Geosci. Model Dev., 13, 3347–3371, 2020 https://doi.org/10.5194/gmd-13-3347-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Representation of the Denmark Strait overflow in a z-coordinate eddying configuration of the NEMO (v3.6) ocean model: resolution and parameter impacts Pedro Colombo1, Bernard Barnier1,4, Thierry Penduff1, Jérôme Chanut2, Julie Deshayes3, Jean-Marc Molines1, Julien Le Sommer1, Polina Verezemskaya4, Sergey Gulev4, and Anne-Marie Treguier5 1Institut des Géosciences de l’Environnement, CNRS-UGA, Grenoble, 38050, France 2Mercator Ocean International, Ramonville Saint-Agne, France 3Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, Paris, France 4P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia 5Laboratoire d’Océanographie Physique et Spatiale, Brest, France Correspondence: Pedro Colombo ([email protected]) Received: 23 September 2019 – Discussion started: 14 January 2020 Revised: 26 May 2020 – Accepted: 9 June 2020 – Published: 30 July 2020 Abstract. We investigate in this paper the sensitivity of the players contribute to the sinking of the overflow: the break- representation of the Denmark Strait overflow produced by a ing of the overflow into boluses of dense water which con- regional z-coordinate configuration of NEMO (version 3.6) tribute to spreading the overflow waters along the Greenland to the horizontal and vertical grid resolutions and to various shelf and within the Irminger Basin, and the resolved verti- numerical and physical parameters. Three different horizon- cal shear that results from the resolution of the bottom Ek- tal resolutions, 1=12, 1=36, and 1=60◦, are respectively used man boundary layer dynamics. This improves the accuracy with 46, 75, 150, and 300 vertical levels. In the given numeri- of the calculation of the entrainment by the turbulent kinetic cal set-up, the increase in the vertical resolution did not bring energy mixing scheme (as it depends on the local shear) and improvement at eddy-permitting resolution (1=12◦). We find improves the properties of the overflow waters such that they a greater dilution of the overflow as the number of vertical more favourably compare with observations. At 300 vertical level increases, and the worst solution is the one with 300 levels the dilution is again increased for all horizontal reso- vertical levels. It is found that when the local slope of the lutions. The impact on the overflow representation of many grid is weaker than the slope of the topography the result is other numerical parameters was tested (momentum advec- a more diluted vein. Such a grid enhances the dilution of the tion scheme, lateral friction, bottom boundary layer param- plume in the ambient fluid and produces its thickening. Al- eterization, closure parameterization, etc.), but none had a though the greater number of levels allows for a better resolu- significant impact on the overflow representation. tion of the ageostrophic Ekman flow in the bottom layer, the final result also depends on how the local grid slope matches the topographic slope. We also find that for a fixed number of levels, the representation of the overflow is improved when 1 Introduction horizontal resolution is increased to 1=36 and 1=60◦, with the most drastic improvements being obtained with 150 levels. Oceanic overflows are gravity currents flowing over topo- With such a number of vertical levels, the enhanced verti- graphic constraints like narrow straits, channels, or sills, as cal mixing associated with the step-like representation of the well as down topographic slopes. Overflows carry dense topography remains limited to a thin bottom layer represent- waters formed in marginal seas or on continental shelves ing a minor portion of the overflow. Two major additional through intense air–sea exchanges (cooling, evaporation) from their source regions into the great ocean basins where Published by Copernicus Publications on behalf of the European Geosciences Union. 3348 P. Colombo et al.: Denmark Strait overflow in a z-coordinate eddying configuration of NEMO (v3.6) they join the general ocean circulation (Legg et al., 2006, breaking) have even smaller scales (from metres down to the 2009). Overflows are often structured as plumes or boluses millimetric scale) and cannot be resolved in present ocean of dense fluid a few hundred metres thick, accelerated toward models (Girton and Standford, 2003). Their effects are rep- great depths by gravity (Magaldi and Haine, 2015; Koszalka resented by a vertical turbulence closure scheme, the aim of et al., 2017; Almansi et al., 2017; Spall et al., 2019). As they which is to achieve a physically based representation of this cascade down over distances that may reach up to a few hun- small-scale turbulence. However, models using fixed geopo- dred kilometres, they entrain ambient waters through advec- tential levels as vertical coordinates (i.e. z-level models) are tion and intense shear-driven mixing processes. After reach- known to generate spurious (i.e. excessive and non-physical) ing a depth close to a neutral buoyancy level and a quasi- diapycnal mixing when moving dense water downslope. The geostrophic equilibrium, the entrainment of ambient water is link of this spurious mixing with the staircase-like represen- significantly reduced and the overflow becomes a neutrally tation of the bottom topography peculiar to these models is buoyant bottom density current (Legg et al., 2009). well-established (Winton et al., 1998; Wang et al., 2008). The Overflows of importance because of their contribution to parameterization of overflows in these models has been the the general circulation are those associated with the fol- topic of a number of studies (Beckmann and Döscher, 1997; lowing: the Denmark Strait and the Faroe Bank Channel Campin and Goosse, 2012; Killworth, 1977; Song and Chao, where dense cold waters formed in the Arctic Ocean and 2000; Danabasoglu et al., 2010; Wang et al., 2015). A large the Nordic Seas flow into the North Atlantic (Girton and number of idealized model studies, many of them conducted Standford, 2003; Brearley et al., 2012; Hansen and Øster- in the DOME framework (Dynamics of Overflows Mixing hus, 2007); the Strait of Gibraltar where dense saline wa- and Entrainment; Legg et al., 2006, 2009), tested the ability ters generated in the Mediterranean Sea overflow into the At- of overflow parameterizations against very-high-resolution lantic Ocean (Baringer and Price, 1997); the strait of Bab-el- simulations in a variety of OGCMs. When used in global Mandeb where the highly saline Red Sea waters flow into the simulations these parameterizations improve overflows, but Gulf of Aden and the Indian ocean (Peters et al., 2005); and still produce deep or bottom water properties that are not yet the continental shelves of the polar oceans (Killworth, 1977; satisfactory if not inadequate (Condie et al., 1995; Griffies et Baines and Condie, 1998), in particular around Antarctica, al., 2000; Legg et al., 2009; Danabasoglu et al., 2010, 2014; where high-salinity shelf waters formed in polynyas ventilate Downes et al., 2011; Weijer, 2012; Heuzé et al., 2013; Wang the Antarctic Bottom Water (Mathiot et al., 2012; Purkey et et al., 2015; Snow et al., 2015). Past model studies performed al., 2018). More reference papers can be found in Legg et with DOME-like idealized configurations also permitted us al.(2009), Magaldi and Haine(2015), and Mastropole et al. to gain an understanding of the dynamics of overflows and (2017). Altogether, these overflows feed most of the world the sensitivity of their representation in models to physical ocean deep waters and play an important role in distribut- and numerical parameters (see Reckinger et al., 2015, for ing heat and salt in the ocean. For the case of the Denmark exhaustive references and a synthesis of the main findings). Strait overflow (DSO hereafter), it feeds the Deep Western Significant differences between models due to the type of Boundary Current in the North Atlantic and so contributes vertical coordinate system were pointed out (e.g. Ezer and to the Atlantic Meridional overturning cell and the global Mellor, 2004; Legg et al., 2006; Wang et al., 2008; Laanaia thermohaline circulation (Dickson and Brown, 1994; Beis- et al., 2010; Wobus et al., 2011; Reckinger et al., 2015). mann and Barnier, 2004; Hansen and Østerhus, 2007; Dick- Numerical modelling of dense water cascades with son et al., 2008; Yashayaev and Dickson, 2008; Danabasoglu OGCMs designed to simulate the large-scale circulation still et al., 2010; Zhang et al., 2011; von Appen et al., 2014). This represents a challenge, especially because the hydrostatic ap- world-ocean-wide importance of the overflows makes their proximation on which these models rely removes the vertical representation a key aspect of ocean general circulation mod- acceleration from the momentum equation. This results in a els (OGCMs). misrepresentation of the diapycnal mixing processes (Özgök- A variety of physical processes of different scales are in- men, 2004) and requires, to represent their effects, a turbu- volved in the control of overflows and their mixing with lence closure scheme. Magaldi and Haine(2015) compared the ambient waters (Legg et al., 2008). Dynamical pro- high-resolution (2 km) hydrostatic and non-hydrostatic sim- cesses (e.g. hydraulic control and/or jump at sills or straits, ulations of dense water cascading in a realistic model config- mesoscale instability of the dense water plume, interactions uration of the Irminger Basin. They found that for 2 km hor- of the plume with overlaying currents) have length scales of izontal resolution, the parameterization of the non-resolved a few kilometres in the horizontal and a few tens of me- turbulence used in the hydrostatic model accurately repre- tres in the vertical. Such scales of motion are not resolved sented the effects of the lateral stirring and vertical mix- in present large-scale coarse-resolution (non-eddying) ocean ing associated with the cascading process.