The 2021 Atlantic Niño and Benguela Niño Events: external forcings and air–sea interactions

Abstract

This paper presents a comprehensive analysis of the extreme Atlantic and Benguela Niño events that occurred during the boreal spring–summer of 2021. We conducted sensitivity experiments with a regional ocean–atmosphere coupled model of the tropical Atlantic to investigate the phenology of these interannual events, unravel their triggering mechanisms, and quantify the contributions of local and remote processes. The results revealed that both the 2021 Atlantic and Benguela Niños were triggered by anomalous atmospheric fluxes at the model southern boundary (32° S), leading to a significant and persistent weakening of the South Atlantic Anticyclone. The associated poleward anomalous coastal wind off Africa reduced coastal upwelling and evaporation south of 15° S, initiating the Benguela Niño. Then, the relaxation of the equatorial trade winds forced a downwelling equatorial Kelvin wave, which warmed the eastern equatorial region, marking the onset of the Atlantic Niño. The equatorial event reached full maturity in July 2021 through ENSO-like air-sea interactions in the equatorial basin, enhanced by the atmospheric connection associated with low-level winds converging toward the distant coastal warming. While air–sea interactions in the tropical Atlantic acted as a negative feedback for the coastal warming, the ocean connection with the equatorial variability through the propagation of equatorially-forced downwelling coastal waves intensified the coastal warming, peaking end of May 2021. Overall, this research provides valuable insights into the complex dynamics of Atlantic and Benguela Niños, emphasizing the interconnectedness between these two systems. This has important implications for improving Earth system models which currently struggle to simulate these extreme events.

Sea level variability in Gulf of Guinea from satellite altimetry

“…At the same time, the shape of the coastline and regional bathymetry, which are critical for CTW propagation, play a key role in modifying the dynamics of temperature anomalies along the coast. Changes in river dynamics and coastline morphology can alter the interplay between CTWs and other oceanic processes 37 , further influencing the observed SST/SLA dynamics. These patterns persist across regions and months, highlighting the critical role of local processes, including CTWs, in shaping the intricate temperature anomalies observed in the GoG and the southeast Atlantic.…”

“…7 ). Illig and Bacherely 37 observed a long-term oceanic memory resulting from free-propagating downwelling equatorial waves forced before 2021. These waves warmed the slightly colder than normal surface layer near the equatorial waveguide during the first trimester of 2021, leading to neutral conditions by March 2021, just before the onset of an equatorial event.…”

“…A sizable portion of the EKW energy is transported poleward as CTWs reach the eastern boundary along the zonally oriented coasts of both the GoG and Angola-Namibia coast 12 . The warmest Atlantic Niño events of the entire satellite observation era has been recorded in late 2019 36 and boreal summer 2021 20 , 37 , 38 , of which both events were also reported as a Benguela Niño.…”

The Atlantic Niño Mode: A Thermodynamic or a Dynamic Phenomenon?

“…This suggests that the high correlation between SST anomaly changes in the ABA and the Atl3 may be due to coastally trapped internal waves propagating from the ABA to the Atl3. Illig et al (2020) and Illig and Bachèlery (2024) have shown that when interannual wind-stress forcing is limited to the western-central equatorial Atlantic, Atlantic Niño events occur before coastal events, making it difficult to explain the fact that heat in the Atl3 comes from the south. However, if wind-stress anomalies are also present in the coastal region, the coastal warming occurs before the Atlantic Niño, providing a possible explanation for the result that heat in the Atl3 originates from the south.…”

Planktonic foraminiferal assemblages as tracers of paleoceanographic changes within the Northern Benguela current system since the Early Pleistocene

“…6). Several authors (Walter, 1937;Boyd and Thomas, 1984;Boyd et al, 1987; During the Benguela Niño (Niña) events, an increase (decrease) in sea surface temperatures (SSTs) occurs in the eastern equatorial and south-east Atlantic Ocean due to the relaxation (intensification) of the trade winds (Rouault et al, 2007;Rosell-Melé et al, 2014;Illig and Bachèlery, 2024). The reduction in intensity of the trade winds during the Benguela Niño induces warmer SST in the equatorial Atlantic and the ingress of warm Angola water in the BUS (Hisard, 1980;Philander, 1990;Illig et al, 2004).…”