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Abstract
A warming and freshening trend of the mixed layer in the upper southeastern tropical Atlantic Ocean (SETA) is observed by the Argo float array during the time period of 2006–2020. The associated ocean surface density reduction impacts upper-ocean stratification that intensified by more than 30% in the SETA region since 2006. The initial typical subtropical stratification with a surface salinity maximum is shifting to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. During the same period isopycnal surfaces in the upper 200 m are shoaling continuously. Observed wind stress changes reveal that open ocean wind curl-driven upwelling increased, however, partly counteracted by reduced coastal upwelling due to weakened alongshore southerly winds. Weakening southerly winds might be a reason why tropical surface waters spread more southward reaching further into the SETA region. The mixed layer warming and freshening and associated stratification changes might impact the marine ecosystem and pelagic fisheries in the Angolan and northern Namibian upwelling region.
Highlights
The southeastern tropical Atlantic (SETA: 10–20◦S, 5◦W–15◦E) encompasses a highly-productive ecosystem off Angola which is located just north of one of the major global coastal upwelling regions: the Benguela upwelling system (Ostrowski et al, 2009; Kopte et al, 2017)
Siegfried et al (2019) showed that the Angola Current is fed with tropical South Atlantic Central Water (SACW) via the eastward flowing Equatorial Undercurrent (EUC), the South Equatorial Undercurrent (SEUC) and the South Equatorial Countercurrent (SECC) (Figure 1a)
Note that for the analysis of the SETA region (10◦S–20◦S, 5◦W–15◦E) we extended the time series with Argo observations until November 2020, in order to overcome a gap in the data in this particular region in the mid of the year of 2019 (Argo, 2020)
Summary
The southeastern tropical Atlantic (SETA: 10–20◦S, 5◦W–15◦E) encompasses a highly-productive ecosystem off Angola which is located just north of one of the major global coastal upwelling regions: the Benguela upwelling system (Ostrowski et al, 2009; Kopte et al, 2017). Siegfried et al (2019) showed that the Angola Current is fed with tropical South Atlantic Central Water (SACW) via the eastward flowing Equatorial Undercurrent (EUC), the South Equatorial Undercurrent (SEUC) and the South Equatorial Countercurrent (SECC) (Figure 1a). These currents and the location of the ABF undergo large seasonal variations and the area influenced by the presence of tropical SACW varies as well (Rouault, 2012).
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