[MITgcm-support] Open boundaries and sponge layers

[Stanislav Martyanov]

Hello Dimitris!

>> If they are daily velocities, the largest tidal components would either
be aliased (if snapshots) or heavily damped (if daily mean). I have not
been following this thread, but can’t you just use:
https://github.com/MITgcm/MITgcm/blob/master/pkg/obcs/obcs_add_tides.F ?

Yes, I do use this subroutine to set tidal forcing (through phases and
amplitudes at the boundaries).

Having looked at this subroutine once more, I've just realised that I might
have been wrong about the nudging mechanism in MITgcm. Tidal oscillations
ARE added to the boundary non-tidal currents in this subroutine, and AFTER
that, if the sponge is ON, the internal model's solution is pulled towards
these new boundary values (inside the sponge layers) that already contain
both daily-mean currents and tidal oscillations. So tides do not get
smoothed out inside the sponge layers due to the use of GLORYS's daily-mean
non-tidal currents, because the tidal component has already been added to
boundary values.

Thank you for the help!

Regards,
Stanislav

сб, 19 сент. 2026 г. в 12:18, Stanislav Martyanov <martyanov.sd at gmail.com>:

> Hello Jody,
>
> Thank you for your reply!
>
> Unfortunately, I do not have hourly nudging fields to use. In fact, all I
> have is daily-mean fields from the GLORYS reanalysis (which includes
> velocities). And I set them as boundary values in a Dirichlet fashion
> (MITgcm's 'prescribe' option). And these values are simultaneously the
> nudging fields itself when the sponge option is active.
>
> If I had hourly velocity fields, there wouldn't be a problem then :-)
>
> Meanwhile, MITgcm's sponge layers use these boundary values and nudge the
> inner solution towards them inside some bands along the open boundaries.
> Thus, I can not figure out how to keep tidal forcing while using daily-mean
> velocities (which do not contain tidal oscillations) and still using
> MITgcm's sponge layers..
>
> >> If you don’t like the forcing fields being so large,
> >> you could modify the code to add the tidal velocities easily enough.
> The size wouldn't be a problem, but I simply do not have hourly data.
>
> And the code already deals quite well with tidal velocities in
> corresponding subroutines; it efficiently takes them out from the volume
> balance procedure, for example. If I'm not mistaken, you suggest to tune
> the sponge procedure itself to somehow make it keep tides? But at this
> moment the total current velocities already include the tidal component, I
> believe.. Anyway, it is something for me to think about..
>
> Or just switch the sponge layers off entirely.. Speaking of which, what is
> an accumulated community's experience - do people always use
> MITgcm's sponge in their regional applications?
>
> Regards,
> Stanislav
>
>
> сб, 19 сент. 2026 г. в 01:58, Jody Klymak <jklymak at uvic.ca>:
>
>> Hi Stanislav
>>
>> If I had nudging fields for the boundary conditions, I’d simply prescribe
>> them every hour instead of every day, and add the tidal and daily ocean
>> velocities together to get the forcing.  Yes your nudging fields are 24
>> times larger, but…
>>
>> If you don’t like the forcing fields being so large, you could modify the
>> code to add the tidal velocities easily enough.
>>
>> Cheers,   Jody
>>
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>>
>
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