Reduced Spurious Vertical Mixing in MPAS-Ocean Model

Summary

In the ocean, vertical diffusion is several orders of magnitude smaller than horizontal diffusion. Ocean models have difficulty in reproducing low values of vertical diffusion due to spurious mixing intrinsic to the numerical algorithms. Recent work supported by the Department of Energy shows that spurious vertical mixing may be reduced by several advanced techniques.

The Model for Prediction Across Scales-Ocean (MPAS-Ocean) was validated against five long-standing ocean models using five domains, ranging from simple idealized test cases to real-world simulations. MPAS-Ocean produces results commensurate with the other models, validating the functionality of the new model. In addition, MPAS-Ocean produced less spurious mixing than other models, by up to a factor of ten, as measured by the resting potential energy. This result is due to a combination of the vertical coordinate, hexagon-type horizontal grid, and a tracer advection scheme designed for these grids. Ocean models are often categorized by their vertical coordinate. The Arbitrary Lagrangian-Eulerian method (ALE) of the MPAS-Ocean model offers great flexibility, so users can choose from numerous vertical coordinates: z-level (fixed), z-star (expands with sea surface), z-tilde (grid moves with fast waves), sigma (terrain-following), and idealized isopycnal (density surfaces). All of these modes were validated in idealized test cases and compared to other ocean models, including the Parallel Ocean Program (POP), Modular Ocean Model (MOM), MIT General Circulation Model (MITgcm), Regional Ocean Modeling System (ROMS), and Hallberg Isopycnal Model (HIM). The z-type coordinates were validated using real-world cases. MPAS-Ocean performed similarly or better than long-standing ocean models, and certain configurations of the vertical coordinate dramatically reduced the spurious mixing. Thanks to improved algorithms, MPAS-Ocean will better represent physical mixing processes in climate simulations, leading to more accurate climate studies.

References

Petersen, M.R., D. W. Jacobsen, T. D. Ringler, M. W. Hecht, and M. E. Maltrud. 2015. “Evaluation of the Arbitrary Lagrangian–Eulerian Vertical Coordinate Method in the MPAS-Ocean Model,” Ocean Modelling 86, 93–113. DOI: 10.1016/j.ocemod.2014.12.004.