Article Dans Une Revue Scientific Reports Année : 2025

Can a single coastal tide gauge help predict internal solitary wave activity offshore

Résumé

Internal solitary waves (ISWs) propagate in stratified waters, enhancing diapycnal mixing, sediment and mass transport on shelves. They have typical wavelengths of hundreds of meters and tens of minutes periods, requiring high resolution and high frequency measurements for their sampling. But such in-situ measurements are scarce and ISWs remain largely unpredictable. We evidence that large amplitude ISWs propagating in the Strait of Gibraltar have a signature in the sea level at the Tarifa tide gauge and we propose an algorithm to automatically detect them. We validate the algorithm with insitu measurements and satellite Sentinel-1 images. Thanks to the accuracy of this method, we analyse the ISW emission in the SoG from 2015 to 2023 from tidal to seasonal scales. This promising new method of in-situ ISW measurements offers potential benefits by supplying data on ISWs at various locations. These data will enhance our understanding of ISW dynamics, their parameterization and prediction.

Large amplitude Internal Solitary Waves (ISWs hereafter) propagate in stratified waters over long distances. They are specific internal waves achieving a balance between non-linear steepening and non-hydrostatic dispersion. ISWs can be generated by tide-topography interactions at shelf break or sills 1-4 , by upwelling front oscillations 5 , at river plume fronts 6,7 and even by eddies 8 . In addition they can result from the non-linear steepening of internal tides as in the South China Sea 9 . ISWs are of great importance for vertical mixing 10,11 , sediment transport on shelves 1,12-14 and threaten the safety of underwater infrastructures.

The Strait of Gibraltar (SoG hereafter) is the only dynamical connection between the Atlantic Ocean and the semi-enclosed Mediterranean Sea. The density difference between the relatively cold and salty Mediterranean waters and the warm and fresh Atlantic waters drives a baroclinic two-layer flow 15 . The Mediterranean waters flow westward in the Mediterranean Outflow while the Atlantic waters flow eastwards as a jet in the upper layer. The exchange flow maintains a strong pycnocline between both water masses which deepens westward in the SoG 16 . This permanent pycnocline plays the role of an internal-wave guide. The strong barotropic tide 17-19 and the complex bathymetry of the SoG favors the generation of internal tides and Internal Solitary Waves (ISWs). Large amplitude ISWs have long been described in the SoG, from in-situ measurements 20-30 , satellite or airborne observations 31-36 and more recently with numerical modeling 30,37-41 . They are generated at Camarinal Sill (CS hereafter), the main topographic feature, by the release of a hydraulic jump created during sufficiently strong tidal outflows (westward tidal current during the flood) and they propagate eastward to the Alboran Sea.

Apart from these impressive eastward propagating ISWs, other ISWs develop in the SoG. Watson et al. 31 observed northward propagating wave off the coast of Ceuta, Brandt et al. 35 identified westward propagating wave west of CS, Richez et al. 32 reported cross-strait ISWs. Recently, Roustan et al. 29 (RBGL23 hereafter)

evidenced the existence of reflected wave propagating northwestward from the Moroccan shelf-break towards the Spanish shelf associated to the well-known eastward propagating ISWs. They named these waves R-waves and the corresponding incident waves I-waves, names that we retain in this study. The authors suggested that these R-waves have a signature in the sea level measurements at the Tarifa tide gauge.

In-situ measurements of ISWs are scarce and short in time as they generally require the deployment of Acoustic Doppler Current Profilers (ADCPs) or Conductivity-Temperature-Depth (CTD) mooring lines sampling at relatively high frequency involving very high energy supply. This complexity is enhanced in the SoG characterized by heavy maritime traffic and harsh environment. We present here a unique opportunity to obtain

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hal-04947191 , version 1 (14-02-2025)

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Jean-Baptiste Roustan, Lucie Bordois, Jesús García-Lafuente, Franck Dumas, Francis Auclair, et al.. Can a single coastal tide gauge help predict internal solitary wave activity offshore. Scientific Reports, 2025, 15, pp.3551. ⟨10.1038/s41598-024-83165-1⟩. ⟨hal-04947191⟩
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