Oxygen Variability in the Offshore Northern Benguela Upwelling System From Glider Data

Area of Science:

• Oceanography
• Marine Ecology
• Biogeochemistry

Background:

• Low-oxygen events (hypoxia) significantly impact marine ecosystems but are poorly understood in the offshore northern Benguela Upwelling System (BenUS) due to their episodic nature.
• Traditional shipboard measurements struggle to capture the variability and drivers of these events in the BenUS.

Purpose of the Study:

• To investigate the variability and drivers of low-oxygen events in the offshore northern Benguela Upwelling System.
• To analyze the influence of water mass dynamics on oxygen concentrations using high-resolution data.

Main Methods:

• Utilized 4 months of high-resolution glider data collected offshore at 18°S between February and June 2018.
• Analyzed subsurface oxygen (O2) concentrations at depths ranging from 100-500 m.
• Investigated the role of water mass origins (Angola vs. southern Benguela) and oceanographic features like eddies.

Main Results:

• Oxygen concentrations are controlled by the subsurface alternation of low-oxygen Angola-derived water and oxygenated southern water.
• Intermittent hypoxia (O2 < 60 μmol kg-1) occurred approximately 30% of the time, driven by Angola-derived tropical water pulses.
• Hypoxic events were persistent at 300-450 m and sporadic with weekly durations at 100-300 m.
• Extreme hypoxia (minima of 16 μmol kg-1) was linked to an anticyclonic eddy with no surface signature.

Conclusions:

• Physical drivers, particularly subsurface water mass dynamics and eddy activity, dominate short-term oxygen variability in the offshore northern BenUS.
• A dynamic coupling exists between the Angola and Benguela systems, influencing oxygen levels.
• Long-term, high-resolution monitoring is crucial for understanding future changes in tropical oxygen minima and lateral fluxes in this region.