[MITgcm-support] Ocean currents veering due to bathymetry

[Martin Losch]

Hi Kunal,

I have no idea of the loca hydrography and circulation and don’t know what to expect, but there are few things that may be problematic in your setup:

- check the forcing: the surface stresses are aligned with the grid direction, i.e. ustress is not directed towards the east, but towards increasing model index “i” (which is sometimes called “east” in the code). So your wind stress needs to be rotated according to your grid orientation before it is read by the model. Probably you have already thought about this
- with your obcs settings there is no net flow through the open boundaries. Is that what you want? If the current system is only local, that this is OK but a quick google search tells me that the southward WICC should actually leave your domain. You’d need some sort of boundary conditions for that. I would suggest that you look into the Stevens boundary conditions (described in the documenation) where you can specify a barotropic flow field normal to the boundaries.
- a 10-day simulation period may be very short for any system to develop
- there’s no buoyancy forcing. Is that important or do you only want wind driven circulation?

small things that you may want to adjust:
- your viscosities are very high
- your horizontal grid aspect ratio is very high (dx/dy = 2891.9791/939.661 = 3), that could cause numerical problems (e.g. CFL problems or stability problems with viscosity). If you can still change that ratio to 1, I would do that
- for delZ/delR the rule of thumb is that delZ(k+1)/delZ(k) < 1.4, i.e. there shouldn’t be too large jumps in the grid spacing so that the numerical approximations are not compromised
- hFacMin = 0.9 is close to turning off the partial cells. I’d use something like 0.3 or even 0.1 if possible. with your 5m grid spacing this may be problematic but you can set minimum thickness with hFacMinDz/hFacMinDr (=5 or 10)

- OBCSfixTopo=.FALSE., is not a good choice, use the default (.True.)

Hope that helps,
Martin




> On 24. Jul 2020, at 06:22, kunal madkaiker <kunal.madkaiker02 at gmail.com> wrote:
> 
> Dear MITgcm users,
> 
> I am trying to simulate ocean currents along the West coast of India in a high resolution grid (360x780) for the month of September. For now, I am providing monthly climatology of August Temperature and Salinity as initial forcing along with September zonal and meridional wind stress. Currently, using the Orlanski scheme for obcs and no boundary forcings. Model is simulated for 10 days.
> 
> But the currents are not coming correct. The West Indian Coastal Current (WICC) which flows equatorward (during Sept) isn't being captured at all. After plotting the bathymetry, I observed that the currents are being veered northward by the Arabian sea continental shelf (Please refer to Picture1.png attached (Day 04-10)). Is that possible? My bathymetry bin file is just a 2D mesh with depth values. Also, my coastal currents are very weak.
> 
> Attaching my data,data.obcs and data.exf file. Please let me know if any additional information is required from my end.
> Kindly advise. All suggestions are welcomed.
> 
> Regards
> Kunal
> <data.obcs><bathymetry.png><data><data.exf><Picture1.png>_______________________________________________
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