coupling is also performed every 30min. FE-
Toaccountfortheseuncertainties,aswellastoinvestigate
SOMisrunwithatimestepof10minforthelow-resolution
the importance of resolution on FESOM’s performance, we
mesh and 9min for the high-resolution mesh. The sea-ice
have prepared two meshes: “low resolution” (Fig. 1b) and
model operates on the same time step as the ocean compo-
“high resolution” (Fig. 1c). The high-resolution mesh has
nent.
approximately double the number of 2-D nodes as the low-
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1260 K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions
2.3 Smoothingofbathymetryandice-shelfdraft doesnotdirectlyconsiderwatercolumnthickness,forwhich
somelargegradientsmayremain.
The smoothing procedure in FESOM is the same as de-
Steep bathymetry can be problematic for terrain-following scribed by Nakayama et al. (2014). The source bathymetry
coordinate ocean models, as it has the potential to intro- and ice-shelf draft are first averaged over 4min (1/15◦) in-
duce pressure gradient errors (Haney, 1991). Both ROMS tervals.ThenGaussianfiltersareappliedtobothfields,with
(Shchepetkin,2003)andFESOM(Wangetal.,2008)arede- spatiallyvaryingradiiscaledbythedesiredfinalresolution.
signed to minimise this issue with the splines density Jaco- For this reason, high-resolution regions of the domain re-
bianmethodforthecalculationofthepressuregradientforce, ceivelesssmoothingthanlower-resolutionregions.Theice-
whichreduceserrorscomparedtothestandarddensityJaco- shelf draft undergoes one pass of the Gaussian filter, while
bian method. Nevertheless, a particular challenge arises at thebathymetryundergoesfourpasseswithlargerradii.Fol-
ice-shelffronts,whichinrealityareclifffacesthatcanreach lowing interpolation to the unstructured mesh, the ice-shelf
severalhundredmetresindepthbutwhichmodelsmustrep- draftfieldreceivesselectivesmoothingtosatisfythecritical
resentasslopingsurfaces.Thissubstantialchangeinsurface steepnesslimitationofHaney(1991)atallpoints.Thispro-
layerdepthoveraslittleasonegridcellcreatessteeplyslop- cedure limits the slope of the ice-shelf draft, and extremely
ingverticallayerswithalargepressuregradient,andnumer- highresolutionmaybenecessarytopreservesteepslopes.
ical errors in the pressure gradient calculation could drive Another region of concern is the grounding line, where
spuriouscirculationpatternsacrossthegivenice-shelffront. water column thickness approaches zero and vertical layers
In both models, some amount of smoothing of the converge.Estimatesofpressuregradienterror,suchasthatof
bathymetryandice-shelfdraftisnecessaryfornumericalsta- Haney (1991), scale inversely with the vertical layer thick-
bility and to reduce pressure gradient errors. On the other ness and can therefore diverge near the grounding line. To
hand, excessive smoothing could alter the geometry of the alleviate this problem, a minimum water column thickness
ice-shelf cavities to the point where circulation is affected. of 50m is enforced. In both models, the bathymetry is ar-
An oversmoothed ice-shelf front would be too shallow and tificially deepened where necessary to satisfy this condition
gently sloping, providing a pathway for warm surface wa- duringthesmoothingprocess.
ters to easily enter the cavity, where in reality a physical
barrier exists. Near the grounding lines at the backs of ice- 2.4 Oceanmixing
shelf cavities, oversmoothing would remove the deepest ice
whichmeltsmosteasily(LewisandPerkin,1986).Inthissit- ROMSincludesseveraloptionsfortraceradvection(Shchep-
uation,thewatercolumnthicknesswouldbeoverestimated, etkin and McWilliams, 2005), and the choice of advection
allowing for greater transport of warm water to the ground- scheme is known to impact the simulation. The centred
ing line. In a coupled ice-sheet/ocean model, Timmermann and Akima fourth-order tracer advection schemes are dom-
and Goeller (2017) demonstrated that increased water col- inated by dispersive error, which can lead to undershoots
umnthicknessduetoathinningiceshelfcanmorethancom- of the freezing point and spurious sea-ice formation in our
pensateforthereducedmeltingexpectedfromtheelevatedin MetROMS configuration (Naughten et al., 2017). On the
situfreezingpointattheice-shelfbase.Therefore,adelicate otherhand,theupwindthird-ordertraceradvectionschemeis
balancemustbestruckwhensmoothingmodeltopographies, dominatedbydissipativeerror,whichcanresultinhighlev-
inordertoachievethemostaccuratesimulation. elsofdiapycnalmixingforsomesimulations(Lemariéetal.,
We prepared the MetROMS and FESOM domains us- 2012;Marchesielloetal.,2009).Indeed,problematicdiapyc-
ingbathymetry,ice-shelfdraft,andland/seamasksfromthe nalmixingrelatedtotheupwindthird-orderschemewasob-
RTopo-1.05 dataset (Timmermann et al., 2010). MetROMS servedindecadal-scalesimulationswithourconfigurationof
follows a three-step smoothing procedure similar to that of MetROMS (not shown). Therefore, the 25-year MetROMS
Lemarié et al. (2012). First, the “deep ocean filter” con- simulation we present here uses the Akima fourth-order
sists of a single pass of a Hanning filter (window size 3) tracer advection scheme, combined with explicitly parame-
over the bathymetry h, with variable coefficients designed terisedLaplaciandiffusionappliedalongisoneutralsurfaces,
toremoveisolatedseamounts.Next,aselectiveHanningfil- atalevelstrongenoughtosmoothoutmostdispersiveoscil-
terisrepeatedlyappliedtobothlog(h)andlog(z ),where lations. This configuration shows minimal spurious sea-ice
ice
z istheice-shelfdraft,untiltheslopeparameterr =|h − formation,comparabletoasimulationwithflux-limited(i.e.
ice i
hi+1|/(hi+hi+1)satisfiestheconditionr <0.25everywhere locallymonotonic)upwindthird-orderadvection(Naughten
(and similarly for z ). This selective filter has coefficients etal.,2017),andexhibitslessspuriousdiapycnalmixingthan
ice
scaled by the gradient of h or z , meaning that regions theupwindscheme.Thediffusivitycoefficientis150m2s−1,
ice
which are already smooth enough will not become over- which applies to the largest grid cell (approximately 24km
smoothed. Finally, both h and z undergo two final passes resolution)andisscaledlinearlyforsmallercells.Advection
ice
of a regular Hanning smoother to remove 2-D noise. Note ofmomentumusestheupwindthird-orderschemeinthehor-
thatthisseparatetreatmentofbathymetryandice-shelfdraft izontal and the centred fourth-order scheme in the vertical
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K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions 1261
(ShchepetkinandMcWilliams,2005),andiscombinedwith tions with our FESOM configuration (not shown). At least
parameterised biharmonic viscosity along geopotential sur- on the 5-year timescale, hydrography in the offshore Wed-
faces,withacoefficientof107m2s−1(scaledbygridsizeas dell Sea was very similar between KPP and the modified
withdiffusivity). Pacanowski–Philanderscheme.Itispossiblethatlongersim-
FESOMcomputesadvectionofmomentumusingthechar- ulationswouldshowmoredivergence,andthiswarrantsfur-
acteristicGalerkinmethod,andadvectionoftracersusingthe therinvestigation.
explicitsecond-orderflux-corrected-transportscheme(Wang
et al., 2014). The Laplacian approach is used to explicitly 2.5 Ice-shelfthermodynamics
parameterisebothdiffusivityandviscosity,withcoefficients
600 and 6000m2s−1, respectively. These values apply to a With terrain-following coordinates, it is relatively straight-
referenceareaof5800km2andarescaledtotheareaofeach forwardtoincludeice-shelfcavitiesinanAntarcticdomain.
InbothROMSandFESOM,alloftheterrain-followingver-
triangularelement,scalingwiththesquarerootfordiffusiv-
ticallayerssubductbeneaththeiceshelves.Thepressureof
ity and linearly for viscosity. At 10km resolution (element
area of 100km2), the resulting diffusivity is 78.8m2s−1, the ice-shelf draft must be considered in the calculation of
compared to 62.4m2s−1 in ROMS. The analogous viscos- thepressuregradient.ROMSverticallyintegratesthedensity
of water displaced by ice, and assumes the density of this
ity terms cannot be directly compared between ROMS and
displacedwaterisalinearfunctionofdepth,withcoefficient
FESOM,sincetheydonotusethesameparameterisation.
∂ρ =4.78×10−3kgm−4 andinterceptgivenbythedensity
Aflow-dependentSmagorinskyviscositytermisalsoap- ∂z
inthefirstmodellayer.FESOMcomputesthepressuregra-
plied in FESOM (Smagorinsky, 1963, 1993; Wang et al.,
dient force from the vertically integrated horizontal density
2014). In z-coordinate regions, tracer diffusion is rotated
gradientandassumesthatthehorizontalpressuregradientis
along isoneutrals, and the Gent–McWilliams eddy parame-
zero at the ice-shelf base. High-order interpolation for den-
terisation is used (Gent and McWilliams, 1990; Gent et al.,
sityisdoneintheverticaltocomputehorizontaldensitygra-
1995;Wangetal.,2014).Inσ-coordinateregions,diffusivity
dientsasaccuratelyaspossible.
andviscosityarebothappliedalongconstant-σ surfaces.
ROMS and FESOM simulate ice-shelf thermodynamics:
ForweaklystratifiedregionssuchastheSouthernOcean,
theheatandsaltfluxesassociatedwithmeltingandrefreez-
the choice of vertical mixing parameterisation can have a
ingattheice-shelfbase.However,anynetmeltingorfreez-
significant impact on simulated convection (Timmermann
ing is not actually applied to the ice-shelf geometry. It is
and Beckmann, 2004). MetROMS employs the Large–
assumed that glacial flow of the ice shelf, surface accumu-
McWilliams–Doney interior closure scheme (Large et al.,
lation, and basal melting are in dynamic equilibrium such
1994) which includes the K-profile parameterisation (KPP)
thatthegeometryremainsconstant.Removingthisassump-
boundary layer parameterisation. We implement the same
tionnecessitatescouplingwithanice-sheetmodel,whichhas
KPP modification as in Dinniman et al. (2011), which im-
recently been accomplished for FESOM (Timmermann and
posesaminimumsurfaceboundarylayerdepthbasedonsur-
Goeller, 2017) and is under development for ROMS (Glad-
facestress,inthecaseofstabilisingconditions.Thismodifi-
stone et al., 2017). Ice-sheet/ocean coupling is an emerging
cationisdesignedtoaddressproblemswithexcessivestratifi-
fieldofclimatemodelling,andthefirstgenerationofmodels
cationduringperiodsofrapidsea-icemelt,andfollowsprin-
willbecomparedandevaluatedaspartoftheMISOMIPex-
ciples similar to the FESOM vertical mixing parameterisa-
periments (Marine Ice Sheet-Ocean Model Intercomparison
tion discussed below. A shallow bias in mixed layer depths
Project)(Asay-Davisetal.,2016).
during the melt season is problematic for the accurate sim-
Both ROMS and FESOM (Galton-Fenzi, 2009; Galton-
ulation of Southern Ocean water masses, particularly in the
Fenzi et al., 2012; Timmermann et al., 2012) implement
WeddellSea(TimmermannandBeckmann,2004).
the three-equation parameterisation of Hellmer and Olbers
The vertical mixing scheme in our configuration of FE-
(1989)refinedbyHollandandJenkins(1999).Theheatand
SOM (Timmermann et al., 2009) consists of the Richard-
saltexchangecoefficientsγ andγ havetheform
son number dependent parameterisation of Pacanowski and T S
Philander(1981),modifiedtohavemaximumverticaldiffu- u∗ u∗
sivitiesandviscositiesof0.05m2s−1.Overadepthdefined γT = κ+12.5 Pr23 −6, andγS = κ+12.5 Sc23 −6, (1)
by the Monin–Obukhov length, calculated as a function of
surfacestressandbuoyancyforcing,anextra0.01m2s−1 is wherePristhePrandtlnumberandScistheSchmidtnumber
applied to both vertical diffusivity and viscosity. This com- (bothdimensionlessconstants),andu∗isthefrictionvelocity
bination was found by Timmermann and Beckmann (2004) inms−1,calculatedas
toproducethemostrealisticrepresentationofwatermasses (cid:18)q (cid:19)
intheWeddellSea,avoidingtheexcessiveopen-oceancon- u∗=max Cd(cid:0)u2+v2(cid:1),u∗min , (2)
vection which is characteristic of traditional convective ad-
justment. We also tested the KPP parameterisation (with- where C is the drag coefficient (3×10−3 in ROMS, 2.5×
d
out the modification used by MetROMS) in short simula- 10−3 in FESOM), u and v are the horizontal ocean veloc-
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1262 K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions
ity components in the uppermost vertical layer, and u∗ is Our configuration of CICE uses the “mushy” thermody-
min
a lower bound for u∗ which represents molecular diffusion namicsscheme ofTurneret al.(2013a).Italso includesthe
(10−3inROMS,2.5×10−4inFESOM).Whiletheeffectof level-icemeltpondparameterisationofHunkeetal.(2013),
thedifferentdragcoefficientbetweenthemodelsislikelyto and the Delta–Eddington radiation scheme (Briegleb and
be negligible, the larger minimum u∗ in ROMS will cause Light, 2007). In FESOM, sea-ice thermodynamics follows
strongermeltinginlocationswithveryweakflow,suchasat Parkinson and Washington (1979) with the zero-layer ap-
thegroundingline(Gwytheretal.,2016). proachtoheatconduction(Semtner,1976).
The turbulence term κ in Eq. (1) has a different formula- For sea-ice dynamics, CICE uses elastic–anisotropic–
tionbetweenthetwomodels.FESOMfollowsaverysimilar plastic rheology (Tsamados et al., 2013) with the ridging-
approachtoJenkins(1991)bywhich basedicestrengthformulationofRothrock(1975).FESOM
has elastic–viscous–plastic rheology (Bouillon et al., 2013)
(cid:18) (cid:19)
D
κ =2.12 log u∗ −3, (3) including a linear formulation of ice strength with coeffi-
ν cientP∗=15000Nm−2.Sea-icetransportfollowsanincre-
where D=10m is a reference boundary layer depth, and mentalremappingapproachinCICE(LipscombandHunke,
ν=1.95×10−6m2s−1isthekinematicviscosity.ROMSin- 2004),withtheridgingparticipationandredistributionfunc-
tions of Lipscomb et al. (2007). FESOM uses a backward
steadusesasimplifiedversionofMcPheeetal.(1987)’sap-
Eulerimplicitadvectionschemeforsea-icetransport.
proachbywhich
2.5 log(cid:18)(cid:0)5300m|−f2|s(cid:1)(u∗)2(cid:19)+7.12 ifu∗>10−3ms−1 2W.7hileSMureftaRcOeMexSchaanndgFeEsSchOeMmeareforcedwiththesameat-
κ= and ,
0 o|fth|e>rw1i0se−8s−1 mdmiofofsdeperhlseb.raiOcseusdrtatocenon(sfitehgeeuSrbaeutcilotk.n3f.oo2fr)m,FtuhEleaSeOreMsimulptuilnseegmsseucnrotfenadscteabnflytuxethexes-
(4) change coefficients for heat and momentum fluxes, while
MetROMS’ exchange coefficients vary in time and space.
wheref istheCoriolisparameterins−1.
For ocean/atmosphere fluxes (in ROMS), these coefficients
While refreezing is implicit in the three-equation formu-
are based on the COARE (Coupled Ocean-Atmosphere Re-
lation, none of our configurations include an explicit frazil
sponse Experiment) protocol (Fairall et al., 1996). For sea-
ice model such as that of Smedsrud and Jenkins (2004) or
ice/atmospherefluxes,CICEincludesastability-basedatmo-
Galton-Fenzietal.(2012).
spheric boundary interface (Hunke et al., 2015). These dif-
ferences in bulk formulae may affect the simulations, par-
2.6 Seaice
ticularlythemomentumfluxeswhichhaveconsequencesfor
ACC transport, Ekman pumping, and sea-ice formation and
MetROMS includes the sea-ice model CICE (Hunke et al.,
drift. A comparison of ocean surface stress (not shown) re-
2015) which is a multi-layer, multi-category model widely
veals that these momentum fluxes are typically stronger in
usedinglobalcoupledmodelsaswellasregionalanduncou-
MetROMSbyupto30%.
pledsetups.OurconfigurationofCICEhassevenicelayers
plusonesnowlayer,andfiveicethicknesscategories.Itisex-
ternally coupled to ROMS, i.e. runs on separate processors,
3 Experimentaldesign
with communication driven by the coupler MCT (Larson
etal.,2005;Jacobetal.,2005).Therearesixbaroclinicocean For this intercomparison, we simulated the 25-year pe-
time steps (5min) for each sea-ice time step (30min), and riod of 1992–2016 using three model configurations:
thecouplerexchangesfieldseverysea-icetimestep.Having MetROMS,low-resolutionFESOM,andhigh-resolutionFE-
longertimestepsfortheseaicethanfortheoceaniscompu- SOM(Fig.1).
tationallyfavourable,butitalsointroduceslagsinocean/sea-
iceinteractions,becausethecoupledfieldsaretime-averaged 3.1 Initialconditions
overtheprevious30min.
FESOM’s sea-ice model is described by Danilov et al. All simulations are initialised using monthly averaged ob-
(2015). It has a single ice layer (plus one snow layer) and servational or reanalysis products for January 1992. Initial
a single thickness category. It is internally coupled with the oceantemperatureandsalinityaretakenfromtheECCO2re-
ocean, running on the same processors and the same time analysis(Menemenlisetal.,2008;Wunschetal.,2009),and
step.WhiletheFESOMsea-icemodelisgenerallylesscom- extrapolatedintoice-shelfcavitiesusinganearest-neighbour
plexthanCICE,itnonethelesshasbeenshowntoreproduce methodinCartesianspace.Initialoceanvelocityandseasur-
key features of observed Arctic and Antarctic sea ice (Tim- faceheightaresettozero.
mermannetal.,2009).
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K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions 1263
Sea ice is initialised using the National Oceanic and At- distributionconstrainedbyobservations.MartinandAdcroft
mospheric Administration (NOAA)/National Snow and Ice (2010)’smonthlyclimatologyoficebergmeltisinterpolated
DataCenter (NSIDC)ClimateData Recordfor PassiveMi- to each time step in our simulations and repeated annually.
crowave Sea Ice Concentration (Meier et al., 2013). Wher- Riverrunofffromothercontinentsisnotconsidered.
ever the observed Antarctic sea-ice concentration exceeds
0.15,themodelisinitialisedwithconcentration1,icethick- 3.3 Surfacesalinityrestoring
ness of 1m, and snow thickness of 0.2m. This is the same
ApersistentfeatureofmanySouthernOceanmodels(Kjells-
methodusedbyNaughtenetal.(2017)andissimilartothat
son et al., 2015; Heuzé et al., 2015; Sallée et al., 2013;
ofKjellssonetal.(2015).FESOM,havingaglobaldomain,
Turner et al., 2013b; Goosse and Fichefet, 2001) is spu-
also requires initial conditions for Arctic sea ice. We fol-
riously deep convection in the Weddell Sea, leading to an
low the same method as for the Antarctic but set the initial
unrealistic open-ocean polynya as warm Circumpolar Deep
icethicknessto2m,sinceArcticseaicetendstobethicker
Water is brought to the surface. The possible causes of this
(Kwok and Cunningham, 2008; Worby et al., 2008). Initial
widespreadmodelbiasincludeinsufficientsurfacefreshwa-
sea-icevelocityissettozero.
terflux(Kjellssonetal.,2015)aswellasinsufficientsummer
Ourexperimentsdonotincludeaproperspinuptoaquasi-
mixedlayerdepths(TimmermannandBeckmann,2004).In
equilibrium state. For the purposes of this intercomparison
both circumstances, salinity in the subsurface Winter Water
aroundtheAntarcticmarginandcontinentalshelf,aswellas
layer increases until the weakly stratified water column be-
in the ice-shelfcavities, we argue a full spinup is not worth
comesunstableandoverturns.
thecomputationalexpense.Theprocesseswefocuson–on-
MetROMSispronetodeepconvectionintheWeddellSea,
shoreflow,denseshelfwaterformation,andocean/ice-shelf
and while tuning of the sea-ice dynamics and ocean verti-
interaction–equilibratemuchmorequicklythantheinterior
calmixinghelpedtodelaytheonsetofconvection,theonly
ocean.Forexample,area-averagedbasalmeltratesinourex-
permanentsolutionwefoundwassurfacesalinityrestoring.
perimentsstabilisewithin5–10yearsformosticeshelves.
Suchrestoringaffectsthesaltbudgetandmaycontributeto
driftinthetotalsaltcontentoftheocean,althoughitprevents
3.2 Atmosphericforcing
driftinthesurfacelayer.Thismayimpactthedensitystruc-
tureoftheSouthernOcean,andparticularlytheACC,aswell
MetROMS and FESOM are both forced with the ERA-
asdampinginterannualvariability.However,theseshortcom-
Interim atmospheric reanalysis (Dee et al., 2011) using 6-
ings were deemed preferable to spurious deep convection
and 12-hourly fields over the years 1992–2016. Due to dif-
for the purposes of our analysis. We restore MetROMS to
fering implementations of model thermodynamics, the two
the World Ocean Atlas 2013 monthly climatology of sur-
models are forced with different combinations of atmo-
face salinity (Zweng et al., 2013), linearly interpolated to
spheric variables. Both models utilise 6-hourly fields for
eachmodeltimestepandrepeatedannually.Restoringhasa
near-surfaceairtemperature,pressure,andwinds,whichare
timescaleof30daysandaffectstheuppermostlayer,whose
linearlyinterpolatedtoeachtimestep.Near-surfacehumid-
thicknessistime-varyingbutgenerallyrangesfrom1to3m.
ity is derived from ERA-Interim’s 6-hourly fields for dew-
We exclude the Antarctic continental shelf from this restor-
point temperature; this conversion is performed in advance
ing(definedasregionssouthof60◦Swithbathymetryshal-
forMetROMSbutatruntimeforFESOM.Bothmodelsread
lower than 1500 m, as well as all ice-shelf cavities), as sig-
12-hourly fields for precipitation (split into rain and snow)
nificantfresheningofAntarcticBottomWateroccursother-
andevaporation,whicharenotinterpolatedintimebutrather
wise.Giventherelativelyscantobservationsonthecontinen-
applied at a constant rate with a step change every 12h, as
talshelfmakinguptheWorldOceanAtlasproducts,restor-
theyrepresenttotalfluxesoverthegiven12hperiod.
inginthisregionmaynotbeappropriate.
MetROMSdiagnosesincomingshortwaveradiationfrom
FESOM does not develop spurious deep convection
ERA-Interim’s 6-hourly total cloud cover, which is interpo-
in the Weddell Sea, even for long simulations without
latedtoeachtimestep.Incominglongwaveradiationiscal-
restoring. Possible reasons for this differing behaviour be-
culatedinternally.InFESOM,incomingshortwaveandlong-
tween MetROMS and FESOM are discussed in Sect. 4.2.1.
wave radiation are read directly from ERA-Interim, as 12-
Nonetheless,weapplythesamesurfacesalinityrestoringto
hourlyfieldswhichareappliedasstepchanges.
FESOMaswedotoMetROMS,sothattheexperimentsare
To account for the influence of iceberg calving on the
as similar as possible. Restoring in FESOM is scaled with
Southern Ocean freshwater budget, both models are forced
a constant depth of 10m, which is the depth of the surface
with an additional surface freshwater flux representing ice-
layer in z-coordinate regions, neglecting free surface varia-
berg melt. For this field, we use the output of Martin and
tions.Wedonotrestorenorthof30◦S,asthisregionisout-
Adcroft (2010), who modelled icebergs as interactive La-
sidetheMetROMSdomain.
grangian particles in the ocean component of a general cir-
culation model (GCM) simulation. The initial sizes of ice-
bergs at calving fronts were determined from a statistical
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1264 K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions
3.4 Northernboundaryconditions For our simulations, zonal transport through Drake Pas-
sage is calculated at 67◦W over the period 2002–2016.
MetROMS,withitsregionalcircumpolardomain,haslateral The first 10 years of the simulation (1992–2001) are ex-
boundaryconditionsat30◦S.TheECCO2reanalysis(Men- cluded as spinup. This time-averaged Drake Passage trans-
emenlis et al., 2008; Wunsch et al., 2009) provides temper- port, including the standard deviation in annual averages,
ature, salinity, and meridional velocity (v) as monthly av- is 126.8±3.4Sv in MetROMS, 158.6±2.8Sv in low-
erages over the transient period of 1992–2016. Sea surface resolution FESOM, and 152.6±3.1Sv in high-resolution
height is taken from the AVISO annual mean climatology FESOM. Compared to the observations of Donohue et al.
(AVISO,2011)whichisasingletimerecord.Notethattides (2016), the values from all three of our simulations
arenotconsidered,asdiscussedfurtherinSect.5. are too low, especially in MetROMS. This occurs de-
WefollowthemethoddescribedinNaughtenetal.(2017) spite MetROMS’ stronger surface stress than in FESOM
to ensure stability at the open boundary: zonal velocity u is (Sect. 2.7). Additionally, the MetROMS and low-resolution
clamped to zero, the bathymetry is modified to be constant FESOMsimulationsexhibitdownwardtrendsinDrakePas-
inlatitudeoverthenorthernmost15rowsofthedomain,and sage transport over 2002–2016, which are statistically sig-
a sponge layer is applied over these rows (in which the dif- nificant at the 95% level: −0.28Svyr−1 in MetROMS and
fusivity coefficient linearly increases to 10 times its back- −0.17Svyr−1inlow-resolutionFESOM.Thisweakeningof
ground value, and the viscosity coefficient to 100 times). the ACC may be driven by degradation of Southern Ocean
Northern boundary conditions are applied using the Chap- interior water masses due to spurious diapycnal mixing, as
man scheme for sea surface height (Chapman, 1985), the discussed in Sect. 4.1.4. Furthermore, drift in the density
Flather scheme for barotropic v (Flather, 1976), and the structuremayresultfromnon-closureofthesurfacefreshwa-
radiation-nudgingschemeforbaroclinicv,temperature,and terbudget,whichisgloballyunconstrainedbythebulk-flux
salinity(Marchesielloetal.,2001). approach of our simulations. Since interior Southern Ocean
Thepresenceoflateralboundaryconditionsderivedfrom processesoperateonmuchlongertimescalesthanourexper-
observationsmaygiveMetROMSanadvantagefortheaccu- iments,andwouldrequirelongspinupstoequilibrate,simu-
rate simulation of Southern Ocean water masses, compared lated ACC transport should be interpreted with caution and
toFESOMwhichhasaglobaldomain.However,thisinter- isnotthefocusofthispaper.
comparison focuses on the continental shelf and ice-shelf
cavities. These regions are relatively far field from 30◦S, 4.1.2 Mixedlayerdepth
comparedtotheACCandtheinteriorSouthernOceanwhich
aremoretightlycoupledtotheboundaryconditions.Forthe The surface mixed layer representsthe portion of the ocean
relatively short (25-year) simulations shown here, it is un- whichisdirectlyinfluencedbytheatmosphere.Thedepthof
likelythatcontinentalshelfwatermasseswillbesignificantly themixedlayerisakeyindicatorofthestrengthofconvec-
influenced by nudging at 30◦S. Longer simulations would tion,andheatlosstotheatmosphereresultingfromconvec-
likelyshowalargerresponse. tionwillinfluencewatermassproperties.Regionsofstrong
sea-iceformation,suchascoastalpolynyas,arecharacterised
bydeepwintertimemixedlayers.
4 Results
Wecalculatemixedlayerdepthusingthedensitycriterion
ofSalléeetal.(2013):theshallowestdepthatwhichthepo-
4.1 Ocean
tentialdensityisatleast0.03kgm−3 greaterthanatthesur-
4.1.1 DrakePassagetransport face(orattheice-shelfinterface,inthecaseofice-shelfcav-
ities). Summer (DJF) and winter (JJA) mixed layer depths
The ACC has the strongest transport of any ocean current ineachsimulation,averagedovertheperiod2002–2016,are
in the world and is key to the thermal isolation of Antarc- shown in Fig. 2 for the entire Southern Ocean, and Fig. 3
tica. Transport of the ACC is influenced by the Southern zoomedintotheAntarcticcontinentalshelf.Figure2alsoin-
Hemisphere westerly winds as well as the density structure cludesclimatologicalobservationsbyPellicheroetal.(2017)
of the Southern Ocean. By convention, zonal transport of recalculatedtousethesamedefinitionofmixedlayerdepth
the ACC is evaluated through Drake Passage and is time- as the models. We have not included these observations in
averaged to remove the seasonal cycle. With respect to ob- Fig.3,astheyarelessreliableonthecontinentalshelfdueto
servations, Drake Passage transport was previously thought insufficientmeasurements.
to lie around 134Sv (Cunningham et al., 2003). However, In the ACC in summer (top row of Fig. 2), MetROMS
recent improvements in measuring systems have suggested showsaringofdeepermixedlayersaround100msurround-
a higher value. As part of the cDrake project (Dynamics ingtheregionstratifiedbysea-icemeltwater.Thisspatialpat-
andTransportoftheAntarcticCircumpolarCurrentinDrake ternagreeswellwithobservations,butthemagnitudesome-
Passage), Donohue et al. (2016) estimated a Drake Passage whatdisagrees,asMetROMS’mixedlayersaretoodeepin
transportof173.3±10.7Sv. the ACC and too shallow elsewhere. FESOM has a much
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K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions 1265
Figure2.Mixedlayerdepth(m),calculatedastheshallowestdepthwherepotentialdensityisatleast0.03kgm−3greaterthanatthesurface
orice-shelfbase.ResultsareshownforMetROMS,low-resolutionFESOM,andhigh-resolutionFESOMaveragedovertheyears2002–2016
forsummer(DJF)andwinter(JJA),aswellasclimatologicalobservationsbyPellicheroetal.(2017)recalculatedwiththesamedefinition
ofmixedlayerdepth.Notethedifferentcolourscaleforsummerandwinter.
more uniform summer mixed layer depth which is 45m
(corresponding to the fourth layer in z-coordinate regions)
throughoutmostoftheACC,andgenerallyshallowerinthe
σ-coordinate region of the continental shelf. Both models
havesignificantlydeepermixedlayersinwinter(bottomrow
of Fig. 2; note the different colour scale) with the largest
values in the northern branch of the ACC where mode and
intermediatewaters subduct.Observationsindicate thisfea-
ture should be strongest in the Pacific and Australian sec-
tors. MetROMS shows local maxima in both regions, but
the magnitude in the Pacific sector (approximately 250m)
is still quite low. FESOM only captures this feature in the
Pacific sector, but here it attains mixed layer depths in ex-
cessof500mwhichexceedsobservations.Thisoverestima-
tion is less pronounced at high resolution. Elsewhere in the
Figure 3. As Fig. 2 for each model simulation, zoomed into the
ACC, FESOM’s mixed layer depths (approximately 100m)
Antarcticcontinentalshelf.
more or less agree with observations, while in MetROMS
they are too deep (approximately 200m). The tendency of
MetROMS to have deeper mixed layers than FESOM may
front,nearMcMurdoSound,inagreementwithobservations
beinfluencedbythedifferingsurfacestressbetweenthetwo
(Jacobs et al., 1979). A small region of dense water forma-
models(Sect.2.7).
tioninwesternPrydzBay,adjacenttotheAmeryIceShelf,
Zooming into the continental shelf, the water column is
is also present in both models. These regions are in agree-
largelystratifiedinsummer(toprowofFig.3)butshowsac-
ment with observed bottom water formation sites (Foldvik
tiveregionsofdensewaterformationinwinter(bottomrow
et al., 2004; Gordon et al., 2015; Herraiz-Borreguero et al.,
of Fig. 3; note the different colour scale). Both MetROMS
2016). FESOM also exhibits deep mixed layers (>500m)
andFESOMformdensewaterintheinnerRossandWeddell
in the Amundsen Sea, which were observed in 2012 but
seas, with regions of mixed layer depth exceeding 500m.
are not a consistent feature of this region (Dutrieux et al.,
Convection appears to be stronger in FESOM where these
2014). The presence of CDW on the Amundsen Sea conti-
regionsaredeeperandmorewidespreadduetostrongersea-
nentalshelfissensitivetomixedlayerdepth,asacompletely
ice production (Sect. 4.2.3). In the Weddell Sea, dense wa-
destratified water column filled with cold shelf water will
ter formation is split into two regions on either side of the
prevent the development of a warmer bottom layer (Petty
Filchner–RonneIceShelffront,withshallowermixedlayers
et al., 2013, 2014). This mechanism has been proposed as
inthemiddle.IntheRossSea,bothmodelsshowsomewhat
a cause of cold biases in Amundsen Sea ice-shelf cavities,
strongerconvectiononthewesternsideoftheRossIceShelf
and subsequent underestimation of ice-shelf melt rates, in
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1266 K.A.Naughtenetal.:IntercomparisonofAntarcticice-shelf,ocean,andsea-iceinteractions
◦
Figure4.Temperature–salinitydistributionsouthof65 SforMetROMS,low-resolutionFESOM,andhigh-resolutionFESOM,averaged
overtheyears2002–2016,andcolouredbasedondepth(notethenon-linearcolourscale).Eachgridbox(inMetROMS)ortriangularprism
(in FESOM) is sorted into 1000×1000 temperature and salinity bins. The depth shown for each bin is the volume-weighted average of
the depths of the grid boxes or triangular prisms within that bin. The dashed black line in each plot is the surface freezing point, which
has a slightly different formulation between MetROMS and FESOM due to the different sea-ice thermodynamics schemes. The dotted
greylinesarepotentialdensitycontoursinkgm−3−1000.Labelsshowdifferentwatermasses:AABWindicatesAntarcticBottomWater,
WSBWindicatesWeddellSeabottomwater,RSBWindicatesRossSeabottomwater,CDWindicatesCircumpolarDeepWater,MCDW
indicatesModifiedCircumpolarDeepWater,LSSWindicateslow-salinityshelfwater,HSSWindicateshigh-salinityshelfwater,AASW
indicatesAntarcticSurfaceWater,andISWindicatesIce-ShelfWater.Slantedlabelsbelowthefreezingpointlineshowspecificiceshelves’
contributionstoISW.
FESOM (Nakayama et al., 2014). In our simulations, these edge of the Amery Ice Shelf, which is a region of refreez-
deep mixed layers have some dependence on resolution, as inginMetROMSbutnotinFESOM(Sect.4.3.3).Duetothe
theycovernearlytheentireAmundsenSeaatlowresolution lackofobservationsinice-shelfcavities,thetruemixedlayer
butaremorerestrictedtotheice-shelffrontsathighresolu- depthsintheseregionsareunknown.
tion.Similarly,low-resolutionFESOMexhibitslocallydeep-
ened mixed layers (approximately 250m) near the southern 4.1.3 Watermassproperties
entrance of George VI Ice Shelf in the Bellingshausen Sea,
whilethisfeatureisabsentathighresolution. Ice-shelf melt rates and sea-ice formation both influence,
Mixed layer depths in ice-shelf cavities show no signifi- and are influenced by, water mass properties on the conti-
cantseasonality(notethedifferentcolourscalesforsummer nental shelf. Figure 4 plots the temperature–salinity (T/S)
andwinterinFig.3)andaregenerallyshallow(<50m)ex- distributionsouthof65◦Sineachsimulation,averagedover
cept near regions of persistent refreezing, which forms ma- 2002–2016,andcolour-codedbasedondepth.Inthissection,
rineice.Thisprocessincreasessalinityattheice-shelfbase weidentifythedifferentwatermassesrepresentedinFig.4,
asfreshwaterisremovedintheformoffrazilice,providing and compare their properties between the two models. Due
abuoyancyforcing.Regionsofmarineiceformationarede- toascarcityofyear-roundmeasurementsonthecontinental
tailed in Sect. 4.3, but theirsignature can be seen here. The shelf, it is not feasible to create a comparable figure using
most affected region is the central Ronne Ice Shelf, which observations. However, limited observations of some water
hasmixedlayerdepthsof300–400minMetROMS,50–80m masses exist and are compared to the simulated water mass
inlow-resolutionFESOM,and70–120minhigh-resolution propertiesinthetextbelow.
FESOM. Refreezing in this region is indeed stronger and Justabovethesurfacefreezingtemperature(dashedblack
morewidespreadinMetROMSthaninFESOM(Sect.4.3.1). lines in Fig. 4, approximately −2◦C) are two subsurface
All three simulations exhibit mixed layer depths exceeding water masses (100–500m depth). Low-salinity shelf water
50m in much of the Ross Ice Shelf, which has large areas (LSSW, <34.5psu) and high-salinity shelf water (HSSW,
ofrefreezing(Sect.4.3.5).OnlyMetROMSshowsincreased >34.5psu) are both the result of sea-ice formation, but
mixed layer depths (approximately 70m) along the western HSSW is more affected by strong brine rejection. LSSW
shows similar properties in all three simulations, with min-
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