Characterization of the Shallow Oxygen Minimum Zone of the Eastern Tropical North Atlantic

Abstract

The Eastern Tropical North Atlantic (ETNA) hosts a shallow Oxygen Minimum Zone (sOMZ) extending roughly from 50 to 250 m depth, where oxygen concentrations typically fall to 40–60 μmol L⁻¹. Despite its ecological and biogeochemical importance, the spatial and temporal characteristics of the hypoxic events that shape this sOMZ remain poorly understood. Here we use a high-resolution physical-biogeochemical model to investigate the occurrence, intensity, and structure of hypoxia within the upper 300 m of the ETNA. A combined Eulerian–Lagrangian framework is applied to identify and track Lagrangian Hypoxic Events (LHEs) and to classify them according to the dominant physical processes sustaining them. Coastal and offshore hypoxia exhibit distinct dynamics. Coastal hypoxia is widespread, predominantly seasonal, and strongest along the continental margin, where events are intense but vertically confined near the seabed. Coastal hypoxia peaks in summer–autumn, consistent with seasonal reductions in ventilation, though the underlying mechanisms are not directly diagnosed here. In contrast, offshore hypoxia is primarily controlled by mesoscale eddies that form near the coast, trap low-oxygen waters, and intensify such conditions as they propagate westward. Offshore LHEs display greater spatial extent and vertical reach, particularly during autumn and winter. Although eddy-driven hypoxia dominates offshore volumes, large coastal events with long duration also contribute substantially to the structure of the sOMZ. Together, these results provide the first detailed assessment of hypoxic event dynamics in the ETNA's sOMZ and highlight the need for improved observations to quantify their ecological and biogeochemical impacts.