[MITgcm-support] Temperature undershoots using scheme 77 and GM

[Christopher L. Wolfe]

Hi all,

I'm running a low resolution (dx = 80 km) basin scale model and I'm  
finding that my abyss is getting filled with water that's colder than  
any surface water by .001 to .01 degrees. I'm using a linear equation  
of state, no salt, no sea-ice, nothing fancy. I'm using advection  
scheme 77 and GM in skew-flux form. Scheme 77 is not supposed to be  
able to generate local interior maxima and GM is not supposed to be  
able to create new water masses, so I'm at a loss as to what's causing  
the undershoots. The problem seems to get worse at low diffusivities.  
I use small (1e-5) isotropic Laplacian diffusivity, which would be  
unstable if I wasn't using a flux limiter ... but I am. I run a high- 
resolution version of the model with everything the same but no GM and  
don't get any undershoots, so I suspect GM.

Any thoughts as to what could be causing these undershoots?

I'm using the latest version of the OS7MP scheme. My data and  
data.gmredi files are included below.

Thanks,
Christopher

data:
# ====================
# | Model parameters |
# ====================
#
# Continuous equation parameters
  &PARM01
  tRef=-9.9, -9.9, -9.9, -9.9, -9.9, -9.9, -9.9
       -9.9, -9.9, -9.9, -9.9, -9.9, -9.9, -9.9
       -9.9, -9.9, -9.9, -9.9, -9.9, -9.9,
  viscAh=10E3,
  viscAz=0.25E-3,
  no_slip_sides=.TRUE.,
  no_slip_bottom=.FALSE.,
  diffK4T=0.E9,
  diffKhT=0.8E-4,
  diffKzT=0.8E-4,
  f0=-1.3683e-04,
  beta=3.4208e-11,
  tAlpha=2.E-4,
  sBeta =0.,
  gravity=10.,
  rhoConst=1000.,
  rhoNil=1000.,
  heatCapacity_Cp=3900.,
  rigidLid=.FALSE.,
  implicitFreeSurface=.TRUE.,
  saltAdvection=.FALSE.,
  saltForcing=.FALSE.,
  saltStepping=.FALSE.,
  eosType='LINEAR',
  nonHydrostatic=.FALSE.,
  momAdvection=.TRUE.,
  implicitViscosity=.TRUE.,
  implicitDiffusion=.TRUE.,
  ivdc_kappa=10.,
  readBinaryPrec=64,
  tempAdvScheme=7,
  staggerTimeStep=.TRUE.,
  bottomDragLinear=1.1135E-3,
  debugLevel=-1,
  useJamartWetPoints=.TRUE.,
  useSingleCpuIo=.TRUE.,
  &

# Elliptic solver parameters
  &PARM02
  cg2dMaxIters=400,
  cg2dTargetResidual=4.E-7,
  cg3dMaxIters=400,
  cg3dTargetResidual=4.E-7,
  &

# Time stepping parameters
  &PARM03
  nIter0=0,
  nTimeSteps=3155760,
  deltaT=4000,
  abEps=0.1,
# write out every 50 years
  pChkptFreq=1.57788e9,
  chkptFreq=1.57788e8,
  dumpFreq=1.57788e9,
  monitorFreq=1.57788e7,
  tauThetaClimRelax=764400.,
  cAdjFreq=0,
  pickupStrictlyMatch=.FALSE.,
  &

# Gridding parameters
  &PARM04
  usingCartesianGrid=.TRUE.,
  usingSphericalPolarGrid=.FALSE.,
  dXspacing=80.e3,
  dYspacing=80.e3
  delZ=  12.0505,  14.9159,  18.4328,  22.7337,  27.9692,
         34.3067,  41.9245,  51.0025,  61.7057,  74.1598,
         88.4181, 104.4189, 121.9376, 140.5412, 159.5568,
        178.0710, 194.9769, 209.0770, 219.2372, 224.5642
  &

# Input datasets
  &PARM05
  thetaClimFile='N85_SST.bin',
  bathyFile='topo_two_noeddy.bin',
  zonalWindFile='windx_two.weakeq',
  meridWindFile=,
  &


data.gmredi:
  &GM_PARM01
   GM_background_K    = 0.9e+3,
   GM_taper_scheme    = 'gkw91',
   GM_maxSlope        = 1.e-2,
   GM_Kmin_horiz      = 50.,
   GM_Scrit           = 4.e-3,
   GM_Sd              = 1.e-3,
  &