The Nucleus for European Modelling of the Ocean (NEMO) is a framework of ocean related engines, namely OPA for the ocean dynamics and thermodynamics (blue ocean), LIM 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).
It 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.
Ocean dynamics (NEMO-OPA)
OPA is the physical ocean component of NEMO containing the dynamics and thermodynamics. It’s name comes from the French OPA model version 8.2 of the 1990’s but has since been extensively developed by the NEMO consortium.
OPA is primitive equation model adapted to regional and global ocean circulation problems down to kilometric scale. Prognostic variables are the three-dimensional velocity field, a linear or non-linear sea surface height, the temperature and the 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 distribution of variables is a three-dimensional Arakawa C-type grid.
Various physical choices are available to describe ocean physics, so as various HPC functionalities to improve performances.
Within NEMO, the ocean dynamics is interfaced with a sea-ice component (LIM), with the passive tracer and biogeochemical components (TOP/PISCES) and, via the OASIS coupler, with several atmospheric general circulation models. It also supports two-way grid embedding via the AGRIF software.
Sea Ice (NEMO-LIM)
The sea-ice component of NEMO takes in account ice dynamics, thermodynamics, brine inclusions and subgrid-scale thickness variations. It is designed for global to regional applications, up to ~10 km of effective resolution.
Our next major evolution towards a pan-European sea ice model with expanded capabilities is to be expected at the 2020 horizon.
- 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, LIM follows the dynamical component NEMO-OPA.
- 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 LIM, 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.
LIM 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.
An option to switch back to the single-category sea-ice (or 2-levels) provides a cheaper sea ice model.
Biogeochemistry (NEMO-TOP / PISCES)
TOP (Tracers in the Ocean Paradigm) handles oceanic passive tracers. At present, this component provides the physical constraints and boundaries 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 masses ventilation
- a built-in biogeochemical model (PISCES) to simulate lower trophic levels ecosystem dynamics in the global ocean
- a prototype tracer module (MY_TRC) to enable user-defined cases or the coupling with alternative biogeochemical models (see e.g. BFM)