Frontal Dynamics in the Alboran Sea: 1. Coherent 3D Pathways at the Almeria‐Oran Front Using Underwater Glider Observations

Ocean fronts are areas that can support phytoplankton production through fertilization in the sunlit layer and the subduction of biogeochemical properties from the surface to the interior of the ocean. The Almeria‐Oran (AO) front is formed from the juxtaposition of fresh inflowing Atlantic waters and more saline re‐circulating Mediterranean waters. A fleet of three gliders flying in parallel lines was deployed across the AO to obtain observations in the CALYPSO project. These observations were combined with remote sensing and modeling simulations, thus providing a novel approach to identifying the three‐dimensional transport and the submesoscale across‐front circulation. The resulting 33 cross‐front sections reveal spatial and temporal changes in the frontal boundary, with isopycnals steepening and/or relaxing. The observations revealed strong horizontal density gradients (up to ∼1.4 kg m−3) and the spatial variability was observed over different length scales (∼10–45 km). The potential vorticity decreased across the front due to the vorticity component in the horizontal density gradient direction. The predominant cyclonic relative vorticity on the dense side of the AO is associated with downwelling processes. The biogeochemical observations also suggest vertical transport along coherent pathways through baroclinic instability. Phytoplankton biomass enhancement occurs as a result, and is subducted below the euphotic layer. The observed oxygen filaments show upwelling and downwelling, providing a mechanism for oxygenating deeper layers and reducing the ventilation of deep low‐oxygenated waters. Understanding the mechanisms of vertical transport can help us evaluate the dynamics of ocean fronts and their impacts on biological carbon storage. Plain Language Summary Ocean fronts are spots of biochemical activity and have an important role in supplying nutrients to the euphotic zone and transport heat and carbon from the surface to the interior. Their spontaneous nature and the intensification of the ageostrophic circulation leads to upwelling/downwelling in the frontal area. The Almeria‐Oran (AO) front has formed from the juxtaposition of fresh inflowing Atlantic waters and more saline re‐circulating Mediterranean waters. The low salinity Atlantic Water has the leading influence on density and generates strong density gradients in the upper ∼200m. Combing glider observations from three parallel gliders with remote sensing and modeling simulations, we can under