The OASIS coupler is a software allowing synchronized exchanges of coupling information between numerical codes representing different components of the climate system. Current OASIS developers are CERFACS (Toulouse, France) and Centre National de la Recherche Scientifique (Paris, France)
OASIS3-MCT, the version of the OASIS coupler interfaced with the Model Coupling Toolkit (MCT) from the Argonne National Laboratory, offers today a fully parallel implementation of coupling field regridding and exchange. Low-intrusiveness, portability and flexibility are OASIS3-MCT key design concepts as for all previous OASIS versions. OASIS3-MCT is a coupling library that needs to be linked to the component models, with the main function of interpolating and exchanging the coupling fields between these components.
OASIS3-MCT supports coupling of general two-dimensional fields. Unstructured grids and 3D grids are also supported using a one-dimension representation of the two- or three-dimensional structures. Thanks to MCT, all transformations, including regridding, are executed in parallel on the set of source or target component processes and all coupling exchanges are now executed in parallel directly between the components via Message Passing Interface (MPI). OASIS3-MCT also includes a mixed OpenMP/MPI version of the SCRIP library, allowing fully parallel calculation of the remapping weights in the initialisation phase. OASIS3-MCT also supports file I/O using NetCDF, allowing an easy switch between the coupled and forced modes; the implementation of this functionality is however non-parallel with the reading/writing of the fields performed by the master process only.
In spite of the significant changes in underlying implementation, usage of OASIS3-MCT in the component code has largely remained unchanged with respect to OASIS3.3. To communicate with another model, or to perform I/O actions, a component model needs to include a few specific calls to the OASIS3-MCT coupling library, using the same API as in OASIS3.3. The namcouple configuration file is also largely unchanged relative to OASIS3, although few options are either not used or not supported.
The scalability tests done with OASIS3-MCT at high number of cores and the fact that it supports unstructured grids allow us to conclude that this coupler offers today a fully parallel and efficient coupling solution answering the short- and mid-term needs of the European climate modelling community.
The current developments of OASIS3-MCT are funded in the framework of the EU IS-ENES3 – Infrastructure for the European Network for Earth System modelling – Phase 3 project (GA # 824084), the ESiWACE Center of Excellence (grant agreement no. 823988).