Add some marine variables

[svahl991]

Description

We need some marine variables added to the ESM standard since we wish to use them in generic JEDI code for use with SOCA. I have attempted to follow the conventions used for the corresponding air variables.

Issues

Variables needed for https://github.com/JCSDA-internal/vader/pull/208

[svahl991]

Tagging @travissluka, @shlyaeva, @Dooruk

[svahl991]

There's a failing check on this PR that should be resolved by commit 2cfaeb5, but the test didn't get re-triggered.

[svahl991]

Note that I'm aware of the apparent inconsistency between adding the names potential_temperature_of_sea_water and sea_water_temperature (instead of temperature_of_sea_water). But I was attempting consistency with the corresponding air variables, which are named in this inconsistent way as well.

[gold2718]

I have a question about units but this otherwise seems fine.

[gold2718]

I am familiar with ppt for seawater salinity but not ppt m. What is this?

[svahl991]

I don't know. (I'm a software engineer, not a scientist, so I'm probably not the best person to create this PR.) It could well be incorrect. I just copied the units in use for the existing sea_water_salinity_in_diurnal_thermocline variable, figuring they would have the same units. But, since I don't understand the diurnal_thermocline part, maybe that's incorrect. I'm happy to correct it to just ppt if needed.

[svahl991]

After a quick google search, it seems like the units for sea_water_salinity should either be ppt (parts per thousand), psu (practical salinity units, which seems to be the same numerical value as ppt), or possibly even just percent. Sorry for not looking into this matter more carefully originally. Can someone with ocean expertise advise on which units should be used here?

[travissluka]

short answer: use PSU, and state "practical salinity of sea water" in the description

long answer:
Technically PSU is not a unit, so i've seen places where the unit is "1", but who cares.
Also, "salinity" is ambiguous, there are 2 main types of salinity 1) "practical salinity" which is what ocean models historically used to use, and is a measure of conductivity and 2) "absolute salinity" which is the more recent standard that models are now starting to use, and has units of g/kg

edit: and, fyi, ppt is close to psu, but technically not the same

[Dooruk]

We should stick with PSU which is the current standard for measurements. It may be confusing because in Gibbs Seawater (GSW) toolbox, absolute salinity (AS) is used but there is a conversion from pracitcal salinity (PS) to AS.

[svahl991]

Thanks @travissluka. Since the goal of ESM naming is to avoid ambiguity, should the name of this variable be changed to sea_water_practical_salinity instead of sea_water_salinity? It is possibly only a matter of time until the more recent kind of salinity is added to the ESM naming, so we might as well prepare so we don't have to change names later. Or it is even simple enough to add both names now.

[svahl991]

Tagging @twsearle

[svahl991]

I took a look at the CF conventions, and added their names for these two quantities. It looks to me like CF uses ppt, not PSU for the units of sea_water_salinity. But I stuck with PSU here since that is what @Dooruk and @travissluka recommended.

[svahl991]

OK, I looked again, closer, and CF naming has three names:

• sea_water_salinity with units of 1e-3 (which I think means the same as ppt?)
• sea_water_practical_salinity with units of 1 (which Travis says is the same as PSU)
• sea_water_absolute_salinity with units of g kg-1

Do we want to duplicate all three names in ESM? I need scientific guidance here.