MOM 5 (Modular Ocean Model)

MOM5 is a numerical ocean model based on the hydrostatic primitive equations, making use of the B-grid in the horizontal and generalized level coordinates for the vertical.

numericaloceanhydrostatic primitive equationB-grid



Initial contribute: 2019-12-29


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Application-focused categoriesNatural-perspectiveOcean regions

Detailed Description

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MOM5 is a numerical ocean model based on the hydrostatic primitive equations, making use of the B-grid in the horizontal and generalized level coordinates for the vertical.

Development is led by scientists at NOAA/GFDL in collaboration with scientists worldwide. Much of the ongoing activity is centred in Australia.

Version 5 of MOM (MOM5) is an open source project released under the GPL license. Contributions are welcome.

Quoted from: Elements of the Modular Ocean Model (MOM) 2012 release with updates 

      The Modular Ocean Model evolved from numerical ocean models developed in the 1960’s-1980’s by Kirk Bryan and Mike Cox at GFDL. Most notably, the first internationally released and supported primitive equation ocean model was developed by Mike Cox (Cox (1984)). Although somewhat common today, it was actually quite revolutionary in 1984 to freely release, support, and document code for use in numerical ocean modeling. The Cox-code provided scientists worldwide with a powerful tool to investigate basic and applied questions about the ocean and its interactions with other components of the climate system. Previously, rational investigations of such questions focused on idealized models and analytical methods. Many researchers embraced the Cox-code, thus fostering a wide community of users and developers that further enhanced the features and robustness of the code. This community approach has been fundamental to all versions of the Cox-code and subequent releases of MOM, with the underlying assumption that the scientific integrity of the code progresses more rapidly through input from a wide suite of researchers employing the code for a variety of scientific and operational applications. Quite simply, the Cox-code started what has today become a right-of-passage for every high-end numerical model of dynamical earth systems.

      Upon the untimely passing of Mike Cox in 1989 (Bryan, 1991), Ron Pacanowski, Keith Dixon, and Tony Rosati at GFDL rewrote the Cox-code with an eye on new ideas of modular programming using Fortran 77. The result was the first version of MOM (Pacanowski et al. (1991)). Version 2 of MOM (Pacanowski (1995)) introduced the memory window idea, which was a generalization of the vertical-longitudinal slab approach used in the Cox-code and MOM1. Both of these methods were driven by the desires of modelers to run large experiments on machines with relatively small memories. The memory window provided enhanced flexibility to incorporate higher order numerics, whereas slabs used in the Cox-code and MOM1 restricted the numerics to second order accuracy. MOM3 (Pacanowski and Griffies (1999)) even more fully exploited the memory window with a substantial number of new physics and numerics options. 

      MOM4 has origins dating back to a transition from vector to parallel computers at GFDL, starting in 1999. Other related codes successfully made the transition some years earlier (e.g., The Los Alamos Parallel Ocean Program (POP) and the OCCAM model from Southampton, UK). New computer architectures generally allow far more memory than previously available, thus removing many of the reasons for the slabs and memory window approaches used in earlier versions of MOM. Additionally, the loop structure can be quite opaque with the memory windows, making it relatively difficult to introduce new algorithms, especially for the novice. Hence, for MOM4.0, the memory window was jettisoned in favor of a horizontal 2D domain decomposition. The project to convert MOM3 to MOM4.0 took roughly four years of coding and testing.

      After gaining some experience on parallel machines with MOM4.0, and after developing the IPCC AR4 coupled climate model CM2.1 at GFDL (Griffies et al., 2005; Delworth et al., 2006; Gnanadesikan et al., 2006), development focused on a generalized level coordinate version of MOM, allowing the code to be used with depth based Boussinesq vertical coordinates or pressure based non-Boussinesq vertical coordinates. This effort led to the MOM4p1 project. During development and use of MOM4p1, a wide suite of new diagnostics were developed in support of the evolving applications toward climate and biogeochemistry modeling. Additionally, MOM4p1 has incorporated tools required for use in regional and coastal applications (Herzfeld et al., 2011).

      MOM4p1 continued to evolve from its initial release in 2007 toward the end of 2011. The most recent release took place in 2012, which represents the first release of MOM5. For many applications, the 2012 release of MOM is quite similar to the December 2009 release of MOM4p1. However, the 2012 MOM release has two notable enhancements to the underlying model framework.

      • The 2012 MOM release has a C-grid layout for the horizontal gridding of the discrete model fields. The C-grid has many advantages for fine resolution models and for representing land/sea boundaries (see Section 9.1). Hence, there is much interest at GFDL and within the MOM community to allow MOM to support both the B-grid and C-grid. It is anticipated that the bulk of he fine resolution modeling with MOM at GFDL will transition from the B-grid to the C-grid during late 2012 and beyond. Note that that the C-grid available in the initial release of MOM5 is a proto-type, with extensive testing remaining to be performed over the course of 2012 and beyond. Users intent on applying the C-grid for their purposes should recognize the early stages of this code. 

      • The 2012 MOM release is coupled to a dynamically active Lagrangian submodel as documented by Bates et al. (2012a,b). The interactive Lagrangian parcels provide a fundamentally new means to represent/parameterize vertical convection and gravity driven downslope processes. It is anticipated that much effort will be devoted over the next few years towards development and understanding of the utility of solving a coupled set of Eulerian and Lagrangian equations that interact through the exchange of mass, tracer, and momentum.

      • Further work has continued to refine the many physical parameterizations in MOM.

      • The 2012 MOM release has signficantly new diagnostic facilities allowing researchers to probe mechanisms for water mass transformation and steric changes to sea level, amongst the growing suite of other diagnostic features.

      The Cox-code and each version of MOM have an associated manual or user guide. Besides describing elements of the code and its practical use, these manuals aim to rationalize model methods, algorithms, and parameterizations. Absent such documentation, the code could present itself as a black box, thus greatly hindering its utility to the curious and skeptical scientific researcher. As the code grows and evolves, it is a nontrivial task to keep code and documentation consistent. Hence, visions for complete and updated documentation are unrealized, with elements of the documentation incomplete and/or not fully consistent with the code. Nonetheless, the present document, as well as the earlier MOM documents, should provide ample opportunity to understand many details of the code, thus facilitating its use for simulating the ocean.



NOAA/GFDL (2019). MOM 5 (Modular Ocean Model), Model Item, OpenGMS,


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