Core engines

The Nucleus for European Modelling of the Ocean (NEMO) is a framework of 3 related “engines”:

  • OCE for the ocean dynamics and thermodynamics (“blue ocean”)
  • SI3 for the sea-ice dynamics and thermodynamics (“white ocean”)
  • TOP for the on/offline oceanic tracers transport and PISCES for the biogeochemical processes (“green ocean”).

NEMO is intended to be a flexible tool for studying the ocean and its interactions with the other components of the earth climate system (atmosphere, sea-ice, biogeochemical tracers) over a wide range of space and time scales. To cite these engines, please refer to the reference manuals listed here.

Ocean dynamics (NEMO-OCE)

OCE is the physical ocean component of NEMO. It is a primitive equation model adapted to simulate regional and global ocean circulation up to kilometric scales. The prognostic variables are: the three-dimensional velocity field, linear or non-linear sea surface height, temperature and salinity.

In the horizontal direction, the model uses a curvilinear orthogonal grid and in the vertical direction, a full or partial step z-coordinate, or s-coordinate, or a mixture of the two. The variables are distributed on a three-dimensional Arakawa C-type grid.

Various physical choices are available to describe ocean physics, along with various HPC functionalities to improve performances.

The ocean dynamics component of NEMO is interfaced with the sea-ice component (SI3), the passive tracer and biogeochemical components (TOP/PISCES), and optionally with several atmospheric general circulation models via the OASIS coupler. The framework also supports two-way grid embedding via the AGRIF software.

Sea Ice (NEMO-SI3)

The sea-ice component of NEMO (Sea Ice modelling Integrated Initiative – SI3 ) takes into account the  ice dynamics, thermodynamics, brine inclusions and subgrid-scale thickness variations. It is designed for applications at scales from global to regional, up to ~10 km of effective resolution.

Physical guidelines:

  • Sea ice is frozen seawater, in tight interaction with the underlying ocean. Hence, the sea ice and ocean model components must be as consistent as possible. In practice, SI3 follows the dynamical component NEMO-OCE.
  • We like to either prescribe the atmospheric state or to use an atmospheric model. For consistency and simplicity of the code, we use the exact same formulation in both cases.
  • Different resolutions and time steps can be used, with some parameters to be adjusted.  In SI3, we try to achieve a resolution and time-step independent code, by imposing a priori scaling on the resolution / time step dependence of such parameters.
  • Energy, mass and salt must be conserved as much as possible.

SI3 is a curvilinear grid, finite-difference state-of-the-art implementation of the classical AIDJEX model, combining:

  • the conservation of momentum for an elastic-viscous-plastic continuum;
  • energy and salt-conserving halo-thermodynamics;
  • an explicit representation of sub-grid scale ice thickness variations.

Biogeochemistry (NEMO-TOP)

TOP (Tracers in the Ocean Paradigm) handles oceanic passive tracers. At present, this component provides the physical constraints and boundary conditions for oceanic tracers transport and represents a generalized, hardwired interface toward biogeochemical models to enable a seamless coupling.
In particular, transport dynamics are supplied by the ocean dynamical core thus enabling the use of all available advection and diffusion schemes in both on- and offline modes.

TOP is designed to handle multiple oceanic tracers through a modular approach and it includes different sub-modules:

  • the ocean water age module (AGE) tracks down the time-dependent spread of surface waters into the ocean interior
  • inorganic carbon (e.g. CFCs) and radiocarbon (C14b) passive tracers can be modelled to assess ocean absorption timescales of anthropogenic emissions and further address water mass ventilation
  • a built-in biogeochemical model (PISCES) to simulate lower trophic levels ecosystem dynamics in the global ocean
  • a tracer module (MY_TRC) to enable user-defined cases or the coupling with alternative biogeochemical models (see e.g. BFM)