`p4zbc`

Overview

The p4zbc module handles the reading, interpolation, and application of time-varying external nutrient fluxes from atmospheric deposition, river inputs, sediment mobilization, hydrothermal vents, and sea ice supplies.

Version: NEMO 5.0
Model: PISCES (version NEMO 5.0)
Subroutines: p4z_bc, p4z_bc_init

`p4z_bc`

Main routine that applies external nutrient sources at each time step.

1. Atmospheric Dust Deposition

External nutrient input from dust deposition which dissolve in the water column (calculated from the 2\(^{nd}\) ocean level)

Activation: ll_dust = .TRUE.

Affected Tracers:

Iron (Fe): jpfer
Phosphate (PO4): jppo4 (p4z and p5z only)
Silicate (Si): jpsil (p4z and p5z only)

Key Features:

Reads dust deposition from external NetCDF files
Applies vertical dissolution profile for Fe, PO4, Si: exponential decay with 250m e-folding depth
Iron solubilization: 3.5% of dust iron is soluble
Dissolution rate: 1/(250 days)
Sinking speed: configurable via wdust parameter

Mineral Fractions:

Iron: mfrac / mMass_Fe
Phosphate: 1.e-3 / mMass_P
Silicate: 0.269 / mMass_Si

2. River Input

External input of nutrients from river.

Activation: ll_river = .TRUE.

Affected Tracers:

Nitrate (NO3): jpno3
Total Alkalinity (TALK): jptal (indirect effect)

Key Features:

Reads river nutrient concentrations from boundary condition files
Distributes input over river runoff depth or surface layer
Adjusts alkalinity due to nitrogen input

Implementation:

With runoff (ln_rnf = .TRUE.): Distributes over nk_rnf levels
Without runoff: Applies to surface layer only

3. Nitrogen Deposition

Activation: ll_ndepo = .TRUE.

Affected Tracers:

Nitrate (NO\(_3\)): jpno3
Ammonium (NH\(_4\)): jpnh4 (p4z and p5Z only)
Total Alkalinity (TALK): jptal

Key Features:

Atmospheric deposition of oxidized and reduced nitrogen
Applied to surface layer only
Accounts for alkalinity effects:
- NO\(_3\) decreases TALK
- NH\(_4\) increases TALK

4. Sea-Ice Iron Source

Activation: ln_ironice = .TRUE.

Affected Tracers:

Iron (Fe): jpfer

Key Features:

Simple parameterization based on Lancelot et al, 2009¹
Constant Fe concentration in sea ice: icefeinput
Fe flux computed from ice formation/melting rate
Limited to prevent complete Fe depletion: maximum 99% removal

5. Sediment Iron Mobilization

Activation: ln_ironsed = .TRUE.

Affected Tracers:

Iron (Fe): jpfer

Key Features:

Coastal and shelf iron supply
Spatially variable based on:
- Distance from coast
- Bathymetry
- Continental slope geometry
Active only in upper 50m when available
Uses pre-computed mask from external file (pre-computed sediment Fe field)
Denitride function applied for intensity modulation

6. Hydrothermal Vent Iron

Activation: ln_hydrofe = .TRUE.

Affected Tracers:

Iron (Fe): jpfer
Weak ligands (LGW): jplgw (if ln_ligand = .TRUE.)

Key Features:

Reads 3D helium-3 flux from external files
Converts to iron using Fe:3He ratio (hratio)
Optional ligand co-release at ratio lgw_rath
3D implementation

Input parameters

Parameter	Type	Intent	Description
`kt`	INTEGER	IN	Ocean time step index
`Kbb`	INTEGER	IN	Time level index (before)
`Kmm`	INTEGER	IN	Time level index (now/middle)
`Krhs`	INTEGER	IN	Right-hand side index

Unit Conversions

Dust: kg/m²/s → mol Fe/m³/s via mfrac/mMass_Fe
Hydrothermal: nmol/cm²/s → mol/m³/s with volume and time adjustments
Time scaling: rfact converts per-second to per-timestep rates

7. Output Diagnostics

The module provides the following diagnostic fields via iom_put:

Field Name	Units	Description
`Irondep`	mmol/m²/s	Atmospheric iron deposition
`Po4dep`	mmol/m²/s	Atmospheric phosphate deposition
`Sildep`	mmol/m²/s	Atmospheric silicate deposition
`No3dep`	mmol/m²/s	Atmospheric nitrate deposition
`Nh4dep`	mmol/m²/s	Atmospheric ammonium deposition
`pdust`	kg/m²/s	Surface dust concentration
`Ironice`	mmol/m²/s	Iron flux from sea ice
`Ironsed`	mmol/m³/s	Sediment iron supply
`HYDR`	mmol/m³/s	Hydrothermal iron input

`p4z_bc_init`

Initialization routine for external nutrient sources. The module reads the files and computes the budget called at the first timestep.

Execution Sequence

Read namelist parameters in nampisbc
Determine which sources are enabled
Allocate required arrays
Initialize field reading structures for dust and hydrothermal inputs
Compute sediment iron mask if ln_ironsed = .TRUE.
Write configuration summary to output

Input parameters

Parameter	Type	Intent	Description
`Kmm`	INTEGER	IN	Time level index (now/middle)

Namelist Parameters

Variable	Type	Units	Description
`sedfeinput`	REAL(wp)	-	Coastal iron release rate
`icefeinput`	REAL(wp)	mol/L	Iron concentration in sea ice
`wdust`	REAL(wp)	m/d	Sinking speed of dust particles
`mfrac`	REAL(wp)	-	Mineral content of dust
`lgw_rath`	REAL(wp)	-	Weak ligand ratio from hydrothermal sources
`hratio`	REAL(wp)	-	Fe:3He ratio for hydrothermal vent iron supply
`distcoast`	REAL(wp)	(m)	Distance off coast for sediment iron calculation

Data Array Allocation

Variable	Dimensions	Description
`dust`	(jpi, jpj)	Surface dust deposition field
`ironsed`	(jpi, jpj, jpk)	Coastal iron supply from sediments
`hydrofe`	(jpi, jpj, jpk)	Hydrothermal vent iron supply
`zcmask`	(jpi, jpj, jpk)	Coastal/island iron mask

Control Flags

Variable	Type	Description
`ln_ironsed`	LOGICAL	Enable iron input from sediments
`ln_hydrofe`	LOGICAL	Enable iron input from hydrothermal vents
`ln_ironice`	LOGICAL	Enable iron exchange with sea ice
`ll_bc`	LOGICAL	Master switch for boundary conditions
`ll_dust`	LOGICAL	Enable atmospheric dust input
`ll_river`	LOGICAL	Enable river nutrient input
`ll_ndepo`	LOGICAL	Enable atmospheric nitrogen deposition

Document Version: 1.0
Last Updated: October 2025
Source code: NEMO 5.0

C. Lancelot, A. de Montety, H. Goosse, S. Becquevort, V. Schoemann, B. Pasquer, and M. Vancoppenolle. Spatial distribution of the iron supply to phytoplankton in the southern ocean: a model study. Biogeosciences, 6(12):2861–2878, 2009. URL: https://bg.copernicus.org/articles/6/2861/2009/, doi:10.5194/bg-6-2861-2009. ↩