An extreme cold sea surface temperature event occurred in the Atlantic cold tongue region in boreal summer 2009. It was preceded by a strong negative Atlantic meridional mode event associated with north‐westerly wind anomalies along the equator from March to May. Although classical equatorial wave dynamics suggest that westerly wind anomalies should be followed by a warming in the eastern equatorial Atlantic, an abrupt cooling took place. In the literature two mechanisms—meridional advection of subsurface temperature anomalies and planetary wave reflection—are discussed as potential causes of such an event. Here, for the first time we use in situ measurements in addition to satellite and reanalysis products to investigate the contribution of both mechanisms to the 2009 cold event. Our results suggest that meridional advection is less important in cold events than in corresponding warm events, and, in particular, did not cause the 2009 cold event. Argo float data confirm previous findings that planetary wave reflection contributed to the onset of the 2009 cold event. Additionally, our analysis suggests that higher baroclinic modes were involved.

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.

The Rockall Trough (RT, Figure 1) serves as a key pathway for the poleward transport of oceanic heat, salt, carbon and nutrients. In the eastern subpolar North Atlantic (SPNA) the North Atlantic Current (NAC) consists of several branches with the southeastern branch, the warmest and saltiest, flowing through the RT. In addition, Eastern North Atlantic Water flows from the Bay of Biscay into the RT along the continental shelf break in the European Slope Current (ESC) (

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°N, 23°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°W at the NEUC core position.