ETA and SSH Interpretation in LLC4320

The “Pre-SWOT” LLC4320 data available from PODAAC contains a variable Eta which I interpret to be sea-surface height with a nod to the shallow-water origin of the calculation.

Does anyone know of any documentation indicating exactly how this quantity is measured, i.e., with respect to a given datum or other reference?

This is important for me as I compare this quantity with other sources of SSH information. Any documentation links would be most appreciated.

Maybe this part of the MITgcm documentation helps: 4.5. Global Ocean Simulation — MITgcm checkpoint68v-2-gd8c5b89 documentation

Note that this quantity is coming from a model and is not “measured”, but rather a variable that evolves in the model.
It will be best to compare it to the absolute dynamics topography that you can get from the AVISO dataset (or any other SSH product).

Thanks for your response. My use of the word “measured” was confusing and I definitely missed the word “anomaly” in the metadata.

{'long_name': 'Model sea level anomaly',
 'units': 'm',
 'comments_1': 'Model sea level anomaly WITHOUT corrections for global mean density (steric) changes, inverted barometer effect, or volume displacement due to submerged sea-ice and snow  ',
 'comments_2': 'Note: ETAN should NOT be used for comparisons with altimetry data products because ETAN is NOT corrected for (a) global mean steric sea level changes related to density changes in the Boussinesq volume-conserving model (Greatbatch correction, see sterGloH) nor (b) sea level displacement due to submerged sea-ice and snow (see sIceLoad). These corrections ARE made for the variables SSH and SSHNOIBC.',
 'grid_location': 'c',
 'coverage_content_type': 'modelResult',
 'valid_min': -5.0,
 'valid_max': 5.0}

Given that I am now interpreting this as an anomaly, do we know if this an anomaly from a global average at that time step, that year, a zonal anomaly, etc.