Are we underestimating eddy-wave interactions in the Mediterranean Sea? Insights from the BioSWOT-Med 2023 cruise

The breaking of near-inertial wave (NIW) trapped in anticyclones after strong wind events is a well-known pathway for kinetic energy dissipation below the mixed layer in the ocean and one of the mechanisms by which the ocean responds to modified wind patterns under climate change. In the Mediterranean Sea, where turbulence is generally low far from topographic boundaries, NIW trapping has been documented only in few large (>100 km) and energetic mesoscale features. Whether NIW trapping is restricted to these few isolated and semi-permanent features or is a more widespread phenomenon remains a key open question, whose answer is hindered by the difficulty of tracking in space and time typical Mediterranean eddies of low energy and small radii. 

Here we present an in-situ experiment conducted during the BioSWOT-Med cruise (doi.org/10.17600/18002392) that addressed this problem by surveying a moderately energetic small meander (<50 km, Ro ≈ 0.5 ~ Fr) of the North Balearic front assisted by the first high-resolution SSH images of the SWOT mission. We explore how the front modulates the evolution of the turbulence below the mixed layer after experiencing two consecutive strong wind events. We show that the turbulence remains low in the front and its cyclonic side while turbulence is greatly enhanced in the anticyclonic side. The latter side is dominated by a small anticyclone (~30 km of diameter) that trapped NIWs down to 300 m, generating intense shear and turbulence reaching up to several 10^-8 W/kg. Estimations of vertical kinetic energy fluxes induced by NIWs are about one order of magnitude stronger than previous estimations outside anticyclones (8–10 mW/m² vs 0.5–2.5 mW/m²) and about 3 times the estimation of Kunze et al. (1995) in a mesoscale anticyclone of the Gulf Stream (~3 mW/m2). More generally, these results suggest that moderately energetic fine-scale fronts and eddies are as important to structure the flows and the turbulence as strong fronts and eddies found in western boundary current and upwelling systems, addressing new challenges for their parameterization in Earth system models.