Reconstructing the dynamics of the Almeria-Oran front using 3 glider vehicles over the spring period of 2018

Numerical simulations, remote sensing and in-situ observations in the western Mediterranean in spring 2018 indicate the variability of the position and shape of the Almeria-Oran (AO) Front. The AO front is formed from the juxtaposition of fresh inflowing Atlantic waters and more saline re-circulating Mediterranean waters at the eastern boundary of the eastern Alboran Gyre. Previous published work and contemporary remote sensing and modelling suggest that wave-like perturbations to the frontal boundary propagate rapidly along the front, increasing the risk of asynopticity, a generic problem in ocean observation. Here, we examine the oscillations in the density slope across the front to determine the phase speed and the wavenumber of the fastest growing mode of instability using the high-resolution nature of the glider observations. Our estimations show a phase speed of ~ 20 cm/s and the wavelength of the Almeria-Oran front to be ~ 80km, similar to previous studies. Having carefully examined the resulting potential and relative vorticity distributions, which will also be presented, we are confident of the ability of our approach to reconstruct the time/space evolution of the AO front. The Q-vector formulation of the omega equation was used to estimate the vertical velocities of the sub-mesoscale ageostrophic circulation. Our results, combined with remote sensing observations and modelling, provide a novel quantitative approach to estimate the vertical velocity field and determine observational errors using high resolution glider data. The biogeochemical observations also support the vertical transport as revealed by an enhancement of Chl a and dissolved Oxygen concentrations at the frontal interface. The study results provide critical insights about the vertical transport pathways at the AO front. A better understanding of vertical transport will help us not only to evaluate the dynamic importance of ocean fronts, but also to understand their impacts on the biological storage of carbon through the three-dimensional transport pathways that they provide.