IFS-NEMO

IFS–NEMO couples the atmospheric model Integrated Forecasting System (IFS) with its ECLand land-surface scheme to the NEMO (Nucleus for European Modelling of the Ocean) ocean and sea-ice model and the ecWAM surface wave model within a single executable. As described in IFS model description, the models components are coupled in a sequential way, with the IFS running first. In terms of coupling infrastructure, the IFS-NEMO system has three main specificities which distinguish it from IFS-FESOM:

1. IFS-NEMO includes direct coupling between the ocean model (NEMO here) and the ecWAM wave model (see more detail below).

2. IFS-NEMO uses a fractional coupling approach for the atmosphere–sea-ice interface, where the IFS surface model perceives only the sea-ice concentration and performs its own sea-ice thermodynamics to compute atmosphere -– sea-ice heat fluxes. On the one hand, this pragmatic approach translates into NEMO’s sea-ice module SI3 perceiving fluxes that are inconsistent with its sea-ice state. On the other hand, it does not imply energy or mass leaks and permits mitigating Arctic sea-ice volume biases.

3. In IFS-NEMO, the river runoff is not coupled from the land model to the ocean. The runoff perceived by NEMO is read from a climatological file [Dai and Trenberth, 2002].

Points 2 and 3 are considered as model limitations which are actively being tackled in preparation for DestinE phase 3.

Simulations and IFS description

• Control Simulation
• Historical and Projection Simulations
• IFS model description

Atmosphere component

See IFS model description for a detailed description of the IFS atmospheric model, alongside the land-surface and wave components.

Ocean component

The ocean component is NEMO v4.0.7, a state-of-the-art ocean model solving the hydrostatic Boussinesq primitive equations [Gurvan et al., 2019]. NEMO’s numerical core is based on a split-explicit time-stepping scheme relying on finite difference scheme on a 1/12º (9 km at its coarsest, “eddy-rich” regime) locally quasi-orthogonal curvilinear grid hereinafter eORCA12. It also uses a depth-based coordinate with 75 vertical levels with, 25 of which in the upper 100 m and a 1 m thick uppermost ocean cell.

The NEMO physical core includes many parametrizations, e.g. to account for unresolved scales. A brief overview of the main parameterisations is provided below:

• The vertical diffusion uses a “1.5th-order” turbulent kinetic energy (TKE) based scheme [Gaspar et al., 1990]. The NEMO-ECWAM coupling directly uses surface gravity waves as extra TKE source terms.

• Horizontal diffusion is represented via an iso-neutral biharmonic Laplacian operator inspired by [Griffies et al., 1998].

• Vertical levels vary in thickness [Bruno et al., 2007] with local freshwater variations, and tracers are accordingly updated so that ocean heat and salt content are conserved [Griffies et al., 2001].

• Our NEMO configuration uses the TEOS10 equation of state [Feistel, 2024]. This means that model temperature and salinity are to be understood as conservative temperature (enthalpy per unit volume) and absolute salinity (salt mass concentration). The IFS-NEMO coupling has been specifically adapted to ensure sea-surface temperature consistency across the interface.

Sea-ice component

The NEMO ocean model includes a sea-ice component called SI3 [Vancoppenolle et al., 2023]. Ocean–sea ice continuously exchange heat, freshwater and salt, are coupled at every NEMO time step (360 s here).

SI3’s dynamical core rely on an elastic–viscous–plastic (EVP) rheology [Bouillon et al., 2013]. The model thermodynamics are formulated in an energy-, salt- and freshwater-conserving manner [Bitz and Lipscomb, 1999] over a five-category subgrid-scale ice thickness distribution (ITD) [Bitz et al., 2001]. Finally, SI3’s vertical discretisation employs four ice layers with one snow layer on top.

As said above, IFS-NEMO’s atmosphere–sea ice coupling relies on a fractional coupling approach. In the fractional coupling scheme, the IFS surface model perceives only the sea-ice concentration and performs its own sea-ice thermodynamics to compute net atmosphere–sea-ice heat fluxes that are then sent to SI3.

Initial conditions and forcing

Ocean initialization

A two-phase spin-up was performed to initialize both the control and historical simulations in year 1990. First, a 5-year NEMO (including SI3) stand-alone spinup was carried out over the 1985-1989 period. At its surface, the forced spinup was forced with hourly ERA5 near-surface properties, and fluxes computed from it with a NEMO implementation of the IFS bulk formulas (thus remaining as much consistent as possible as the coupled setup). The forced spinup was initialised at rest on January 1st 1985 with temperatures and salinities coming a 10-year rolling window climatology (1981-1990 here) of EN4.2.2 data [Good et al., 2013] converted to the TEOS10 equation of state. The resulting full ocean state was then used to initialize a two-year coupled IFS-NEMO simulation at the production (TCo2559-eORCA12) resolutions, keeping the radiative forcings at 1990 levels. Finally, the resulting coupled initial state was used as initial conditions for all model components, and for both control and historical simulations.

Atmosphere initial conditions and forcings

These are described in IFS model description.