Meso and submesoscale oxygen and particle variability in the northern Benguela Upwelling System from glider and model data

The northern Benguela Upwelling System is characterized by significant oxygen and particle anomalies due to the lateral influx of both oxygenated water from the south and low-oxygen water that flows south across the northern Angola-Benguela front (ABF). Mesoscale features developing at the front and in the shelf region of the upwelling system further modulate these anomalies. Here we present the results of a study based on high-resolution glider data collected from the surface to 1000 m depth in February – June 2018 at 100 km off the coast of the northern Benguela (18°S). These in-situ data are further interpreted and generalized using high-resolution model output from the physical Regional Ocean Modeling System (ROMS) coupled to the Biogeochemical Ecosystem Cycling (BEC) model. Using the glider data, we discuss the prevalence of low oxygen events characterized by O₂ < 120 µmol (sub-lethal level), O₂ < 60 µmol (hypoxia) and O₂ < 30 µmol (severe hypoxia) as a function of depth and time. We present two different impacts of eddies on oxygen concentrations: extreme hypoxia associated to a subsurface anticyclone generated at the Benguela shelf, and mixing between high and low oxygen water at the rim of a large surface anticyclone generated at the ABF. Through a spike analysis of the glider data, we study the distribution of large and small particles as a function of depth and time, and relate them to the identified mesoscale structures. We find that the subsurface eddy corresponds to a local low in small particle concentrations, while the frontal anticyclone is associated to a deep export event of both small and large particles. Further, we find that the region is characterized by a deep particle layer between 300 m and 500 m. Using the model output, we identify the drivers of this deep particle layer and deep export events and discuss the relative role of physics and biology in the determination of the vertical distribution of particles in the region of study.