In 1991, CERFACS started the development of a software interface to couple existing ocean and atmosphere numerical General Circulation Models. Today, different versions of the OASIS coupler are used by about 45 modelling groups all around the world on different computing platforms1. OASIS sustained development is ensured by a collaboration between CERFACS and the Centre National de la Recherche Scientifique (CNRS) and its maintainance and user support is presently reinforced with additional resources coming from the IS-ENES2 European project (# 312979).
The current OASIS3-MCT internally uses MCT, the Model
Coupling Toolkit2 (Larson et al 2005)
(Jacob et al 2005), developed by the Argonne National Laboratory in the USA. MCT implements fully parallel regridding, as a parallel matrix vector
multiplication, and parallel distributed exchanges of the coupling
fields, based on pre-computed regridding weights and addresses.
Its design philosophy, based on flexibility and minimal invasiveness,
is close to the OASIS approach.
MCT has proven parallel performance and is, most notably, the
underlying coupling software used in National Center for Atmospheric
Research Community Earth System Model (NCAR CESM).
OASIS3-MCT is a portable set of Fortran 77, Fortran 90 and C routines. Low-intrusiveness, portability and flexibility are OASIS3-MCT key design concepts. After compilation OASIS3-MCT is a coupling library to be linked to the component models, and which main function is to interpolate and exchange the coupling fields between them to form a coupled system. OASIS3-MCT supports coupling of 2D logically-rectangular fields but 3D fields and 1D fields expressed on unstructured grids are also supported using a one dimension degeneration of the structures. Thanks to MCT, all transformations, including regridding, are performed in parallel on the set of source or target component processes and all coupling echanges are now executed in parallel directly between the component processes via Message Passing Interface (MPI). OASIS3-MCT also supports file I/O using netcdf.
The new version, OASIS3-MCT_3.0 supports coupling exchanges between components deployed in much more diverse configurations than before. It is of course possible to implement coupling exchanges between two components corresponding to two different executables running concurrently on separate sets of tasks, as before, but also between two components running concurrently on separate sets of tasks within one same executable, or between different sub-components defined on separate or overlapping sets of tasks within one executable. It is also now possible to have some or all tasks of a component not participating to the coupling exchanges.
In spite of the significant changes in underlying implementation, usage of OASIS3-MCT in the codes has largely remained unchanged with respect to previous OASIS3 versions. To communicate with another component, or to perform I/O actions, a component model needs to include few specific calls of the Application Programmig Interface (API) OASIS3-MCT coupling library. The namcouple configuration file is also largely unchanged relative to OASIS3, although several options are either deprecated, not used or not supported.
A Graphical User Interface is now available to build the namcouple configuration file for one's specific coupled system. The OASIS3-MCT GUI is an application of OPENTEA, a graphical interface used by different codes and developed at CERFACS. The sources of the GUI are located in the oasis3-mct/util/oasisgui directory; a short description of the OASIS GUI is given in the README file therein and the GUI itself includes all relevant explanations on-line.
The changes between previous versions and the current one are listed in appendix B.
First tests done with up to 16000 cores on the Bullx Curie machine at the Très Grand Centre de Calcul near Paris are very encouraging and it is therefore very likely that OASIS3-MCT will provide an efficient and easy-to-use coupling solution for many climate modelling groups in the few years to come.