Oxygen minimum zones (OMZs) dominate the eastern tropical Atlantic at 300–700 m depth, separated by an equatorial oxygen maximum maintained by strong zonal currents (Brandt et al., 2012). In cooperation with PIRATA, repeat sections and moorings have revealed strong deoxygenation in the upper 400 m of the ETNA since 2006, largely driven by circulation changes, while longer-term trends point to a multidecadal oxygen decline linked to both anthropogenic forcing and natural variability (Brandt et al., 2015; Schmidtko et al., 2017). Mooring data further highlighted the role of equatorial deep jets and mesoscale eddies in driving oxygen variability and supplying oxygen into the OMZ core (Hahn et al., 2014). Extreme low-oxygen “dead-zone” events within anticyclonic eddies have been documented, with major ecosystem impacts (Karstensen et al., 2015). Since 2017, PIRATA buoys provide real-time oxygen records, enabling continuous monitoring of OMZ dynamics and low-oxygen events. Long-term PIRATA observations also revealed a strengthening of the Equatorial Undercurrent (EUC) between 2008 and 2018, increasing subsurface oxygen and counteracting climate-driven deoxygenation in the region (Brandt et al., 2021).
