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% Abstract (English)
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\chapter*{Abstract}

OPA is the ocean engine of NEMO, a framework of ocean related components developed for both research and operational purposes. It is a primitive equation model adapted to regional and global ocean circulation. It is intended to be a flexible tool for studying the ocean and its interactions with the others components of the earth climate system (atmosphere, sea-ice, biogeochemical tracers, ...) over a wide range of space and time scales. Prognostic variables are the three-dimensional velocity field, the 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 z full or partial step 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, including a 1.5 turbulent closure  or a KPP scheme for the vertical mixing. Within NEMO, OPA is interfaced with a sea-ice model (LIM), passive tracer and biogeochemical models (TOP) and, via the OASIS coupler, with several atmospheric general circulation models. In addition, it can be run on many different computers, including shared and distributed memory multiprocessor computers.

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% Abstract (French)
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\chapter*{R\'{e}sum\'{e}}

OPA est le moteur oc\'{e}anique de NEMO, un syst\`{e}me de mod\'{e}lisation de l'oc\'{e}an d\'{e}velopp\'{e} \`{a} des fins de recherche et d'oc\'{e}anographie op\'{e}rationnelle. Ce moteur est un mod\`{e}le aux \'{e}quations primitives de la circulation oc\'{e}anique r\'{e}gionale et globale. Il se veut un outil flexible pour \'{e}tudier sur un vaste spectre spatiotemporel l'oc\'{e}an et ses interactions avec les autres composantes du syst\`{e}me climatique terrestre (atmosph\`{e}re, glace de mer, traceurs biog\'{e}ochimiques...). Les variables pronostiques sont le champ tridimensionnel de vitesse, la hauteur de la mer , la temperature et la salinit\'{e}. La distribution des variables se fait sur une grille C d'Arakawa tridimensionnelle utilisant une coordonn\'{e}e verticale z \`{a} niveaux entiers ou partiels, ou une coordonn\'{e}e s, ou encore une combinaison des deux. Diff\'{e}rents choix sont propos\'{e}s pour d\'{e}crire la physique oc\'{e}anique, incluant notamment une fermeture turbulente d'ordre 1.5 ou un sch\'{e}ma KPP pour le m\'{e}lange vertical. Via l'infrastructure NEMO, OPA est interfac\'{e} avec un mod\`{e}le de glace de mer, des mod\`{e}les biog\'{e}ochimiques et de traceur passif, et, via le coupleur OASIS, \`{a} plusieurs mod\`{e}les de circulation g\'{e}n\'{e}rale atmosph\'{e}rique. En outre, il peut \^etre ex\'{e}cut\'{e} sur de nombreux calculateurs, y compris des calculateurs multiprocesseurs \`{a} m\'{e}moire partag\'{e}e ou distribu\'{e}e.


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% Disclaimer
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\chapter*{Disclaimer}

OPA (an acronym for Ocean PArall\'{e}lis\'{e}) is the ocean component of NEMO (Nucleus for European Modelling of the Ocean (www.locean-ipsl.upmc.fr/NEMO). Like all components of NEMO, it is developed under the CECILL license, which is a french adaptation of the GNU GPL (General Public license). Anyone may use OPA freely for research purposes, and is encouraged to communicate back to the NEMO team its own developments and improvements. The model and the present document have been made available as a service to the community. We cannot certify that the code and its manual are free of errors. Bugs are inevitable and some have undoubtedly survived the testing phase. Users are encouraged to bring them to our attention. The author assumes no responsibility for problems, errors, or incorrect usage of OPA.

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The OPA OGCM reference in papers and other publications is as follows:
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Madec, G., 2007: NEMO ocean engine. \textit{Note du P\^ole de mod\'{e}lisation}, Institut Pierre-Simon Laplace (IPSL), France, NXX, \colorbox{yellow}{YYpp}.\\

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% Foreword
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\chapter*{Foreword}

OPA is the ocean engine of the NEMO. This model, like all research tools, is in perpetual evolution. The present document describes the OPA model include in the release 2.3 of NEMO. This is the release 9 of OPA. This version differs significantly from version 8, documented in \citet{Madec1998}. The major modifications are :\\
(1) transition to full native \textsc{Fortran} 90, deep code restructuring and drastic reduction of CPP keys, \\
(2) introduction of partial step representation of bottom topography \\
(3) partial reactivation of a terrain-following vertical coordinate ($s$- and hybrid $s$-$z$) with the addition of several options for pressure gradient computation \footnote{Partial support of $s$-coordinate: there is presently no support for neutral physics in $s$-coordinate and for the new options for horizontal pressure gradient computation with true equation of state.}, \\ 
(4) more choices for the treatment of the free surface: full explicit, split-explicit , filtered and rigid-lid \\
(5) non linear free surface option (variable level thickness distributed on the whole water column) \\
(6) additional schemes for vector and flux forms of the momentum  advection \\
(7) addition of several advection schemes on tracers \\
 (8) implementation of the AGRIF package (Adaptative Grid Refinement in \textsc{Fortran} ) \\
(9) online diagnostics : tracers trend in the mixed layer and vorticity balance \\
(10) rewriting of the I/O management \\
(11) OASIS 3 and 4 couplers interfacing with atmospheric global circulation models. 

In addition, several minor modifications in the coding have been introduced with the constant concern of improving performance on both scalar and vector computers. 

At the time of this writing, the current release is NEMO 2.3. The new surface module described in this document is not yet part of the current distribution.

