The Influence of Freshwater Input on the Evolution of the 1995 Benguela Niño

• Published in: Journal of geophysical research. Oceans 2024-02, Vol.129 (2), p.n/a
• Main Authors: Aroucha, L. C., Lübbecke, J. F., Körner, M., Imbol Koungue, R. A., Awo, F. M.
• Format: Article
• Language: English
• Subjects: Advection; Availability; barrier layer; Barrier layers; Benguela Niño; Coastal winds; Coasts; Evolution; Fish; Fresh water; Freshwater; freshwater input; geophysics; heat; Heat budget; Heat loss; Inland water environment; Mixed layer; Oceans; Precipitation; rain; Rainfall; remote sensing; River discharge; River flow; River mouth; River mouths; Rivers; Salinity; Salinity effects; Satellite data; Satellite observation; Satellites; Sea surface; Sea surface temperature; Surface boundary layer; Surface layers; Surface salinity; Surface temperature; Surface water; surface water temperature; Temperature gradients; tropical Atlantic; Turbulent mixing; Water discharge; Wind; Wind speed; Wind stress; Wind stress curl; Winds
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020241

Benguela Niño events are characterized by strong warm sea surface temperature (SST) anomalies off the Angolan and Namibian coasts. In 1995, the strongest event in the satellite era took place, impacting fish availability in both Angolan and Namibian waters. In this study, we use direct observations, satellite data, and reanalysis products to investigate the impact that the up‐until‐now unnoticed mechanism of freshwater input from Congo River discharge (CRD) and precipitation had on the evolution of the 1995 Benguela Niño. In the onset phase of the event, anomalous rainfall in November/December 1994 at around 6°S, combined with a high CRD, generated a low salinity plume. The plume was advected into the Angola‐Namibia region in the following February/March 1995 by an anomalously strong poleward surface current generated by the relaxation of the southerly winds and shifts in the coastal wind stress curl. The presence of this low surface salinity anomaly of about −2 psu increased ocean stability by generating barrier layers, thereby reducing the turbulent heat loss, since turbulent mixing acted on a weak vertical temperature gradient. A mixed layer heat budget analysis demonstrates that southward advection of Angolan waters drove the warming at the onset, while reduced mixing played the main role at the event's peak. We conclude that a freshwater input contributed to the SST increase in this exceptionally strong event and suggest that this input can influence the SST variability in Angola‐Namibia waters through a combination of high CRD, precipitation, and the presence of a strong poleward surface current. Plain Language Summary Benguela Niño events are characterized by excessive warming of the sea surface temperature off the Angolan and Namibian coasts. One of the strongest‐ever recorded warm events dates back to 1995, impacting fish availability in both Angolan and Namibian waters. In our research, we investigate if freshwater from rain and from the Congo River could have impacted the evolution of this 1995 Benguela Niño. In the event's early stage, high precipitation and river discharge generated a low salinity pool at the Congo River mouth, which in February/March 1995 was taken to the south by an exceptionally strong surface current, generated by changes in wind strength and direction at the African coast. This low sea surface salinity in a shallow layer in the upper meters of the ocean increased the ocean's stability. As the stabilized waters diminished