The 2019 Benguela Niño

Rodrigue Anicet Imbol Koungue,Arthur Prigent,Peter Brandt,Meike Sena Martins,Joke Lübbecke,Regina R Rodrigues

doi:10.3389/fmars.2021.800103

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.

Highlights

The southeastern tropical Atlantic Ocean hosts the Angola Benguela upwelling system which is one of the most productive marine ecosystems in the world (Chavez and Messié, 2009; Jarre et al, 2015) fuelled by the upwelling of nutrient-rich waters
All those extreme events have already been described in the literature (Florenchie et al, 2004; Reason et al, 2006; Rouault et al, 2007; Ostrowski et al, 2009; Rouault et al, 2009; Lübbecke et al, 2010; Rouault, 2012; Lutz et al, 2013; Imbol Koungue et al, 2017; Rouault et al, 2018; Imbol Koungue et al, 2019) except for the 2019 Benguela Niño which is the focus of this study
Compared to previous Benguela Niño events that occur during boreal spring, the 2019 Benguela Niño developed at the end of the year with sea surface temperature (SST) anomalies exceeding 2◦C in November 2019 in the Angola Benguela front region (Figure 3E)

Summary

Introduction

The southeastern tropical Atlantic Ocean hosts the Angola Benguela upwelling system which is one of the most productive marine ecosystems in the world (Chavez and Messié, 2009; Jarre et al, 2015) fuelled by the upwelling of nutrient-rich waters. During a Benguela Niño (Niña) event, the SSTs can be up to 3◦C higher (lower) than the climatology in the Angola Benguela area (ABA, 8◦E – coast; 10–20◦S) Those extreme events may impact the marine ecosystem, biological productivity and fisheries in the Angola Benguela upwelling system by modulating the upward supply of nutrients (Bachèlery et al, 2016b) as they affect the upwelling intensity and upperocean mixing (Gammelsrød et al, 1998; Boyer et al, 2001; Blamey et al, 2015). Benguela Niños are often associated with floods in Angola and Namibia and strongly enhanced rainfall in the arid Namib desert (Rouault et al, 2003; Hansingo and Reason, 2009), whereas Benguela Niñas often lead to droughts over the Angola Benguela region (Koseki and Imbol Koungue, 2020)

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Conclusion