Contrasting Responses of Dissolved Oxygen in the Northern Benguela Upwelling System to Four Decades of Warming

The Northern Benguela Upwelling System (NBUS) is a highly productive yet low-oxygen region, influenced by the combined effects of circulation and biogeochemical processes. Using data from a high-resolution (∼7 km) coupled physical–biogeochemical model spanning 1980–2020, this study investigates long-term dissolved oxygen (DO) variability in the NBUS. Model results show a total DO loss of ∼1 Tmol in the upper 1,000 m, with a pronounced vertical dipole: the upper 400 m experienced net oxygenation (0.3 Tmol), while the 400–1,000 m layer accounted for significant depletion (1.3 Tmol). Oxygenation in upper layers is linked to enhanced turbulence due to intensified alongshore wind stress, as well as reduced intrusion of oxygen-poor South Atlantic Central Water. In contrast, deoxygenation at depth is primarily associated with increasing ocean heat content (OHC) and vertical stratification, with the OHC increase likely linked to changes in intermediate waters entering the domain. Divergent changes in the oxygen minimum zone (OMZ) include contraction of OMZ20 (waters with DO <20
mmol m−3 ) and deepening of OMZ60 (DO <60 mmol m−3
) and OMZ120 (DO <120 mmol m−3 ) by approximately 120 m. The deepening of OMZ60 and OMZ120, along with bottom water deoxygenation, poses major ecological concerns in the NBUS. Low-oxygen-tolerant pelagic fish are especially at risk, potentially reducing fishery yields. Declining oxygen also threatens benthic biodiversity by eliminating sensitive species and favoring low-oxygen-adapted ones.