[ecco-support] [EXTERNAL] Calculation of ice-ocean freshwater flux from ECCO v4r4

Wed Mar 17 11:55:28 EDT 2021

Alex,

Calculating the salinity, salt, and freshwater budgets in ECCO can be confusing and it doesn't help that several variables have had ambiguous (and sometimes even contradictory) descriptions.  We recently made an effort to improve the variable descriptions, and those will roll out soon in the newest version of the ECCO V4r4 NetCDF files and its accompanying user guide. 

In the interim, let me point you to this file which contains all of the updated variable descriptions: https://github.com/ECCO-GROUP/ECCO-ACCESS/blob/master/metadata/ECCOv4r4_metadata_json/ECCOv4r4_variable_metadata.json

In that structured json file you will find entries for all ECCO variables.  Here is a selection for the freshwater and salinity flux terms:

   "name": "oceFWflx",
      "long_name": "Net freshwater flux into the ocean",
      "units": "kg m-2 s-1",
      "standard_name": "water_flux_into_sea_water",
      "comments_1": "Net freshwater flux into the ocean including contributions from runoff, evaporation, precipitation, and mass exchange with sea-ice due to melting and freezing and snow melting",
      "comments_2": "Note: oceFWflx does NOT include freshwater fluxes between the atmosphere and sea-ice and snow. The variable 'SIatmFW' accounts for freshwater fluxes out of the combined ocean+sea-ice+snow reservoir.",
      "direction": ">0 decreases salinity (SALT)",

   "name": "SFLUX",
      "long_name": "Rate of change of total ocean salinity per m2 accounting for mass fluxes.",
      "units": "g m-2 s-1",
      "comments_1": "The rate of change of total ocean salinity due to freshwater fluxes across the liquid surface and the addition or removal of mass",
      "comments_2": "Note: the global area integral of SFLUX matches the time-derivative of total ocean salinity (psu s-1). Unlike oceFWflx, SFLUX includes the contribution to the total ocean salinity from changing ocean mass (e.g. from the addition or removal of freshwater in oceFWflx). ",

"name": "EXFempmr",
      "long_name": "Open ocean net surface freshwater flux from precipitation, evaporation, and runoff",
      "units": "m s-1",
      "comments_1": "Net surface freshwater flux from precipitation, evaporation, and runoff per unit area in open water (not covered by sea-ice). Excludes freshwater fluxes involving sea-ice and snow.",
      "comments_2": "Note: calculated as EXFevap-EXFpreci-EXFroff.",
      "direction": ">0 increases salinity (SALT)",

You see that oceFWflx  is the freshwater mass flux into the liquid ocean, SFLUX is a net salt mass flux and EXFempmr is freshwater "thickness" flux (m/s) in the open water (read: not sea-ice) part of ocean grid cells.  

To understand how one uses these and other terms to close salt, salinity, and freshwater budgets, Jan-Erik Tesdal and others put together a very nice tutorial: https://ecco-v4-python-tutorial.readthedocs.io/ECCO_v4_Salt_and_salinity_budget.html.  The document still needs a little polishing (suggestions welcome) but it is thorough and is a good place to start.

-Ian

-----Original Message-----
From: <ecco-support-bounces at mit.edu> on behalf of Alexander Haumann <haumann at princeton.edu>
Reply-To: "ECCO support list, wider membership" <ecco-support at mit.edu>
Date: Wednesday, March 17, 2021 at 7:05 AM
To: "ecco-support at mit.edu" <ecco-support at mit.edu>
Subject: [EXTERNAL] [ecco-support] Calculation of ice-ocean freshwater flux from ECCO	v4r4

    Dear ECCO support,

    I am trying to estimate the different components of the surface freshwater flux balance from ECCO v4r4 and I am wondering if you could maybe help me with clarifying some issues that I have.

    A colleague of mine estimated the total surface salt flux variations from SFLUX and I am trying to explain its variability from the different components of the surface freshwater fluxes. However, the sum of the ocean freshwater fluxes does not match the interannual
     variability in the salt flux. My main problems are (1) that I am not quite sure which freshwater flux is being used to obtain SFLUX and would be the equivalent and (2) how to estimate the sea-ice--ocean flux component.

    In terms of (1) there are three variables in the documentation that are referred to as total ocean freshwater flux: oceFWflx, EXFempmr, SIempmr. Which one should I use to match SFLUX? How is SFLUX in the model being calculated from the surface freshwater flux?
     SIempmr seems to be missing from the output files (nctiles_monthly) on the ECCO Drive in v4r4.

    In terms of (2), a colleague of mine recommended me to calculate the sea-ice--ocean flux as  -SIempmr- (SIatmFW-SIsnPrcp). However, for this calculation I would missing the SIempmr from the output. Would that in principal be correct? I have tried to calculate
     it as oceFWflx-SIatmFW, which looks reasonable in terms of spatial pattern but the variability does not match the SFLUX variability. So, I am skeptical that this is the correct approach.

    Thank you very much for your time and help.

    Best wishes,
    Alex

    -- 

    Dr. F. Alexander Haumann, Associate Research Scholar
    Atmospheric and Oceanic Sciences, Princeton University
    & British Antarctic Survey Honorary Researcher
    https://www.ahaumann.net <https://urldefense.us/v3/__https://www.ahaumann.net__;!!PvBDto6Hs4WbVuu7!flbgcHTTrvexZUNiBoyVxKjXsqdDs-N_8FHRfU8BYDPA0e4hgT8ahWft7bzZ9Aoi2IjR$>