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

Abstract

<p>Benguela Niños are events of anomalous Sea Surface Temperature (SST) increase in the Southeastern Tropical Atlantic Ocean. In 1995, the strongest Benguela Niño observed in the satellite era took place. It had a drastic impact on the Angola-Benguela Area (ABA, 8ºS – 20ºS, 8ºE to the coast) ecosystem, including high mortality, poor recruitment, and southward shift of sardine populations, as well as reductions in the number of benthic organisms. Although low Sea Surface Salinity (SSS) values extending as far south as 18ºS have been observed during this event, the role of freshwater input for the SST increase in the 1995 Benguela Niño has not been analyzed yet. In this study, we use satellite data, CTD profiles, and reanalysis products to investigate the impact that freshwater anomalies from anomalously high Congo river discharge (CRD) and precipitation might have had on the evolution of the 1995 Benguela Niño. We find that in the onset phase of the event a freshwater plume from the north was spreading southward towards the Angola-Namibia coastal area, concomitant with signatures of positive Barrier Layer Thickness (BLT) and stratification (N2) anomalies. At the same time, a strong poleward Angola current anomaly was observed. Positive SST anomalies peaked in March when SSS values averaged over the ABA were almost 3 psu lower than normal. Our analysis suggests that the anomalous CRD combined with higher than usual precipitation in November/December 1994 generated a negative SSS plume north of ABA, which was advected into the Angola-Namibia coastal region by the poleward surface current anomaly, increasing ocean stability, and reducing the mixing. A Mixed Layer Heat Budget analysis suggests that both anomalous advection and absence of entrainment contributed to the surface warming while the net surface heat flux provided a damping effect. Thus, the high freshwater input that was advected southwards inhibited the entrainment of cool subsurface waters into the surface mixed layer in the ABA, which contributed to the SST increase in the exceptionally strong 1995 Benguela Niño event.</p>