The 2019 Benguela Niño

Abstract

High interannual sea surface temperature anomalies of more than 2°C were recorded along the coasts of Angola and Namibia between October 2019 and January 2020. This extreme coastal warm event that has been classified as a Benguela Niño, reached its peak amplitude in November 2019 in the Angola Benguela front region. In contrast to classical Benguela Niños, the 2019 Benguela Niño was generated by a combination of local and remote forcing. In September 2019, a local warming was triggered by positive anomalies of near coastal wind-stress curl leading to downwelling anomalies through Ekman dynamics off Southern Angola and by anomalously weak winds reducing the latent heat loss by the ocean south of 15°S. In addition, downwelling coastal trapped waves were observed along the African coast between mid-October 2019 and early January 2020. Those coastal trapped waves might have partly emanated from the equatorial Atlantic as westerly wind anomalies were observed in the central and eastern equatorial Atlantic between end of September to early December 2019. Additional forcing for the downwelling coastal trapped waves likely resulted from an observed weakening of the prevailing coastal southerly winds along the Angolan coast north of 15°S between October 2019 and mid-February 2020. During the peak of the event, latent heat flux damped the sea surface temperature anomalies mostly in the Angola Benguela front region. In the eastern equatorial Atlantic, relaxation of cross-equatorial southerly winds might have contributed to the equatorial warming in November 2019 during the peak of the 2019 Benguela Niño. Moreover, for the first time, moored velocities off Angola (11°S) revealed a coherent poleward flow in the upper 100 m in October and November 2019 suggesting a contribution of meridional heat advection to the near-surface warming during the early stages of the Benguela Niño. During the Benguela Niño, a reduction of net primary production in the Southern Angola and Angola Benguela front regions was observed.

The Extraordinary Equatorial Atlantic Warming in Late 2019

“…The rapid warming in October and November was not limited to the equator but extended southward to most of the southern subtropical Atlantic. Particularly warm SST along the southwest African coast indicates the occurrence of a Benguela Niño, which may have partially contributed to the equatorial wind anomalies (Hu & Huang, 2007; Imbol Koungue et al., 2021). We note, however, that there are basin‐scale surface wind anomalies from September through November that suggest the involvement of a large‐scale process.…”

Seasonal Cycle of Sea Surface Salinity in the Angola Upwelling System

“…(2017) and recently updated by Imbol Koungue et al. (2021) to present the direct velocity observations of Angolan Current at 11°S. The climatology of the moored current is based on the full available mooring data (2013–2021) and the model current is based on the period from 1982 to 2015 (Figure 7).…”

“…A current meter mooring has been deployed at about 11°S off the Angolan coast since 2013 to measure the current speed in the water column up to about 45 m below the sea surface (Imbol Koungue et al., 2021; Kopte et al., 2017). The current velocity is measured by an ADCP mounted on the mooring cable at 500 m depth with an upward looking 75 kHz long Ranger up to 45 m below the sea surface.…”

“…The current velocity is measured by an ADCP mounted on the mooring cable at 500 m depth with an upward looking 75 kHz long Ranger up to 45 m below the sea surface. The mooring data is freely available from July 2013 to May 2021 (Imbol Koungue et al., 2021).…”

Oxygen Variability in the Offshore Northern Benguela Upwelling System From Glider Data

“…The glider data were collected during February–June 2018, that is, during the period spanning from the low upwelling season to the early phase of the winter upwelling season of the northern Benguela (Hutchings et al., 2009 ). SST anomalies for 2018 indicate neutral Benguela Niño conditions (Imbol Koungue et al., 2021 ). Satellite images and horizontal velocity products (Figure 2 ) show a succession of very different conditions in the region surrounding the glider.…”