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

Abstract

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° 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.

Fluctuations of the Atlantic North Equatorial Undercurrent and Associated Changes in Oxygen Transports

“…By combining moored zonal velocities at 5°N, 23°W and meridional ship sections along ∼23°W we obtained a NEUC transport time series from June 2006 to February 2008 and November 2009 to January 2018. In contrast to previous studies (Burmeister et al, 2019;Goes et al, 2013;Hüttl-Kabus & Böning, 2008), neither the moored zonal velocity at 5°N (Figure 1) nor the reconstructed eastward NEUC transport ( Figure 2) exhibits a pronounced seasonal cycle. We find that both time series are dominated by strong intraseasonal eastward flow events which can peak throughout the year (Figures 1 and 2).…”

“…Another explanation may be that some strong eastward flow events are not connected to the well-ventilated western boundary but are instead supplied out of the westward flow of low-oxgen waters north and south of the NEUC. For example, in the meridional section of February 2018, we find strong eastward velocities between 4°N and 5°N at 50 and 100 m depth that are associated with low oxygen concentrations, which might be due to a recirculation of low-oxygen waters typically present south of the NEUC (Figures 3c and 3f Burmeister et al, 2019). As the zonal velocity events with a duration between 1 and 5 months overlap with the dominant frequency band of meridional velocity (30 to 70 days), it is not possible to clearly differentiate between the effect of meridional and zonal velocity on oxygen.…”

“…Only shipboard ADCP (Fischer et al, 2003), hydrography and oxygen sections that cover at least the upper 350 m between 0° and 10°N are used. The ship sections are an extension of the data set used in Burmeister (2019, see Text S1 for more details).…”

“…In general, NEUC transport estimates derived from meridional ship sections are obscured by mesoscale activity (Weisberg & Weingartner, 1988) and interannual variability (Hüttl-Kabus & Böning, 2008). So far, studies based on shipboard velocity observations have not been able to detect a seasonal cycle of the NEUC (Bourlès et al, 1999(Bourlès et al, , 2002Burmeister et al, 2019;Schott et al, 1995Schott et al, , 2003Urbano et al, 2008).…”

“…Until now, the seasonal to long‐term variability of the NEUC has been investigated based on model output (Burmeister et al., 2019; Hüttl‐Kabus & Böning, 2008) or geostrophic velocities derived from a combination of hydrography and satellite data (Goes et al., 2013). In these studies, a seasonal cycle of the NEUC with amplitudes of 1 to 3.5 Sv was identified.…”