Localized General Vertical Coordinates for Quasi‐Eulerian Ocean Models: The Nordic Overflows Test‐Case

• Published in: Journal of advances in modeling earth systems 2024-03, Vol.16 (3), p.n/a
• Main Authors: Bruciaferri, Diego, Guiavarc'h, Catherine, Hewitt, Helene T., Harle, James, Almansi, Mattia, Mathiot, Pierre, Colombo, Pedro
• Format: Article
• Language: English
• Subjects: Accuracy; Algorithms; Coordinate systems; Diapycnal mixing; Dynamic height; Environmental studies; Experiments; Humanities and Social Sciences; Isopycnal mixing; Nordic overflows; Numerical simulations; ocean modeling; Ocean models; Oceans; Simulation; Topography; vertical coordinates
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1029/2023MS003893

A generalized methodology to deploy different types of vertical coordinate system in arbitrarily defined time‐invariant local areas of quasi‐Eulerian numerical ocean models is presented. After detailing its characteristics, we show how the general localization method can be used to improve the representation of the Nordic Seas overflows in the UK Met Office NEMO‐based eddy‐permitting global ocean configuration. Three z*‐levels with partial steps configurations localizing different types of hybrid geopotential/terrain‐following vertical coordinates in the proximity of the Greenland‐Scotland ridge are implemented and compared against a control configuration. Experiments include a series of idealized and realistic numerical simulations where the skill of the models in computing pressure forces, reducing spurious diapycnal mixing and reproducing observed properties of the Nordic Seas overflows are assessed. Numerical results prove that the localization approach proposed here can be successfully used to embed terrain‐following levels in a global geopotential levels‐based configuration, provided that the localized vertical coordinate chosen is flexible enough to allow a smooth transition between the two. In addition, our experiments show that deploying localized terrain‐following levels via the multi‐envelope method allows the crucial reduction of spurious cross‐isopycnal mixing when modeling bottom intensified buoyancy driven currents, significantly improving the realism of the Nordic Seas overflows simulations in comparison to the other configurations. Important hydrographic biases are found to similarly affect all the realistic experiments and a discussion on how their interaction with the type of localized vertical coordinate affects the realism of the simulated overflows is provided. Plain Language Summary Numerical ocean models are arguably one of the most advanced tools the scientific community can use to study the dynamics of the world's oceans. However, the ability of an ocean model to realistically simulate ocean currents depends on the numerical techniques it employs, such as the type of vertical coordinate system. Ocean models typically implement a single type of vertical coordinate throughout the entire model domain, which is often unable to accurately represent the vast variety of physical processes driving the oceans. In this study, we propose a new method that allows different types of vertical coordinates in selected regions of the same model do