Diverging Oxygen Trends in the Northern Benguela Upwelling System over the Last Four Decades

The northern Benguela upwelling system (NBUS), part of the highly productive Benguela Upwelling System, is characterized by persistently low oxygen concentrations, with critical ecological and economic importance. In the NBUS, oxygen variability is governed by complex physical�biogeochemical interactions, particularly the influence of South Atlantic Central Water (SACW); however, these interactions remain poorly represented in global Earth system models due to their coarse spatial resolution. Here, we use a high-resolution (~7 km) coupled physical�biogeochemical model to assess long-term dissolved oxygen dynamics over four decades (1980�2020). Our results show that the NBUS lost a total of �12.3 TeraMoles of oxygen in the upper 1000 m over four decades, with a striking vertical dipole: oxygenation in the upper 400 m (spanning the epipelagic and upper mesopelagic layers, +4.1 TeraMoles) and de-oxygenation in the lower mesopelagic layer (400�1000 m, �16.4 TeraMoles). Oxygen gains in the upper 400 m were linked to reduced biological oxygen demand and declining SACW intrusion, while losses in the lower mesopelagic were driven by ocean warming and increased stratification. The oxygen budget reveals that 92% of DO fluxes in the upper 1000 m are driven by physical transport across lateral, surface, and bottom boundaries, while only 8% stem from local production and respiration. Concurrently, the oxygen minimum zone (OMZ120, where oxygen <120 mmol O2/m3) deepened by ~120 m, particularly offshore, while its core (OMZ20, <20 mmol O2/m3) contracted due to upper-layer oxygenation. These changes indicate rising stress on the NBUS lower mesopelagic zone, with potential consequences for demersal fisheries, benthic ecosystems, and regional nutrient cycling.