eos_t Derived Type

   type :: eos_t
      logical :: is_init = .false.
         !! True between `init` and `destroy`.  Prefer this to
         !! `allocated(...)` — tracks GPU device attachment too.
      integer  :: variant = EOS_VARIANT_LINEAR
         !! Active EOS variant.  Defaults to linear two-tracer
         !! (the simplest implemented branch).  Wright (1997)
         !! rational EOS is also shipped — opt in by setting
         !! `variant = EOS_VARIANT_WRIGHT_97`.  TEOS-10 reserved.
      real(wp) :: rho0 = 1035.0_wp
         !! Boussinesq reference density (kg/m^3).
      real(wp) :: T_ref = 10.0_wp
         !! Reference temperature for linear EOS (degC).
      real(wp) :: S_ref = 35.0_wp
         !! Reference salinity for linear EOS (PSU).
      real(wp) :: alpha_T = 1.7e-4_wp
         !! Thermal expansion coeff (linear EOS), kg/m^3 per degC.
         !! NOTE: this default is ~1000× smaller than the standard
         !! seawater value (~0.17 kg/m³/K).  The small value
         !! suppresses baroclinic feedback from PPM round-off in
         !! tests that don't care about realistic density gradients.
         !! Density-driven tests (e.g. `lock_exchange_2layer`) MUST
         !! override to the realistic value.
      real(wp) :: beta_S = 7.6e-4_wp
         !! Haline contraction coeff (linear EOS), kg/m^3 per PSU.
         !! See `alpha_T` for the rationale on this small default.
      real(wp) :: p_ref = 0.0_wp
         !! Reference pressure (Pa) at which `eos_compute_arrays` evaluates
         !! `ms%rho_layer` — i.e. the pressure the model's POTENTIAL
         !! density is referenced to.  Set from `&ocean_eos_nml p_ref`
         !! (default 0 ⇒ surface/potential density, bit-identical to every
         !! run before the knob existed).  The Wright and Roquet branches
         !! read it; the linear branch has no pressure dependence and
         !! ignores it.
         !!
         !! **It is a SCALAR on purpose and must stay horizontally
         !! uniform.** `rho_layer` is differenced ALONG a layer (the
         !! Montgomery PGF, the FV-lite / FV-MOM6-PCM integrands) and
         !! VERTICALLY (the vmix N² builders); a reference pressure that
         !! varied with `(i,j)` would make two columns of identical water
         !! at the same geopotential depth differ by `∂ρ/∂p · Δp_ref` and
         !! manufacture an along-layer density gradient out of nothing.
         !! A surface load therefore belongs in the IN-SITU pressure
         !! builders (`&ocean_psurf_nml in_eos` →
         !! `multilayer_state_t%p_top`), never here.
         !!
         !! What raising it DOES buy: the thermobaric state at which the
         !! effective α/β are evaluated.  Near the freezing point at
         !! cavity pressures the sign and magnitude of thermal expansion
         !! move appreciably, so a cavity or deep-ocean study is better
         !! referenced to a representative depth (2e7 Pa ≈ 2000 dbar) than
         !! to the surface — the usual σ₂ choice.
         !!
         !! Distinct from `&vcoord_nml rho_ref_pressure`, which references
         !! the RHO / HYCOM target-density COORDINATE and the density-space
         !! diagnostic remap.  They are independent knobs; for a
         !! density-coordinate run they should normally be set to the SAME
         !! value, so the coordinate and the dynamics agree on what
         !! "density" means (they are deliberately not tied together —
         !! a diagnostic remap to σ₂ under a σ₀ dynamics is a legitimate,
         !! if unusual, request).
         !!
         !! Flat POD: read BY VALUE into `eos_compute_arrays`' `_impl`
         !! calls, so a host assignment at configure needs no
         !! `!$acc update device` under `mem:separate` (same contract as
         !! `rho0`).
      real(wp) :: tfr_s = TFR_S_COEFF
         !! Liquidus slope λ1 (degC per PSU) — the `S` coefficient of
         !! `eos_freezing_point`.  Selected as a NAMED SET by
         !! `&ocean_eos_nml tfreeze_set` (`eos_apply_tfreeze_set`), never
         !! knob-by-knob: the three numbers are a fitted triple and
         !! mixing λ1 from one source with λ2 from another is a silent
         !! physics error.  Default = the SIS2 sea-ice set ⇒ bit-identical
         !! to every run before the knob existed.
      real(wp) :: tfr_0 = TFR_0_COEFF
         !! Liquidus intercept λ2 (degC).  Exactly `0.0` for the default
         !! sea-ice set — see `eos_freezing_point` for the (signed-zero
         !! only) bit-identity argument this exactness underwrites.
      real(wp) :: tfr_p = TFR_P_COEFF
         !! Liquidus pressure coefficient λ3 (degC/Pa).
      integer :: ts_convention = TS_POT_PRAC
         !! Tracer T/S convention this EOS expects (TS_POT_PRAC /
         !! TS_CONS_ABS).  Identity for linear + Wright.  This type is
         !! a flat POD — NO allocatable component — so it copies into
         !! registers for free when passed by value into the device
         !! point routines (`!$acc routine seq`).
   contains
      procedure, non_overridable :: init => eos_init
      procedure, non_overridable :: destroy => eos_destroy
   end type eos_t