Localised general vertical coordinates for quasi-Eulerian ocean models: the Nordic overflows test-case

A generalised methodology to deploy different types of vertical coordinate systems in arbitrarily defined time-invariant local areas of quasi-Eulerian numerical ocean models is presented. After detailing its characteristics, we show how the novel localisation 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 models localising different types of terrain-following vertical coordinates in the proximity of the Greenland-Scotland ridge are developed and compared against a control. Experiments include a series of idealised 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 localisation approach proposed here can be successfully used to embed terrain-following levels in a global geopotential levels-based configuration, provided that the localised vertical coordinate chosen is flexible enough to allow a smooth transition between the two. In addition, our experiments show that deploying localised terrain-following coordinates via the multi-envelope method allows the crucial reduction of spurious cross-isopycnal mixing when modelling bottom intensified buoyancy driven currents, significantly improving the realism of the Nordic Seas overflows simulations in comparison to the other models. Important hydrographic biases are found to similarly affect all the realistic experiments and a discussion on how their interaction with the type of localised vertical coordinate affects the accuracy of the simulated overflows is provided.