Insertion of PETSc in the NEMO stack software driving NEMO towards exascale computing

This paper addresses the scientific challenges related to high level implementation strategies which steer the NEMO (Nucleus for European Modelling of the Ocean) code toward the effective exploitation of the opportunities offered by exascale systems. We consider, as case studies, two components of the NEMO ocean model (OPA-Ocean PArallelization): the Sea Surface Height equation solver and the Variational Data Assimilation module. The advantages rising from the insertion of consolidated scientific libraries in the NEMO code are highlighted: such advantages concern both the "software quality" improvement (see the software quality parameters like robustness, portability, resilience, etc.) and the reduction of time spent for software development and maintenance. Finally, we consider the Shallow Water equations as a toy model for NEMO ocean model to show how the use of PETSc objects predisposes the application to gain a good level of scalability and efficiency when the most suitable level of abstraction is used