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Although the core velocity of the Atlantic North Equatorial Undercurrent (NEUC) is low (0.1-0.3 m s(-1)), it has been suggested to act as an important oxygen supply route towards the oxygen minimum zone in the eastern tropical North Atlantic. For the first time, the intraseasonal to interannual NEUC variability and its impact on oxygen are investigated based on shipboard and moored velocity observations around 5 degrees N, 23 degrees W. In contrast to previous studies that were mainly based on models or hydrographic data, we find hardly any seasonal cycle of NEUC transports in the central Atlantic. The NEUC transport variability is instead dominated by sporadic intraseasonal events. Only some of these events are associated with high oxygen levels suggesting an occasional eastward oxygen supply by NEUC transport events. Nevertheless, they likely contribute to the local oxygen maximum in the mean shipboard section along 23 degrees W at the NEUC core position.

Plain Language Summary In the eastern tropical North Atlantic, a zone of low-oxygen waters exists between 100 and 700 m depth due to high oxygen consumption and weak exchange of water masses. Long-term oxygen changes in this zone have been reported with possible impacts on, for example, the ecosystem or the available habitat for fish. Typically, water masses in that region are exchanged via weak eastward and westward currents. As the oxygen concentration in the western Atlantic basin is high, an eastward current such as the North Equatorial Undercurrent (NEUC) may transport oxygen-rich waters into the eastern low-oxygen zone. Given the east-west difference in oxygen concentration, we assume that a stronger NEUC is transporting more oxygen-rich water from the western towards the eastern basin. This is the first study that investigates the variations in NEUC transport based on direct velocity measurements at 5 degrees N, 23 degrees W. In contrast to previous studies based on model simulations or hydrographic data, we do not find a seasonal cycle of the NEUC transport. Instead, changes of the NEUC transport are dominated by bursts of eastward flow which persist for a few months. These eastward flow bursts are only occasionally associated with higher oxygen concentrations.