Interannual Variability of the Atlantic North Equatorial Undercurrent and Its Impact on Oxygen

The North Equatorial Undercurrent (NEUC) has been suggested to act as an important oxygen supply route toward the oxygen minimum zone in the eastern tropical North Atlantic. Observational estimates of the mean NEUC strength are uncertain due to the presence of elevated mesoscale activities, and models have difficulties in simulating a realistic NEUC. Here we investigate the interannual variability of the NEUC and its impact onto oxygen based on the output of a high-resolution Ocean General Circulation Model (OGCM) and contrast the results with an unique data set of 21 ship sections along 23 degrees W and a conceptual model. We find that the interannual variability of the NEUC in the OGCM is related to the Atlantic meridional mode with a stronger and more northward NEUC during negative Atlantic meridional mode phases. Discrepancies between the OGCM and observations suggest a different role of the NEUC in setting the regional oxygen distribution. In the model a stronger NEUC is associated with a weaker oxygen supply toward the east. We attribute this to a too strong recirculation between the NEUC and the northern branch of the South Equatorial Current in the OGCM. Idealized experiments with the conceptual model support the idea that the impact of NEUC variability on oxygen depends on the source water pathway. A strengthening of the NEUC supplied out of the western boundary acts to increase oxygen levels within the NEUC. A strengthening of the recirculations between NEUC and the northern branch of the South Equatorial Current results in a reduction of oxygen levels within the NEUC. Plain Language Summary In the eastern tropical North Atlantic a zone of low-oxygen waters exists between 100 and 700m due to high oxygen consumption and a weak exchange of water masses. Long-term oxygen changes in this zone have been reported with potential impacts on, for example, ecosystems including fish populations. The water masses in that region are exchanged among others via weak eastward and westward currents. The mean eastward-flowing North Equatorial Undercurrent (NEUC) transports oxygen-rich waters from the western basin into the eastern low-oxygen zone, suggesting that a stronger NEUC supplies more oxygen-rich water toward the eastern basin. In this study we investigate the year-to-year variability of the NEUC and its impact on oxygen. For our analysis, we are using ship observations and model simulations. We find some discrepancies between them that we attribute to a too strong recirculation between the NEUC and the westward-flowing current just south of it in the model. This recirculation impacts the variability of the eastward oxygen supply, as the westward current is transporting low-oxygen waters. In the model, a higher recirculation between the currents results in a stronger NEUC transporting lower-oxygen waters, a mechanism for oxygen variability that could not be conjectured from observations so far.