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Configuration Overview

The NAARC_NPD configuration is designed for the Atlantic-Arctic domain with a 1/12-degree horizontal resolution and 75 vertical levels. It is derived from the Near-Present-Day (NPD) global ocean configuration, which uses the eORCA12 grid with a zps vertical coordinate system. To develop the NAARC_NPD configuration, the boundary condition template from the NAARC configuration (by James Harle) has been applied to the NPD global ocean configuration.

NPD configuration: https://github.com/NOC-MSM/NOC_Near_Present_Day

NAARC configuration: https://github.com/NOC-MSM/NAARC

Model version details

NEMO Version: 4.2.2

XIOS Version: 2

HPC System: ARCHER2

Environment: GNU-MPICH

Installation & Compilation: Provided in scripts/setup.sh

Initial and boundary conditions

Initial Conditions:

• Derived from the eORCA25 NPD configuration (1979).

• Initial condition files, Ice_initialization_m01.nc and initcd_m01.nc, were prepared by James Harle. These conditions are extracted from the January 1979 monthly mean of the ORCA25 simulation.

  initcd_m01.nc -> eORCA12_from_eORCA025_npd_grid_T_y1979m01.nc

  Ice_initialization_m01.nc -> eORCA12_from_eORCA025_npd_icemod_y1979m01.nc

Boundary Conditions:

Also prepared by James Harle for his NAARC configuration.

BC files :

• /work/n01/shared/se-eORCA12/NAARC_NA_v2 /work/n01/shared/se-eORCA12/NAARC_NP_v2

Parent files from where BC files are prepared:

• /dssgfs01/scratch/npd/simulations/eORCA025_JRA55/

Key Configuration Insights

Temperature & Salinity Initialization (ln_reset_ts)

The ln_reset_ts parameter initialises temperature and salinity from a restart file. This is particularly relevant during the ice spin-up phase, where ice evolves based on temperature and salinity conditions.

Ice Spin-Up Phase

In this process, the model starts from, say, 2006-01-01 and runs for a year. The generated restart file is then used to re-run the same year, repeating this cycle for 3–5 years to allow proper ice evolution. This approach is primarily used in shelf-enabled mode (se-NEMO). Further details regarding this spin-up can be found in the run scripts provided in the NAARC and NPD git repo:

Links:

https://github.com/NOC-MSM/NAARC/blob/main/scripts/setup/NAARC_setup

https://noc-msm.github.io/NOC_Near_Present_Day/deep_dives/#model-configurations

Model time step

The NPD global ocean configuration runs with a 450-second time step, which is highly efficient for a 1/12-degree resolution model with 75 vertical levels. The time step is also influenced by the vertical coordinate system. The zps coordinate system performs well in this setting.

Source Code Differences

Although both NAARC and NPD configurations use NEMO v4.2.2, their source codes differ slightly due to modifications made by their respective developers.