Oxygen measurements at the southern and northern boundaries of the AMOC

Near the eastern boundary at 34.5ºS (known as SAMBA-E), dissolved oxygen sensors were deployed within the Agulhas Leakage. Two instruments were deployed in October 2020 and recovered in September/October 2022. Dissolved oxygen concentrations typically covary with temperature and salinity indicating the intrusion of different water masses to the study site. Additionally, data suggest the passage of Agulhas Rings past the mooring locations. The dissolved oxygen data have been formatted and accessioned through the South African National Marine Information Management System.

Near the western boundary at 34.5ºS two new full-depth moorings, one under iAtlantic and one under the sister project TRIATLAS, were deployed in December 2022. The iAtlantic mooring includes dissolved oxygen measurements at the core of the Brazil Current. These moorings are significant additions to the western component of the AMOC monitoring array in the subtropical South Atlantic and provide, for the first time, time series observations of mid-depth dissolved oxygen, velocity, temperature, and salinity. The observations will be used primarily to study collocated changes in ocean currents and water mass properties relevant to marine ecosystems in both the Brazil and Agulhas Currents to contribute to regional and global assessments.

At 57.5 ̊N, dissolved oxygen sensors were deployed at the eastern boundary within a branch of the North Atlantic Current. Instruments were first deployed in May 2017 with the latest recovery in July 2022. Data from 65 m show a clear seasonal signal with a maximum dissolved oxygen saturation and concentration associated with the Spring bloom. Additionally, dissolved oxygen saturation shows a secondary peak in October, possibly linked to an Autumn bloom. The lowest concentrations (and saturations) are observed at 960 m within a mid-depth lower oxygen layer. The highest dissolved oxygen variability is not observed at 65 m, but rather at 750 m, which lies in a depth of rapidly changing dissolved oxygen between the well-oxygenated upper waters and lower oxygen layer.

The dissolved oxygen measurements at 57.5 ̊N are being used to derive nutrient and carbon fields to enable the transport of these parameters by the AMOC to be quantified. Nutrient and carbon transports associated with the easternmost branch of the North Atlantic Current are highly variable, showing the value in deriving these from moored observations rather than hydrographic sections.

Future comparisons of measurements in the Brazil Current, at the Agulhas Leakage and from the North Atlantic Current will improve our understanding of exchanges at sensitive gateways in the Atlantic Ocean. In addition, the oxygen sensors deployed on the moorings across SAMBA-E will provide, for the first-time, high-resolution data of the impact that the Agulhas Leakage has on the mid-depth ventilation of this region.