Tropical low oxygen extreme events caused by persistent submesoscale coherent vortices

Submesoscale coherent vortices (SCVs) have been frequently observed in the eastern tropical Atlantic (between 12°S and 12°N) based on moored and shipboard observations. They are located well below the mixed layer with no surface signature and, thus, undetectable by remote sensing making in-situ observations and modeling indispensable. The SCVs persist and are relatively long-lived and coherent, despite the increasing suppression of geostrophic balance and the rapid change in the Coriolis parameter (ß-effect) near the equator. These factors typically suggest predominant wave-like structures in this region. Additionally, the energetic zonal current system, which stretch and shear the vorticity fields, further complicate the formation of closed vortex structures. Ship-based oxygen measurements conducted in the area between 6°-12°N, 24°-18°W reveal that approximately two-third of these SCVs are associated with low oxygen cores with dissolved oxygen concentrations less than 60 µmol kg^-1 (minimum 40 µmol kg^-1). These values are significantly lower than the climatological averages for this depth range (> 80 µmol kg^-1). Both, observed water mass characteristics and the analysis of an eddy-resolving ocean-biogeochemistry model indicate that the majority of SCVs originate from the eastern boundary and may last for longer than half a year. While propagating westward into a higher potential vorticity environment, anticyclonic SCVs with a low PV core are more effectively isolated and feature longer life times than cyclonic SCVs with a high PV core. The vertical structure of the dominating anticyclonic SCVs is characterized by higher baroclinic modes 4-10, associated with a Rossby radius of 34 -13 km respectively, which is in agreement with the observed eddy radius and well below the 1^st baroclinic Rossby radius of deformation in the region (> 100 km). This study does not only increase our understanding of submesoscale dynamics in equatorial regions, but also how SCVs contribute to the formation of hypoxic zones in the open ocean due to their association with low-oxygen extremes. These hypoxic regimes have the potential to directly impact pelagic fish, biodiversity, and biogeochemical cycles.