[MITgcm-support] Stratification creating current without forcing

[Martin Losch]

Hi Nadine,

when you add stratification and still want to have an ocean at rest without any forcing (this includes no meltwater forcing, i.e. SHELFICEheatTransCoeff=0., SHELFICEsaltTransCoeff = 0. or similarly shiCdrag=0), you need to make sure that your iceshelf is in exact hydrostatic balance with the ocean (by specifying an appropriate SHELFICEloadAnomalyFile). This is a little tricky with a non-linear equation of state, but you can find an example in verification/isomip/input/gendata.m how to create data for the  SHELFICEloadAnomalyFile = 'phi0surf.exp1.jmd95z’, in data.shelfice. This file needs to be regenerated each time you change your stratification or your equation of state.

Martin

> On 27. Nov 2019, at 09:45, Nadine Steiger <Nadine.Steiger at uib.no> wrote:
> 
> Dear all,
> 
> I am using an idealized setup of a v-shaped channel in MITgcm with dimensions of 250 km x 140 km x 0.8 km.
> So far, I had no stratification in the water and added now a linear stratification in salinity (still constant temperature).
> 
> To test the setup, I started without any forcing and heat/momentum source, to test if the water stays in rest. Surprisingly, it does not and I get a circulation that increases in strength with time. In the model runs without stratification everything worked fine and the ocean stayed in rest.
> 
> I checked the first time steps and it looks like the salinity reduces at the bottom along the topography, where I also get non-zero velocities. It seems like the reduction in salinity happens exactly in the cells where HFacC is between 0 and 1.
> 
> Does anyone know what is happening there?
> 
> Thanks a lot,
> Nadine
> 
> 
> This is my data-file:
> 
> # Continuous equation parameters
> &PARM01
> Tref = 160*-1.,
> Sref = 160*34.5,
> viscAr=1.E-4,
> viscAh=25.,
> viscA4=0.E-10,
> no_slip_sides=.FALSE.,
> no_slip_bottom=.FALSE.,
> diffKrT=1.E-5,
> diffKhT=5.,
> diffKrS=1.E-5,
> diffKhS=5.,
> bottomDragQuadratic=0.,
> eosType='JMD95Z',
> saltAdvScheme=33,
> tempAdvScheme=33,
> HeatCapacity_cp = 3974.0,
> rhoConst=1027.6,
> rhoNil=1027.6,
> gravity=9.81,
> f0=-1.398E-04,
> beta=0.E-11,
> tAlpha=3.67E-5,
> sBeta=7.89E-4,
> # tAlpha=2.E-4,
> # sBeta =0.E-4,
> convertFW2Salt = 33.4,
> rigidLid=.FALSE.,
> implicitFreeSurface=.TRUE.,
> nonlinFreeSurf = 0.,
> exactConserv=.TRUE.,
> staggerTimeStep=.TRUE.,
> useRealFreshWaterFlux = .TRUE.,
> select_rStar = 0.,
> useSingleCpuIO=.TRUE.,
> hFacMin = 0.5,
> hFacMindr = 0.0,
> hFacInf = 0.2,
> hFacSup = 2.0,
> nonHydrostatic=.FALSE.,
> readBinaryPrec=64,
> useCDScheme = .FALSE.,
> &
> 
> # Elliptic solver parameters
> &PARM02
> cg2dMaxIters=1000,
> cg2dTargetResidual=1.E-13,
> &
> 
> # Time stepping parameters
> &PARM03
> nIter0=0,
> nTimeSteps=60000.,
> deltaT=100.,
> abEps=0.1,
> cAdjFreq = -1.,
> #tauCD = 400000.,
> pChkptFreq=10000.0,
> chkptFreq=1000.0,
> # dumpFreq=50.,
> taveFreq=0.0,
> monitorFreq=1.,
> monitorSelect=2,
> &
> 
> # Gridding parameters
> &PARM04
> usingCartesianGrid=.TRUE.,
> delX=500*500.,
> delY=240*500.,
> delR=160*5.,
> &
> 
> # Input datasets
> &PARM05
> bathyFile='topog.channel',
> hydrogThetaFile='T0.bin',
> hydrogSaltFile='S0.bin',
> &
> 
> -- 
> Nadine Steiger
> PhD Candidate
> University of Bergen | Geophysical Institute
> Bjerknes Centre for Climate Research
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