Southeastern Tropical Atlantic Changing From Subtropical to Tropical Conditions

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.

The 2019 Benguela Niño

“…This upwelling system is marked by the presence of a sharp meridional temperature gradient called the Angola Benguela front (ABF) located in the region between 15 • S and 18 • S (Mohrholz et al, 2001;Veitch et al, 2006) which separates the warm tropical waters in the north to the cold upwelled waters in the south (Figure 1A). The region exhibits high sea surface temperature (SST) variability at a wide range of frequencies varying from sub-monthly to decadal timescales (Bachèlery et al, 2020;Imbol Koungue and Brandt, 2021;Roch et al, 2021). The interannual timescale is marked by the occurrence of extreme warm events, the socalled Benguela Niños (Shannon et al, 1986;Florenchie et al, 2004;Rouault et al, 2007Rouault et al, , 2018Lübbecke et al, 2010;Bachèlery et al, 2016aBachèlery et al, , 2020Imbol Koungue et al, 2017 and their cold counterparts, the Benguela Niñas (Florenchie et al, 2004;Koseki and Imbol Koungue, 2020).…”

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

“…Similar procedure has been applied by Roch et al. (2023) for the isopycnal gridding. For a detailed description on the interpolation procedure the reader is referred to the Supporting Information (Text S1 in Supporting Information ).…”

“…Further quality checks have been carried out following Roch et al. (2023): Profiles have monotonically increasing pressure profiles, reach at least 1,000 dbar and no larger pressure difference than 150 dbar of consecutive pressure measurements is allowed. In addition, profiles are excluded if they contain less than 20 sample points until 2,000 dbar and if no mixed layer depth can be determined.…”

Recent large-scale mixed layer and vertical stratification maxima changes

“…For the interpolation we applied the modified Akima piecewise cubic Hermite interpolation [makima method (MathWorks Inc., 2019;MATLAB, 2019)] which is based on the algorithm from Akima (Akima, 1970) and combines the interpolation schemes of cubic spline and the piecewise cubic Hermite interpolating polynomial (pchip) (MathWorks Inc., 2019). To determine vertical maxima and minima from an individual profile, an interpolation scheme is required which allows for local maxima and minima between measurements (Roch et al, 2021). The makima method enables undulations in contrast to the pchip method and at the same time prevents artificial spline maxima between identical samples.…”