Abstract
Abstract. Ocean biogeochemical models are key tools for both scientific and operational applications. Nevertheless the cost of these models is often expensive because of the large number of biogeochemical tracers. This has motivated the development of multi-grid approaches where ocean dynamics and tracer transport are computed on grids of different spatial resolution. However, existing multi-grid approaches to tracer transport in ocean modelling do not allow the computation of ocean dynamics and tracer transport simultaneously. This paper describes a new multi-grid approach developed for accelerating the computation of passive tracer transport in the Nucleus for European Modelling of the Ocean (NEMO) ocean circulation model. In practice, passive tracer transport is computed at runtime on a grid with coarser spatial resolution than the hydrodynamics, which reduces the CPU cost of computing the evolution of tracers. We describe the multi-grid algorithm, its practical implementation in the NEMO ocean model, and discuss its performance on the basis of a series of sensitivity experiments with global ocean model configurations. Our experiments confirm that the spatial resolution of hydrodynamical fields can be coarsened by a factor of 3 in both horizontal directions without significantly affecting the resolved passive tracer fields. Overall, the proposed algorithm yields a reduction by a factor of 7 of the overhead associated with running a full biogeochemical model like PISCES (with 24 passive tracers). Propositions for further reducing this cost without affecting the resolved solution are discussed.
Highlights
Ocean biogeochemical and ecological models are key tools for numerous applications in oceanography
The choice between the two definitions is done at the precompilation stage of the code with a cpp key key_crs: Secondly, we present how we manage the grid and dynamic variables used in the passive transport component
The multi-grid algorithm has been implemented in Nucleus for European Modelling of the Ocean (NEMO) ocean general circulation model (OGCM) (Madec et al, 2017) version 3.6 in which the OPA (Océan Parallélisé) ocean model is coupled to the LouvainLa-Neuve sea ice model (LIM) v3 (Rousset et al, 2015; Vancoppenolle et al, 2009) and its passive tracer transport component TOP (Tracer in the Ocean Paradigm)
Summary
Ocean biogeochemical and ecological models are key tools for numerous applications in oceanography. Ocean fine-scale dynamics is known to affect the structure of ecosystems (d’Ovidio et al, 2015) and to impact the response of ocean biogeochemical cycles to environmental changes (Dufour et al, 2013) These findings are motivating the ongoing increase in the spatial resolution of ocean components of climate and operational models (Hewitt et al, 2017). A multi-grid approach has been proposed by Aumont et al (1998), where the output from the ocean circulation model used to drive the biogeochemical model are coarse grained, so that the biogeochemical model runs at a lower spatial resolution. Because the existing implementation of multi-grid tracer transport for the NEMO ocean model is not meant to be run at the runtime with the physical ocean component, its application is strongly limited by the I/O requirements and storage capacity. The algorithm is tested in a series of global ocean model simulations of varying complexity
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