The tropical-subtropical coupling in the Southeast Atlantic from the perspective of the northern Benguela upwelling system

Abstract

In the Benguela upwelling system, the environmental conditions are determined to a large extent by central water masses advected from remote areas onto the shelf. The origin, spreading pathways and fate of those water masses are investigated with a regional ocean model that is analysed using Eulerian passive tracers and on the basis of Lagrangian trajectories. Two major water masses influencing the Benguela upwelling system are identified: tropical South Atlantic Central Water (SACW) and subtropical Eastern South Atlantic Central Water (ESACW). The spreading of tropical waters into the subtropical Benguela upwelling system is mediated by equatorial currents and their continuation in the Southeast Atlantic. This tropical-subtropical connection has been attributed to signal propagation in the equatorial and coastal waveguides. However, there exists an additional spreading path for tropical central water in the open ocean. This mass transport fluctuates on a seasonal scale around an averaged meridional transport in Sverdrup balance. The inter-annual variability of the advection of tropical waters is related to Benguela Niños, as evidenced by the 2010/2011 event. The northern Benguela upwelling system is a transition zone between SACW and ESACW since they encounter each other at about 20°S. Both water masses have seasonal variable shares in the upwelled water there. To summarise the main pathways of central water mass transport, an enhanced scheme for the subsurface circulation in the Southeast Atlantic is presented.

Regional atmospheric response to the Benguela Niñas

“…The Angola‐Benguela Frontal Zone (ABFZ) is one of remarkable characteristics in the Tropical and south Atlantic Ocean dividing the tropical warm water associated with the warm poleward‐flowing Angola Current (e.g., Kopte et al ., 2017) and subtropical cold water due to the equatorward‐flowing Benguela current and coastal upwelling system (e.g., Mohrholz et al ., 2004; Junker et al ., 2015; Tchipalanga et al ., 2018; Vizy et al ., 2018; Koseki et al ., 2019; Siegfried et al ., 2019). In particular, the ABFZ plays a vital role for the regional/local marine ecosystem and fishery resources (e.g., Auel and Verheye, 2007; Chavez and Messié, 2009; Jarre et al ., 2015) providing the enriched nutrient from the sub‐surface ocean to the ocean surface.…”

Exploratory Lagrangian-Based Particle Tracing Using Deep Learning

“…Research related to reduced Lagrangian representations that enable time-varying vector fields has advanced along multiple axes. These include in situ sampling techniques ( [2,45,51,53]), post hoc reconstruction strategies ( [3, 8, 10, 26]), theoretical and empirical error analysis ( [9, 29, 49]), feature extraction ( [16,17,20,31,54]), and application to various domains ( [42,52,57]). In this paper, we study the use of deep learning to perform post hoc reconstruction.…”

Southeastern Tropical Atlantic Changing From Subtropical to Tropical Conditions

“…During these interannual warm events, the ABF is shifted anomalously poleward (Florenchie et al, 2004;Rouault et al, 2007Rouault et al, , 2018Lübbecke et al, 2010). Thereby, tropical SACW extends into the northern part of the Benguela upwelling region (Lübbecke et al, 2010;Siegfried et al, 2019).…”

“…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). In the SETA region the thermal Angola-Benguela Front (ABF) evolves around 15-17 • S where warm, tropical waters encounter cooler, subtropical waters from the Benguela upwelling system (Shannon et al, 1987;Siegfried et al, 2019). At the ABF the poleward Angola Current meets the northward Benguela Coastal Current (Siegfried et al, 2019, indicated schematically in Figure 1a).…”

“…At the ABF the poleward Angola Current meets the northward Benguela Coastal Current (Siegfried et al, 2019, indicated schematically in Figure 1a). 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 hence the area influenced by the presence of tropical SACW varies as well (Rouault, 2012).…”