Equatorial Atlantic Ocean dynamics across time scales from sustained velocity observations between 2001-2025

The tropical Atlantic Ocean is characterized by energetic mean currents, pronounced equatorial wave activity, and enhanced seasonal upper-ocean instability. Variability and extreme events in this region exert a strong influence on weather and climate over adjacent continents and contribute to atmospheric teleconnections with the other tropical ocean basins. Despite this importance, sustained observations of equatorial Atlantic Ocean dynamics remain limited.

To address this gap, a moored observatory has been operated at 0°, 23°W since December 2001, with evolving configurations. In its current design the observatory consists of a subsurface mooring equipped with Acoustic Doppler Current Profilers measuring velocities in the upper 900 m and a McLane Moored Profiler sampling the 900-3,300 m depth range. Additional single-point current meters are deployed at variable depths. These time series are combined with current meter measurements from the nearby PIRATA moored surface buoy and with full-depth shipboard velocity profiles collected during regular French and U.S. PIRATA mooring service cruises.

In this presentation, we provide updated perspectives and new results on the structure, variability and trends of the zonal and meridional velocity components of the equatorial Atlantic circulation. For instance, in the upper ocean, the eastward Equatorial Undercurrent (EUC) exhibits a marked decline in volume transport since 2018, following a decade of strengthening transport and enhanced oxygen ventilation of the eastern equatorial Atlantic between 2008 and 2018. Consistent with this decadal variability, the mean core depth of the EUC has shoaled by approximately 10 m relative to 2018.

Variability in the meridional velocity component is dominated by intraseasonal time scales (~10-50 days), reflecting the presence of tropical instability waves (TIWs) in the upper 100 m, and equatorial Yanai waves at greater depths. While observations suggest an apparent increase in TIW activity, which are on average most pronounced from July to September, a detailed seasonal analysis indicates that the inferred long-term trends largely arise from a systematic shift toward an earlier onset of TIW activity rather than from a sustained intensification.

Since 2022, a combined full-depth velocity product has been released shortly after each mooring recovery (Tuchen et al., 2022). Maintaining long-term observations such as the equatorial moored observatory at 23°W is logistically and financially demanding, but remains essential for detecting and interpreting decadal variability and long-term trends in the tropical Atlantic.

 

Tuchen, F. P., Brandt, P., Hahn, J., Hummels, R., Krahmann, G., Bourlès, B., Provost, C., McPhaden, M. J., and Toole, J. M. (2022): Two Decades of Full-Depth Current Velocity Observations From a Moored Observatory in the Central Equatorial Atlantic at 0°N, 23°W. Front. Mar. Sci. 9:910979. https://doi.org/10.3389/fmars.2022.910979