Towards a numerical configuration of the Strait of Gibraltar at the laboratory scale: The HERCULES project

The Strait of Gibraltar is the site of a rich and complex physics where flows of different densities interact with a strongly rugged topography. These interactions produce many small-scale processes such as the development of hydraulic jumps, shear instabilities, internal solitary waves, which induce strong mixing and impact the Mediterranean outflow water masses. The dynamics of this gravity current emerging from the Strait and flowing along the canyons of the Gulf of Cadiz is conditioned by its prior evolution inside the Strait. In turn, this dense gravity current plays a crucial role on the large-scale oceanic circulation as it mixes with overlying North Atlantic waters thus modifying the properties of the deep-water masses and therefore, likely the regional and basin-scale circulations. Better understanding and describing these small-scale processes is essential to improve operational oceanic models and climate models since they occur below the grid scale of these models and therefore require parameterizations. Recently, the LEGI team has implemented a realistic setup of the Strait of Gibraltar with the adjacent Gulf of Cadiz and Alboran Sea on the Coriolis Platform at Grenoble, including the barotropic forcing (tide), the baroclinic one (lock-exchange), the Earth’s rotation and a realistic topography. The aim is to bring a better understanding of the small-scale physics which take place in this area. In addition to this experimental approach, centimetric resolution of hydrostatic and non-hydrostatic numerical simulations at the laboratory scale were carried out using the numerical code CROCO (Coastal and Regional Ocean COmmunity model – https://www.croco-ocean.org). A specificity of this code is to be able to efficiently resolve sub-mesoscale processes and to relax both the hydrostaticity and Boussinesq assumptions using a non-hydrostatic and compressible Navier-Stokes solver. The purpose of this numerical approach is manifold: explore ranges of parameters that could not be studied experimentally, develop diagnostic tools, investigate the impact of non-hydrostatic effects, evaluate numerical schemes and parameterizations, investigate more specifically each physical process. However, setting up such a realistic numerical configuration is challenging. For example, if we are interested in the purely barotropic forcing, it is essential to represent the tidal forcing identically as in the experiment. Likewise, when we focus on the purely baroclinic forcing, the inherent steep slopes related to the experimental setup put strong constraints on the numerical code. From these numerical simulations, non-hydrostatic effects on the gravity current dynamics are estimated and analyzed at the laboratory scale. They significantly modify hydraulic jumps formation, overflow transport and deep waters circulation in the Gulf of Cadiz. Boundary conditions, vertical resolution, explicit and implicit vertical mixing or viscous effects are all numerical factors that influence gravity current dynamics. All these features must be carefully studied since they impact both the fate of the Mediterranean waters when they flow into the Atlantic Ocean and the fate of the Atlantic waters flowing into de Mediterranean basin. The aim of this presentation is to present the work carried out up to date in the development of these numerical configurations and to present the associated preliminary results.