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The NEMO oceanic model is widely used among the climate community. It is used with different configurations in more than 50 research projects for both long and short-term
simulations. Computational requirements of the model and its implementation limit the exploitation of the emerging computational infrastructure at peta and exascale. A deep revision and analysis of the model and its implementation were needed. The paper describes the performance evaluation of the model (v3.2), based on MPI parallelization, on the MareNostrum platform at the Barcelona Supercomputing Centre. The analysis of the scalability has been carried out taking into account different factors, such as the I/O system available on the platform, the domain decomposition of the model and the level of the parallelism. The analysis highlighted different bottlenecks due to the communication overhead. The code has been optimized reducing the communication weight within some frequently called functions and the parallelization has been improved introducing a second level of parallelism based on the OpenMP shared memory paradigm.

This work has been funded by the EU FP7 IS-ENES project (project number: 228203) and partially by HPC-EUROPA2
project (project number: 228398) with the support of the European Commission Capacities Area Research Infrastructures Initiative.The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Barcelona Supercomputing Center, namely prof. Jose Maria Baldasano, prof. Jesus Labarta and their stuff members.
The research leading to these results has received funding from the Italian Ministry of Education, University and Research and the Italian Ministry of Environment, Land and Sea under the GEMINA project.

• jel: C63