Observations of the Almeria-Oran front by underwater gliders

Fleets of underwater gliders were used to observe the Almeria-Oran front during 2018 and 2019 in an effort to resolve three-dimensional flow including the submesoscale across-front circulation cell. The gliders were deployed to occupy repeated sections oriented across the front in lanes spaced along the front. In 2018, three gliders were deployed during 24 May – 3 August for 190 glider-days, covering 4000 km in 1100 dives to 700 m, and collecting over 30 across-front sections in three lanes separated by 17.5 km. In 2019, eight gliders were deployed during 20 March – 20 May for 400 glider-days, covering 9300 km in 2400 dives, and collecting over 60 sections in five lanes separated by 8.75 km. Measured variables included temperature, salinity, velocity, chlorophyll fluorescence, and acoustic backscatter. The Almeria-Oran front was distinguished by a salinity front roughly 20 km wide, with salty water of Mediterranean origin on the east side and fresh Atlantic water to the west. The along-front jet approached 1 m/s at the surface, and an across-front convergence was apparent in the surface layer. The biological variables chlorophyll fluorescence and acoustic backscatter were suggestive of a downward flow on the dense side of the front. Objective maps were calculated towards the goal of estimating gradients and diagnosing vertical velocity. Along-front streamers of high salinity and high fluorescence indicated subduction of surface water. Vorticity was estimated from the objective maps, yielding a positively skewed distribution, consistent with past measurements and theory. One glider was programmed to track a subsurface isotherm with the objective of following active subduction. The glider adjusted buoyancy autonomously, staying within a vertical root-mean-square distance of 5 m from the desired isotherm. A series of one-day tracks were done, with the largest downward velocities observed to be roughly 50 m/day.