xeos

Lightweight, xarray-enabled wrappers for seawater equations of state.

Ocean models (MOM6, MITgcm, MPAS-Ocean, Oceananigans) differ in the equation of state (EOS) they use, and many let you change it at run time. Python post-processing then often applies a different EOS than the simulation did, silently corrupting derived quantities like density, thermal expansion, and water-mass transformation diagnostics. xeos lets you pick the EOS that matches your run — by the model's own selector string — and apply it to xarray/dask data through one uniform API.

It stays lightweight on purpose: the polynomial/rational equations of state are vendored as small numpy kernels, so the core install needs only numpy + xarray. TEOS-10 (via gsw) is an optional extra.

Install

pip install xeos              # core (numpy + xarray): all vendored EOS
pip install xeos[teos10]      # adds TEOS-10 via gsw
pip install xeos[complete]    # gsw + numba acceleration

Usage

Match your model run by its selector string:

import xeos

# MOM6 run with EQN_OF_STATE = "WRIGHT_FULL"
eos = xeos.from_model("MOM6", "WRIGHT_FULL")
rho = eos.rho(theta, salt, pressure)     # xarray DataArrays in, labeled DataArray out
a   = eos.alpha(theta, salt, pressure)   # thermal expansion
b   = eos.beta(theta, salt, pressure)    # haline contraction

# MITgcm eosType = 'JMD95Z'
xeos.from_model("MITgcm", "JMD95Z").rho(theta, salt, p)

# MPAS-Ocean config_eos_type = 'jm'
xeos.from_model("MPAS-Ocean", "jm").rho(theta, salt, p)

# Oceananigans TEOS10EquationOfState
xeos.from_model("Oceananigans", "TEOS10EquationOfState").rho(CT, SA, p)

Or address an EOS directly:

xeos.equation_of_state("jmd95").rho(t, s, p)
xeos.rho(t, s, p, eos="wright97-full")          # one-off functional form
xeos.list_eos()                                  # what's available

Inputs may be scalars, numpy arrays, or xarray DataArrays (dask-backed arrays stay lazy). Pressure is sea pressure in dbar by default.

Conventions

xeos does not silently convert inputs. Each EOS declares the temperature and salinity it expects: TEOS-10 and the Roquet polynomials use conservative temperature + absolute salinity; the others use potential temperature + practical salinity (see eos.temperature / eos.salinity). Explicit conversion helpers live in xeos.conventions (these need the gsw extra).

Supported equations of state

xeos.list_eos() returns the current set. As of now:

• linear — configurable (MOM6/MITgcm/Oceananigans LINEAR)
• wright97-full, wright97-reduced — Wright 1997 (MOM6 WRIGHT_FULL, WRIGHT_RED/WRIGHT_REDUCED). The bare MOM6 WRIGHT selector also resolves to wright97-reduced with a warning: it is the corrected reduced fit, whereas MOM6's legacy WRIGHT runs the uncorrected kernel (see "not yet implemented").
• jmd95 — Jackett & McDougall 1995 (MITgcm JMD95Z/JMD95P; also MOM6 UNESCO/JACKETT_MCD, which are this fit — not EOS-80)
• unesco — UNESCO/EOS-80, Fofonoff & Millard 1983 (MITgcm UNESCO)
• mdjwf — McDougall et al. 2003 (MITgcm MDJWF)
• teos10-poly55 — Roquet 55-term polynomial / TEOS-10 density form (Oceananigans TEOS10EquationOfState, MOM6 ROQUET_RHO/NEMO)
• roquet-spv — Roquet 55-term specific-volume form (MOM6 ROQUET_SPV)
• roquet-{linear,cabbeling,cabbeling-thermobaricity,freezing,second-order,simplest-realistic} — idealized second-order Roquet forms (Oceananigans RoquetSeawaterPolynomial(:…))
• mpas-linear, mpas-jm, mpas-wright — MPAS-Ocean / E3SM config_eos_type = linear/jm/wright; mpas-jm and mpas-wright reuse the jmd95 and wright97-reduced kernels (MPAS-O's jm/wright are the same EOS)
• teos10 — TEOS-10 via gsw (its 75-term Roquet polynomial, not the exact Gibbs function; MOM6/MITgcm TEOS10)

Not yet implemented (planned, slot into the same registry): MOM6 JACKETT_06 and the legacy-buggy WRIGHT kernel (the bare WRIGHT selector currently resolves to the corrected wright97-reduced with a warning), and MITgcm POLY3 (per-level runtime coefficients).

Full literature references with DOIs are in the usage docs.

Development

pip install -e .[test]
pytest                       # validates vendored kernels against frozen fixtures

Test "truth" values are generated from authoritative reference packages in a pinned, separate environment and frozen into xeos/tests/reference/truth.json; the test suite reads that file and stays lightweight. See xeos/tests/reference/README.md to regenerate.