[MITgcm-support] Question About Sea Surface Height (Eta) Behavior in MITgcm

Thu Jul 10 07:21:03 EDT 2025

Hello,

Thanks everyone for the advice. I’ve repeated the simulation, this time 
with useOBCSbalance = .FALSE.
In the attached figure:
The top panel shows the comparison between the MEDSEA SSH (black), the 
MITgcm output with useOBCSbalance = .TRUE. (green), and with 
useOBCSbalance = .FALSE. (red).
The bottom panel shows the atmospheric pressure used to force the model 
over the domain, as it also influences the SSH.
Additionally, I’ve included another image with a screenshot from a 
mareograph near Tarragona lon=1.21° E, Lat=41.08° N (unfortunately, I 
don’t have access to the raw data, only the screenshot). But, it may 
help to get an idea of the observed nearshore SSH.

The model behaviour has improved, disabling the balance gives the model 
more freedom to follow the low frequency variation, which are 
qualitatively similar to MEDSEA. However, there are still differences in 
the mean SSH values.

Do you have any further suggestions I could try to improve the result?

Best regards,

Jordi

On 9/7/25 17:11, Dimitris Menemenlis wrote:
> So Martin’s guess is correct.  Because of:
>    useOBCSbalance=.TRUE.,
> you are suppressing all volume transport at the boundaries except for 
> tidal transports, which you prescribe on top of that.
>
> D.
>
>> On Jul 9, 2025, at 8:03 AM, Jordi Iglesias <jiglesias at icm.csic.es> wrote:
>>
>> Hi everyone,
>>
>> Thanks a lot for your comments and help. Following your 
>> recommendations, I’m attaching two figures and the namelists.
>>
>> Fig1.jpg shows the MEDSEA parent model, with my domain marked by a 
>> red rectangle, as well as the three points where I plotted the SSH 
>> from the parent model.
>>
>> Fig2.jpg contains three subplots. The top panel corresponds to the 
>> SouthWest of Sicily, where I compare the SSH at that point with and 
>> without tides from the MEDSEA model. I also include the MEDSEA SSH 
>> averaged over my domain (black line) shared in all plots and the 
>> MITgcm output (my result) shown as a black dashed line. I repeat this 
>> comparison in the south of my domain (cyan) and in Gibraltar (red).
>>
>> Note: The Y-axis scales are not shared between subplots, but in all 
>> cases, it's clear that the parent model shows a low frequency SSH 
>> signal that I’m currently unable to reproduce in MITgcm.
>> As you mentioned, probably I should disable the balanced flow option. 
>> Additionally, I’m sharing the namelists data and data.obcs, if you 
>> have any advice or suggestions, I’d be happy to try them.
>>
>> Thanks again for your help!
>> Best regards,
>>
>> Jordi
>>
>>
>> On 8/7/25 17:24, Martin Losch wrote:
>>> Hi Jordi,
>>>
>>> it would help if you could provide your namelist files. And maybe 
>>> the customised *_OPTIONS.h files, if they are very different from 
>>> the standard ones.
>>> Do you use open boundary conditions? Balanced flow is maybe an issue?
>>>
>>> Martin
>>>
>>>> On 8. Jul 2025, at 16:57, Dimitris Menemenlis 
>>>> <dmenemenlis at gmail.com> wrote:
>>>>
>>>> Hi Jodi, perhaps a good place to start would be to understand cause 
>>>> of the larger oscillations in MEDSEA and ROMS, which might then 
>>>> help you diagnose what is missing in your MITgcm configuration. 
>>>>  For example, if the ~10-day oscillation in MEDSEA and ROMS is 
>>>> primarily caused by transport through Gibraltar Strait, then you 
>>>> could look at MITgcm’s transport through Gibraltar Strait to see 
>>>> whether it has been correctly prescribed.
>>>>
>>>> D.
>>>>
>>>>> On Jul 8, 2025, at 6:13 AM, Jordi Iglesias <jiglesias at icm.csic.es> 
>>>>> wrote:
>>>>>
>>>>> Dear MITgcm community,
>>>>>
>>>>> Over the past few months, I’ve been struggling to achieve 
>>>>> realistic sea surface height ("Eta") results in my MITgcm 
>>>>> simulations. I’ve tried several configurations,including 
>>>>> non-hydrostatic, quasi-non-hydrostatic, and nonlinear free surface 
>>>>> options, but the SSH output often differs significantly from the 
>>>>> parent model (the Copernicus Mediterranean analysis, MEDSEA). I’ve 
>>>>> attached an example and also shared a link to illustrate the 
>>>>> issue. I have tides configured, and the ssh is initialized from 
>>>>> MEDSEA data. However, when I compare the SSH output with the 
>>>>> parent model (or with a ROMS simulation, for example), it appears 
>>>>> to oscillate around a constant value that doesn’t evolve much over 
>>>>> time, or changes are very small.
>>>>>
>>>>> A summary of my configuration: Horizontal resolution: 1.6 km, NW 
>>>>> Mediterranean Sea, Atmospheric forcing: ECMWF, initial ssh set 
>>>>> with pSurfInitFile, SphericalPolarGrid, quasiHydrostatic = .TRUE., 
>>>>> nonlinFreeSurf = 3, rigidLid = .FALSE., implicitFreeSurface = .TRUE.,
>>>>>
>>>>> If any additional parameters would help diagnose the issue, I’d be 
>>>>> happy to provide them.
>>>>>
>>>>> The temperature, salinity, and velocity fields are generally in 
>>>>> good agreement with ROMS and MEDSEA, but I’d really like to 
>>>>> improve the SSH performance. Has anyone encountered a similar 
>>>>> issue or found strategies to improve SSH in MITgcm?
>>>>>
>>>>> Thanks in advance for your help!
>>>>>
>>>>> Best regards,
>>>>> Jordi
>>>>>
>>>>> https://saco.csic.es/s/c2fSgN5RWPRcDSQ
>>>>>
>>>>>
>>>>> <Screenshot from 2025-07-08 
>>>>> 14-47-52.png>_______________________________________________
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>>>>> MITgcm-support at mitgcm.org
>>>>> http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support
>>>>
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