Improving nordic overflows representation in global ocean models

Cold dense waters flowing south from the Nordic Seas and the Arctic Ocean form strong bottom intensified gravity currents at the Denmark Strait, Iceland-Faroe ridge, and Faroe-Scotland channel. Such overflows generate water-masses with specific hydrographic features which form the lower limb of the thermohaline circulation, responsible for a large fraction of the ocean heat transport on the Globe.

Gravity current representation in ocean models is sensitive to the choice of the vertical coordinate system. Typically, global ocean models use geopotential z-level coordinates, representing the bottom topography as a series of step-like structures. However, this choice results in excessive entrainment and mixing when simulating gravity currents, even when the partial steps parametrization is employed. Conversely, terrain-following coordinates offers a natural representation of overflows but introduce errors in the computation of the pressure gradient force, making their use in global configurations challenging.

To improve the representation of Nordic overflows in global models, Colombo (2018) proposed the use of a local-sigma vertical coordinate, where model surfaces are terrain-following only in the proximity of the Greenland-Scotland ridge, whilst standard z-level coordinates (with partial steps) are used everywhere else. However, the development of such a mesh is not trivial, especially when defining the transition zone between the two vertical coordinates.

Similarly, to improve the representation of cross-shelf exchange in regional configurations Harle et al. (2013) developed a hybrid vertical coordinate (SZT) where terrain-following computational surfaces smoothly transition to z-level with partial steps below a user defined depth.

Recently, Bruciaferri et al. (2018) introduced the Multi-Envelope (ME) s-coordinate system, where computational levels are curved and adjusted to multiple arbitrarily defined surfaces (aka envelopes) rather than following geopotential levels or the actual bathymetry. This allows the optimisation of model levels in order to best represent different physical processes within sub-domains of the model.

In order to overcome the complexities of the local-sigma method, we propose combining this approach with the flexibility of the SZT and ME methods to generate localised versions of these vertical coordinates. We test this new methodology in the region of the Nordic Sea overflows in a ¼° global NEMO configuration. At first, a series of idealised numerical experiments is conducted to assess the ability of the local-SZT and local-ME grids to minimise both horizontal pressure gradient errors and spurious entrainment of overflow waters. Finally, the skill of the new local-ME and local-SZT systems in reproducing observed properties of the Nordic overflows is assessed and compared with the traditional approach of employing geopotential coordinates with partial steps.

Bruciaferri, D., Shapiro, G.I. & Wobus, F. A multi-envelope vertical coordinate system for numerical ocean modelling. Ocean Dynamics 68, 1239–1258 (2018). https://doi.org/10.1007/s10236-018-1189-x

Harle, J.D. et al. 2013. Report on role of biophysical interactions on basin-scale C and N budgets. Deliverable 6.5, European Basin-scale Analysis, Synthesis and Integration (EURO-BASIN) Project, http://eurobasin.dtuaqua.dk/eurobasin/documents/deliverables/D6.5%20Report%20on%20role%20of%20biophysical%20interactions%20on%20C%20N%20budget.pdf

Pedro Colombo. Modélisation des écoulements d’eaux denses à travers des seuils topographiques dans les modèles réalistes de circulation océanique: une démonstration du potentiel que représente l’hybridation d’une coordonnée géopotentielle et d’une coordonnée suivant le terrain. Sciences de la Terre. Université Grenoble Alpes, 2018.