Standard names: *Ocean surface elevation without the contributions of ocean tides nor atmospheric pressure forcing (invert barometer)*

[abiardeau]

Hello,

I am Aurore BIARDEAU from Mercator Ocean international, working for Copernicus Marine service. Date : 07-03-2024

We are in the process of implementing a new variable in the Copernicus Marine service, and we would be happy to have your feedback on our proposal :

• standard_name : non_tidal_non_ib_elevation_of_sea_surface_height
• unit : m
• definition : "Sea surface height" is a time-varying quantity. The phrase "non_tidal_non_ib_elevation" describes the contribution to sea surface height variability made by processes other than astronomic forcing of the ocean and shallow water resonance of tidal components, or variations in air pressure.

The definition is inspired by standard_name non_tidal_elevation_of_sea_surface_height whose definition corresponds to sea_surface_height + invert_barometer.

I can provide more information from my expert colleagues if needed,

Kind regards, Aurore

[github-actions[bot]]

Thank you for your proposal. These terms will be added to the cfeditor (http://cfeditor.ceda.ac.uk/proposals/1) shortly. Your proposal will then be reviewed and commented on by the community and Standard Names moderator.

[efisher008]

Hi Aurore,

I have added the name to the editor, you can view it here: https://cfeditor.ceda.ac.uk/proposal/5276/edit

I will wait for others to comment on the suitability of the name, although I do think if the term invert_barometer is referenced in the name, it should really be described inside the definition, and possibly in full (i.e. non_invert_barometer instead of non_ib).

Best regards, Ellie

[efisher008]

Hi @abiardeau,

As there don't appear to be any new comments since last month, I'd like to suggest we make the change from non_tidal_non_ib_elevation_of_sea_surface_height to non_tidal_non_invert_barometer_elevation_of_sea_surface_height, and define invert_barometer as a phrase in the CF phrasebank. Perhaps the associated text could be: "Invert barometer refers to the isostatic response of the oceans to atmospheric pressure. This is described by the equation -(change in pressure) / [(seawater density)*(acceleration due to gravity)] (SUITABLE REFERENCE) and is an inverse relationship, as an increase in atmospheric pressure results in a decrease in sea level.". Reference: Wunsch & Stammer, 1997 https://doi.org/10.1029/96RG03037 Ponte, 2006 https://doi.org/10.1175/JTECH1864.1

Please let me know what you think!

Best, Ellie

[JonathanGregory]

Dear Aurore and Ellie

If we can, I think we should relate our sea-level standard names to the sea-level terminology adopted by the IPCC WG1 AR6, which is defined a paper I wrote with a large group of colleagues (Gregory et al., 2019, https://doi.org/10.1007/s10712-019-09525-z), with the aim of trying to clarify some difficult concepts. In the standard name you have proposed (non_tidal_non_ib_elevation_of_sea_surface_height), and the one you used as a model (non_tidal_elevation_of_sea_surface_height) it's not clear to me what the reference level for the "elevation" would be. If the value was zero for "elevation of sea surface height", what level would that indicate? Other standard name for sea surface height state the reference level e.g. sea_surface_height_above_geoid. I think we ought to the same for these quantities.

I agree with Ellie that we should spell out "IB". The term for IB in the 2019 paper is "inverse barometer". However, that's jargon, really, and it might be better for a standard name to say what it means i.e. the depression of SSH by atmospheric pressure. It's described in entry N7, on p11 of the PDF of our paper. (Unfortunately the hyperlinks in the online version don't all work.)

Best wishes

Jonathan

[efisher008]

Hi Jonathan and Aurore,

Thank you for linking the paper @JonathanGregory. This is indeed a useful reference in this case. The description of sea surface height given in this paper is as follows: "The sea-surface height is the geodetic height of the sea surface above the reference ellipsoid (a negative value if below)." The description of the inverse barometer effect in the paper is, if I interpreted correctly: "The time-dependent hydrostatic depression of the sea surface by atmospheric pressure variations, also called inverted barometer". Would you like to revise this definition for use in the phrasebank @JonathanGregory ?

I was able to find literature using the terms 'invert barometer', 'inverted barometer' and 'inverse barometer'. We should decide sensibly at this stage which is most suitable for use and be consistent in its application in future additions to the standard names table.

Best, Ellie

[efisher008]

I would also add that Jonathan's paper mentions a relevant term "IB-corrected sea-surface height", which seems to have a similar, though not identical, meaning to the proposed name non_tidal_non_ib_elevation_of_sea_surface_height (which is a difference quantity, i.e. above geoid or some other reference base?).

[abiardeau]

Dear Ellie and Jonathan,

Thanks a lot for your relevant feedback ! After having discussed with my colleague, I think we can propose this correction : non_tidal_non_invert_barometer_elevation_of_sea_surface_height_above_geoid

[JonathanGregory]

Dear Aurore @abiardeau

Thanks for replying, Aurore. I am still not quite clear what you mean. In your original description, you describe this as a contribution to variability, which I suppose would be a statistic of some sort, but the sea_surface_height_above_geoid is an instantaneous level. "Non-tidal sea surface height" could be mean sea level, so perhaps you mean IB-corrected mean sea level?

Although "inverse barometer" and "inverted barometer" (I haven't read "invert barometer" myself) are commonly used jargon, following the aims of CF standard names I feel we should say something self-explanatory. That was done with sea_surface_height_correction_due_to_air_pressure... names (although in those names I think we could be clearer about what "correction" means). I'm sure we can devise something if you can explain the intended meaning.

Best wishes

Jonathan

[abiardeau]

You are right I was not very clear. We are creating a dataset with the respective contributions to water level of several physical processes. This variable brings together the water heights that do not depend on the tide, the inverse barometer, the steric and the barystatic. Perhaps you have a suggestion ?

[JonathanGregory]

Please could you describe how you obtain this quantity, from measurements and subsequent calculations? At the moment I don't know what you mean, I'm sorry to say.

[slawchune]

Hi Jonathan, I must confess that how the different variables are referenced vertically is not so clear to me. I am going to look for informations.

in the mean time, here's some draft documentation to make things clearer:

Merged_ocean_levels_DESCRIPTION.docx

Thanks for your help, I am sure it will help us to come with a clearer product.

[slawchune]

Hi @JonathanGregory , did you ahave a look to the document ? Perhaps it makes things clearer.

Is that one "non_tidal_non_invert_barometer_elevation_of_sea_surface_height_above_geoid" ok ?

Cheers, Stéphane

[JonathanGregory]

Dear Stéphane

If I understand the document correctly, I think this quantity is the elevation the sea surface would have wrt geoid (not an anomaly in SSH, but an actual value) if the air pressure and tide were "average". Equivalently, it is the sum of ocean dynamic sea level (SSH above geoid for "average" air pressure i.e. IB-corrected, and not including tides) and global mean sea level rise (GMSLR, thermosteric + barystatic NB the document says "barysteric" for "barystatic"). In the document, the quantity is 3 + 4 + 5.

The geoid can be defined in a time-dependent way, with the choice of geopotential defining the geoid varying such that the volume enclosed between the geoid and the sea floor is always equal to the time-mean volume of sea water in the ocean. With that definition, the effect of GMSLR is included in the geoid, and your quantity is simply ocean dynamic sea level. But since 3 (global-mean [thermo]steric) and 4 (barystatic) are not included in 5, I suppose the ocean model ("GLO12 physical ocean system") has a geoid which is constant in the terrestrial reference frame.

If all the above is correct (which it might well not be!) I suggest calling this quantity sum_of_global_mean_sea_level_change_and_ocean_dynamic_sea_level, because I think it's preferable to say what it does include, rather than what it doesn't. Ocean dynamic sea level is defined wrt geoid and its global mean is zero. I think 5 in the document is ocean_dynamic_sea_level (which is not an existing standard name). [Correction: CMIP zos is not ocean dynamic sea level, because it does include the effect of air pressure anomalies if the ocean model includes them. Its global mean is still zero though.]

Expressing it as what it doesn't include I would call it sea_surface_height_above_geoid_assuming_zero_air_pressure_anomaly_and_mean_tide. This is more similar to your proposal, but I think it's less good because it's not obvious that it includes GMSLR.

What do you think?

Best wishes

Jonathan

[slawchune]

Hi Jonathan.

can we reprocess / resume the whole standard name variable set in this issue ? Because we need a certain consistency to ensure that users don't get lost.

5/ sea surface height from GLO12, mainly discussed in this issue.
"I suppose the ocean model ("GLO12 physical ocean system") has a geoid which is constant in the terrestrial reference" --> this is correct. proposed standard names: "_sum_of_global_mean_sea_level_change_and_ocean_dynamic_sealevel" or "_sea_surface_height_above_geoid_assuming_zero_air_pressure_anomaly_and_meantide".

What I am afraid with the first definition is as this quantity doesn't contain the steric and barystatic contribution, is this still correct to talk about global mean sea level change though ?

1/ tides: are these ok ? load_tide : "_sea_surface_height_above_geoid_amplitude_due_to_loadtide" ocean tide : "_sea_surface_height_above_geoid_amplitude_due_to_oceantide"

2/ invert barometer "_change_in_sea_surface_height_due_to_change_in_airpressure" (we work on this in https://github.com/cf-convention/vocabularies/issues/38 , I think we are good on this one

3/ steric

"_global_average_steric_sea_levelchange" ; ok ?

4/ barystatic:

"_global_average_sea_level_changedue to change in ocean mass" ; ok ?

Sum/ total sea level

We just wanted to call it "sea_surface_height_abovegeoid" in the first place ; but I think it would be better to have "total" in the name. Can we work on a new standard name ?

Best wishes and thanks again for all this help ! Stéphane

[slawchune]

up @JonathanGregory :)

[JonathanGregory]

I have not forgotten, Stéphane @slawchune. I was on leave last week. Jonathan

[efisher008]

Hello Stéphane @slawchune and Aurore @abiardeau ,

can we reprocess / resume the whole standard name variable set in this issue ? Because we need a certain consistency to ensure that users don't get lost.

Is your comment in the above post meant as a summary of several active proposals by Aurore in the editor, i.e. issues cf-convention/vocabularies#37 (this one), #288, #287, #286, and #314? I am trying to keep up with recording/tracking these names in the CF editor.

In particular, we should take care that the "complementary" names in issues #285 (non_tidal_non_invert_barometer_elevation_of_sea_surface_height_above_geoid) and #286 (sea_surface_height_above_geoid_due_to_inverse_barometer) don't grow too dissimilar as at the moment they would not very easily be associated.

Best regards, Ellie

[JonathanGregory]

Dear Stéphane

As I wrote in my last contribution, I was assuming you meant 3 + 4 + 5 in your document, because you explicitly excluded just 1 + 2 from it. In your latest, you say "this quantity doesn't contain the steric [3] and barystatic [4] contribution." That is, you mean 5 alone, ocean dynamic sea level. That quantity is defined in this paper as "The local height of the sea surface above the geoid, with the inverse barometer correction applied," i.e. having removed the effect of air pressure variation, and "It is determined jointly by ocean density and circulation," i.e. it does not include tide. We ought to have said so explicitly in the paper.

In the paper, we define the geoid as time-varying such that global-mean ocean dynamic sea level is zero, but the geoid in your model is a different one, which is fixed in the terrestrial reference frame. Does the model assume (as all climate models do) that the geoid and the reference ellipsoid are coincident, disregarding the actual fact that the Earth is not an ellipsoid? If it doesn't distinguish between those surfaces, I think we could call 5 either ocean_dynamic_sea_level_wrt_reference_ellipsoid, or sea_surface_height_above_reference_ellipsoid_assuming_zero_air_pressure_anomaly_and_mean_tide.

I'll write separately about the other ones.

Best wishes

Jonathan

[JonathanGregory]

Dear Stéphane

Thanks for your patience.

Regarding quantity 5 (the subject of this issue), on further reflection I tend to prefer sea_surface_height_above_reference_ellipsoid_assuming_zero_air_pressure_anomaly_and_mean_tide. This is the same meaning as your proposal, but uses an assuming phrase, like many standard names, following the guidelines. I think it makes more sense to keep ocean_dynamic_sea_level as a quantity which must have zero mean, implying a particular definition of the geoid. What do you think?

I agree with you and Ellie on the need for consistency in SL standard names. In addition, to support CMIP7 OMIP, I would like to propose some more. I will open another issue about SL names in general, with links to the ones Aurore opened.

I believe that we have discussed all your quantities now except the tides. They're in cf-convention/vocabularies#40 and cf-convention/vocabularies#41, but we could talk about them here. I have a couple of questions.

• Above, you suggest the word "amplitude" in these quantities. There are already various standard names sea_surface_height_amplitude_due_to_X_tide. The definitions aren't clear, but I presume that "amplitude" means the range from maximum to minimum. Is that what you mean, or do you mean the actual tidal height above some reference level to be specified (ellipsoid, geoid, etc.)?

• I am unclear what is meant by "load tide", I'm afraid. In the 2019 paper, we use the phrase "solid-Earth tide" to mean the effect of gravitational tidal forces on the solid Earth. One of the existing tide standard names uses the phrase earth_tide, which I think must mean the same as "solid-Earth tide". Is the "load tide" the effect on the solid Earth of tidally generated change in the distribution of the weight of the ocean? Is this conventionally included in the solid-Earth tide, or in addition to it?

Best wishes

Jonathan

[slawchune]

Hi Jonathan, thank you very much for having work on this !

sea_surface_height_above_reference_ellipsoid_assuming_zero_air_pressure_anomaly_and_mean_tide works for me for 5 ! It's a bit long, but at least it's precise.

for the tide, the reference is again the geoid. ; do you think we can go with : sea_surface_height_due_to_tide_above_reference_ellipsoid

Load tide : In FES2014 it is describe as the effect of ocean tide on the earth floor : "load tide" the effect on the solid Earth of tidally generated change in the distribution of the weight of the ocean" then

https://www.aviso.altimetry.fr/en/data/products/auxiliary-products/global-tide-fes/description-fes2014.html It is said nowhere that it includes the solid-earth tide. I am afraid that it's not that clear to me. At least, it's geoid-referenced like the ocean tide, but it acts more like a correction terme for a space based observer.

for the total sea level, can we come up with a name ?

Thanks again and sorry for all those uncertainties ! Cheers, Stéphane

[efisher008]

Dear Stéphane,

I have changed your "quantity 5" name from this issue as discussed, the entry in the CF editor can be viewed here: https://cfeditor.ceda.ac.uk/proposal/5276/edit

I removed the reference to "non_tidal_non_ibelevation" as this no longer forms part of the name and added the following suggested phrases: "A phrase "assumingcondition" indicates that the named quantity is the value which would obtain if all aspects of the system were unaltered except for the assumption of the circumstances specified by the condition." "A reference ellipsoid is a regular mathematical figure that approximates the irregular shape of the geoid. A number of reference ellipsoids are defined for use in the field of geodesy."

I hope this is acceptable.

Best regards, Ellie

[JonathanGregory]

Dear Stéphane

I think we've agreed that your model has a time-independent geoid, and assumes that the reference ellipsoid and geoid are identical. In that case, I think the "total" is sea_surface_height_above_reference_ellipsoid, which is an existing standard name. In standard names, if there is no qualification that indicates it's a part of a quantity (with an adjective, or an assuming, for instance), it means the "total". That is, "total" is default, and we don't state it, because one considers it to be a "total" only when considering a particular decomposition. Any quantity might be decomposed and considered a "total" of other things. For the quantity by itself, "total" doesn't add any information; it's relative to context. Do you see what I mean?

The ocean-tide component is a term in your sum. It's a tidal height wrt a tidal reference surface (a "tidal datum"). The tidal component is zero when the tide coincides with the datum. Does the ocean-tide term have a long-term mean of zero? Does the long-term mean of the "total" SSH in a steady climate (no global mean sea level change) coincide with mean sea level? If both are Yes, I believe it means the ocean tide component is tidal_sea_surface_height_above_mean_sea_level, which is an existing standard name.

When I google "load tide", I get a lot of references to laundry detergent. Trying the more specific "ocean load tide" instead, I get the impression that "load tide" isn't a very common term, because actually it's not a tide, as you suggest above - that's convenient jargon, perhaps, because its effect is similar to a tide. Hence I think the answer to my earlier question is that it's not included in the Earth tide. I propose that the "load tide" should be called change_in_sea_floor_height_above_reference_ellipsoid_due_to_ocean_tide_loading. When it's negative, it means the sea floor has been depressed, which lowers SSH wrt reference ellipsoid. Is that the sign you want?

Finally (this time!), with apologies, I've changed my mind again about your quantity 5. I now think it might really be ocean_dynamic_sea_level, which always has zero global mean, regardless of global mean sea level change. Likewise quantity 5 explicitly does not include global mean sea level change (thermosteric 3 + barystatic 4). The name sea_surface_height_above_reference_ellipsoid_assuming_zero_air_pressure_anomaly_and_mean_tide would include GMSLC.

What is the global mean of quantity 5? Is it constant and zero - if not, how does it behave?

Best wishes

Jonathan

[slawchune]

Dear Jonathan,

I see what you mean for the total. Le'ts go for sea_surface_height_above_reference_ellipsoid then :)

change_in_sea_floor_height_above_reference_ellipsoid_due_to_ocean_tide_loading is perfect and the sign sounds correct too to me. The correct term is "tide loading" indeed. We should then change the variable name too

ocean_dynamic_sea_level: for this variable, the global average is 0, so that indeed global mean sea level is not included. I think the term that you propose is good :)

Thanks for all this help ! Best, Stéphane

[JonathanGregory]

Dear Stéphane

Thanks for your collaboration, which has clarified several things for me as well. I think all your and Aurore's @abiardeau issues have been dealt with now, provided tidal_sea_surface_height_above_mean_sea_level is correct for your "ocean tide" term. Do you think it is? Zero for this existing standard name indicates that the tidal height is at the long-term-mean level, because "mean sea level" by definition means at the level of the long-term-mean tide.

Best wishes

Jonathan

[slawchune]

Dear Jonathan, I think tidal_sea_surface_height_above_mean_sea_level suits well our purpose. But for me, the average value of tides on a time cycle (although several very long periods !) ought to be zero. That's why the "above mean sea level" is for right ? i.e. to put an off set value so that this variable also has a zero global average mean.

[JonathanGregory]

Dear Stéphane

Yes, I think we agree, if I have understood you correctly. Suppose SSH wrt reference ellipsoid varied only because of the ocean tide, with no changes occurring in ocean mass, properties, circulation, geopotential or fluxes from the atmosphere. In that case, sea_surface_height_above_mean_sea_level and tidal_sea_surface_height_above_mean_sea_level would be identical. It means the elevation of the SSH wrt MSL due to the tide. Perhaps we should rename it like that, on the pattern of other CF standard names. The long-term mean of this quantity is zero, because MSL is defined as the time-mean level of the sea surface, considering a long enough period that the tides average to zero.

Best wishes

Jonathan

[slawchune]

Hello Jonathan ,

Finally, here's what we're going to put in the documentation, which is the fruit of our discussions. We're really behind schedule on delivering this documentation, so I'll have to freeze it for now. Is everything ok on your side?

Thanks again for all your help in clarifying my ideas.

The standard names are writen like this

Stéphane

---

Dataset : cmems_mod_glo_phy_anfc_merged-sl_PT1H-I (MOL)

Contains all the variables.

total_sea_level [meter] : Total elevation = ocean_tide + invert_barometer + sea_surface_height + global_mean_steric_variation + global_mean_mass_volume_variation sea_surface_height_above_geoid

invert_barometer [meter] Invert barometer change_in_sea_surface_height_due_to_change_in_air_pressure

sea_surface_height [meter] Sea surface height computed without tides and surface pressure force ocean_dynamic_sea_level

ocean_tide [meter] ocean tide elevation tidal_sea_surface_height_above_mean_sea_level

tide_loading [meter] tide_loading elevation change_in_sea_floor_height_above_reference_ellipsoid_due_to_ocean_tide_loading

global_mean_steric_variation [meter] global mean steric variation since initial state (20161005: -5.817441 m) global_average_steric_sea_level_change

global_mean_mass_volume_variation [meter] Global average mass volume sea level change caused by water mass balance since initial state (20161005) global_average_sea_level_change_due to change in ocean mass

[JonathanGregory]

I think that's all fine, Stéphane. Thanks again for working on it. I'm going to propose elsewhere some other changes, but I have been waiting for our discussions to conclude before continuing with them. They include changing global_average to global_mean but that is no problem for your application because, if it were agreed, we would keep the existing names as aliases, as we usually do. Jonathan

[slawchune]

Thanks for everything Jonathan :)

[efisher008]

Dear @slawchune,

Apologies for the delay in getting these names processed and accepted; the CF editor has been experiencing some minor technical issues in the last week which we're troubleshooting at the moment. Once it is fully functional we will be able to mark these as accepted and they will be released in the next update of the standard names table which is planned this summer.

Just one more question: should your name be added as a contributing author to these proposals?

Best wishes, Ellie

[JonathanGregory]

Both Aurore and Stéphane should be added to the list of standard name contributors.

[efisher008]

Dear Aurore @abiardeau and Stéphane @slawchune,

Thank you very much for your patience in working through these proposed names. The following names have been accepted in the CF editor:

• change_in_sea_surface_height_due_to_change_in_air_pressure (proposed in cf-convention/vocabularies#38)
• ocean_dynamic_sea_level (proposed in cf-convention/vocabularies#37)
• tidal_sea_surface_height_above_mean_sea_level (proposed in cf-convention/vocabularies#40)
• change_in_sea_floor_height_above_reference_ellipsoid_due_to_ocean_tide_loading (proposed in cf-convention/vocabularies#41)
• global_average_steric_sea_level_change (proposed in cf-convention/vocabularies#49)
• global_average_sea_level_change_due_to_change_in_ocean mass (proposed in cf-convention/vocabularies#49)

I believe, from reading this issue, that the "total sea level" variable was agreed to be described by the name sea_surface_height_above_reference_ellipsoid instead ofsea_surface_height_above_geoid as in your summary above. I am looking in particular at this comment which seems to be the most recent change. Could you confirm or clarify this please?

Best regards, Ellie

[JonathanGregory]

Dear Ellie @efisher008

Re

I believe, from reading this issue, that the "total sea level" variable was agreed to be described by the name sea_surface_height_above_reference_ellipsoid instead of sea_surface_height_above_geoid as in your summary above.

I think we established that the geoid and the reference ellipsoid are not distinguished in the ocean model in question (like in climate OGCMs in general), so they could use either. They would be equivalent. In the real world, they are different.

Best wishes

Jonathan

[efisher008]

Dear Jonathan @JonathanGregory,

Thank you for your reply. As there is no current need to distinguish these with a new standard name, shall we stick with the existing standard name sea_surface_height_above_reference_ellipsoid for this variable? If a future need arises, the geoid name can then be proposed.

Best, Ellie

[JonathanGregory]

sea_surface_height_above_geoid is also an existing standard name, so they have the choice. :-)

[efisher008]

sea_surface_height_above_geoid is also an existing standard name, so they have the choice. :-)

I see, that is correct @JonathanGregory - thanks for pointing it out!

In this case, all new proposed names have now been accepted and will be published in v86 of the standard names table, planned for this summer. Thanks again to @abiardeau and @slawchune for your proposals. I will also mark the original issues for this suite of names as accepted.

Best wishes, Ellie

[slawchune]

Thank you so much Jonathan and Ellie !