strange disturbance in SSH in the first few seconds

dynamics-numerics-params
,
1

Hi all,
I am facing a strange problem with my CROCO setup and was wondering if anyone can help. Details below:

Horizontal resolution = 200 meters with varying bathymetry ranging from 20 m shallow to 2500 m deep.
theta_s = 5.0, theta_b = 0.1, Tcline = 50.0
Haidvogel and Haney numbers are rx0 = 0.0123958344, rx1 = 0.2893939053. I think these are well within the stable range.

Here is the bathymetry contour plot with masked islands.
image description

I am running this setup with a time step of 20 sec and barotropic time step of 30 sec. The preprocessor options are the following:

      OBC_EAST
      OBC_WEST
      OBC_NORTH
      OBC_SOUTH
      CURVGRID
      SPHERICAL
      WET_DRY
      NEW_S_COORD
      SOLVE3D
      UV_COR
      UV_ADV
      SALINITY
      NONLIN_EOS
      UV_HADV_UP3
      UV_VIS2
      UV_VIS_SMAGO
      TS_HADV_RSUP3
      TS_HADV_C4
      TS_DIF4
      TS_MIX_S
      SPONGE
      VADV_ADAPT_IMP
      LMD_MIXING
      LMD_SKPP
      LMD_BKPP
      LMD_RIMIX
      LMD_CONVEC
      LMD_NONLOCAL
      QCORRECTION
      SFLX_CORR
      CLIMATOLOGY
      ZCLIMATOLOGY
      M2CLIMATOLOGY
      M3CLIMATOLOGY
      TCLIMATOLOGY
      ANA_BSFLUX
      ANA_BTFLUX
      OBC_M2CHARACT
      OBC_M3ORLANSKI
      OBC_TORLANSKI
      DIAGNOSTICS_UV
      UV_MIX_S
      SPLIT_EOS
      M2FILTER_POWER
      VAR_RHO_2D
      HZR
      VIS_COEF_3D
      UV_VADV_SPLINES
      TS_MIX_ISO
      TS_MIX_IMP
      TS_MIX_ISO_FILT
      DIF_COEF_3D
      NTRA_T3DMIX
      TS_VADV_SPLINES
      SPONGE_GRID
      SPONGE_DIF2
      SPONGE_VIS2
      LMD_SKPP2005
      LMD_BKPP2005
      LIMIT_BSTRESS
      NF_CLOBBER

The initial condition is set with no temperature/salinity gradient, and 0 velocities everywhere in the domain. However, within the first few seconds the outputs show a strange buildup in the zeta (sea surface), and the velocities. This buildup accumulates over the first few seconds, and blows up at the 6th second due to unrealistically high velocity. Here is a vertical transect plot of the velocity at the 6th second when it blows.
image description

Right at that point I looked into the depths of the sigma layers. Here is the sigma layer plot at the 6th second. At the point of instability, the surface height shows a strange disturbance, and the sigma layers are also getting distorted because of it. I couldn’t figure out what’s causing this problem.
image description

Then, I then looked into an earlier time step. Here is the velocity at the same transect at 5 seconds.
image description

I think this disturbance is mostly because of the batrotropic component which comes from the sea surface height. Here is the barotropic velocity contour plot at 5 sec:
image description

Does anybody know whats causing this? Any suggestions?

I have tried running a larger grid setup with 3 km resolution and that works perfectly with no problems with SSH. However, with finer resolution I am facing these issues. These strange disturbances in SSH are affecting the velocity, blowing up the simulation. Is there any specific modification that I should do for getting the SSH correct in a fine resolution run?

If you want to look at more details, the preprocessor cppdefs.h and input .in files are attached.

cppdefs.h
cppdefs_dev.h
croco.in

Thank you.