Abstract
Abstract. In this paper, we present a coupling scheme between the Massachusetts Institute of Technology general circulation model (MITgcm) and the Biogeochemical Flux Model (BFM). The MITgcm and BFM are widely used models for geophysical fluid dynamics and for ocean biogeochemistry, respectively, and they benefit from the support of active developers and user communities. The MITgcm is a state-of-the-art general circulation model for simulating the ocean and the atmosphere. This model is fully 3-D (including the non-hydrostatic term of momentum equations) and is characterized by a finite-volume discretization and a number of additional features enabling simulations from global (O(107) m) to local scales (O(100) m). The BFM is a biogeochemical model based on plankton functional type formulations, and it simulates the cycling of a number of constituents and nutrients within marine ecosystems. The online coupling presented in this paper is based on an open-source code, and it is characterized by a modular structure. Modularity preserves the potentials of the two models, allowing for a sustainable programming effort to handle future evolutions in the two codes. We also tested specific model options and integration schemes to balance the numerical accuracy against the computational performance. The coupling scheme allows us to solve several processes that are not considered by each of the models alone, including light attenuation parameterizations along the water column, phytoplankton and detritus sinking, external inputs, and surface and bottom fluxes. Moreover, this new coupled hydrodynamic–biogeochemical model has been configured and tested against an idealized problem (a cyclonic gyre in a mid-latitude closed basin) and a realistic case study (central part of the Mediterranean Sea in 2006–2012). The numerical results consistently reproduce the interplay of hydrodynamics and biogeochemistry in both the idealized case and Mediterranean Sea experiments. The former reproduces correctly the alternation of surface bloom and deep chlorophyll maximum dynamics driven by the seasonal cycle of winter vertical mixing and summer stratification; the latter simulates the main basin-wide and mesoscale spatial features of the physical and biochemical variables in the Mediterranean, thus demonstrating the applicability of the new coupled model to a wide range of ocean biogeochemistry problems.
Highlights
Coupling different models that have been developed to study only limited aspects of the Earth’s systems is becoming increasingly common due to the need to simulate different environmental components – and their interactions – simultaneously (Heavens et al, 2013)
The manual, we aimed to test the coherence of the model with the expected dynamics based on theoretical considerations and to test the model’s performance under different coupling configurations
This experiment was based on a simplified case study that consisted of an idealized domain (2◦ × 2◦ × 280 m closed box) that was forced by steady winds and a seasonal cycle of surface heat and mass fluxes
Summary
Coupling different models that have been developed to study only limited aspects of the Earth’s systems is becoming increasingly common due to the need to simulate different environmental components – and their interactions – simultaneously (Heavens et al, 2013). The numerical implementation of a coupling framework between 3-D hydrodynamic models and biogeochemical models is not a trivial task (Bruggeman and Bolding, 2014) because every model focuses on processes that occur on different temporal and spatial scales and uses different numerical parameterizations and schemes. These models might be coded in different languages or follow different coding “philosophies” with respect to memory allocation, computational schemes, and code workflow. Hydrodynamic and biogeochemical models are often developed by different and highly specialized scientific groups, whereas coupling requires interdisciplinary expertise
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