HERCULES

Predicting Earth’s climate depends on our understanding of the ocean dynamics on a wide variety of interacting scales in time and space. Gravity currents represent one of the key sub-mesoscale processes that drive energy transfer, impact the thermohaline structure and vertical exchange of water masses in the ocean, but their representation remains challenging for numerical models. The targeted study area is the Strait of Gibraltar (columns of Hercules) which connects the Mediterranean to the Atlantic Ocean, located between southern Spain and northern Morocco. This narrow passage is today one of the most frequented maritime routes in the world. The choice of the Strait of Gibraltar and its adjacent areas (Gulf of Cadiz and the Alboran Sea) as targeted study area for this proposal is relevant regarding many aspects . It is particularly suitable for a first exploration of unknown sub-mesoscale regimes since the mechanisms that generate them are easily localized: "intense" turbulent structures can be more easily reproduced in the laboratory and observed within the water column. It also represents a place of formation of large-scale eddy structures. The Strait of Gibraltar is also a good model to better understand the impact (feedback) that turbulent dissipation has on the general circulation in the Mediterranean and North Atlantic basins and the way it may shape the ocean dynamics at a much larger scale. The main objective is to understand the processes underlying the energy transfer between the sub-mesoscale and dissipative scales, and the feedback of small-scale processes on the mesoscale; besides, it will contribute to quantify the impact of small-scale processes on the modulation of non-hydrostatic dynamics, the dynamics of localized exchange fluxes, and its feedback on the larger-scale synoptic dynamics. These objectives will be achieved through high spatial and temporal resolution measurements in the first realistic physical model ever built of the Strait of Gibraltar ; it will encompass the Gulf of Cadiz and Alboran Sea. It will represent a region of 250km x 150km in the world's largest infrastructure dedicated to the study of ocean flows, the Coriolis Platform (LEGI) and will include baroclinic, barotropic (tidal), rotational, and surface wind forcing. It will be a real engineering challenge. These data will be exploited in synergy with high resolution in situ observational data (SHOM) over several campaigns (2014, 2020, 2022) and numerical data (LES) mimicking the experimental physical model. Diagnostic tools will be developed to describe the small-scale turbulent processes - turbulent mixing, potential vorticity evolution, turbulence statistics - necessary for the evaluation of the non-hydrostatic dynamics and its feedback on the mesoscale. The data and tools (experimental, numerical and theoretical) produced in the framework of this project will also provide the necessary data to (i) test the parameterizations of the unresolved processes in the CROCO code related to gravity currents (entrainment, mixing, turbulent processes), (ii) calibrate, test and validate the CROCO code under non-hydrostatic conditions as produced by gravity currents. Although the project is focused on the Gibraltar Strait and the nearby environment, the methodology and approaches validated can be subsequently applied and extended to other locations with similar hydrological and dynamical characteristics and will thus offer a new way of modeling sub-grid processes and their integration into more global models, essential for a correct representation and forecasting of the ocean circulation.