Where two currents meet: Oceanographic processes within the Angola-Benguela Frontal Zone

A modelling lab has been set up in Angola to facilitate knowledge sharing and capacity building

By Dr Juliet Hermes (SAEON Egagasini Node) and Dr Issufo Halo (Nansen Tutu Centre for Operational Oceanography and UCT)

The Benguela ecosystem extends from southern Angola along the west coasts of Namibia and South Africa to the southernmost part of the continent. 

It begins as a northward flow off the Cape of Good Hope, where it skirts the western African coast equatorward – 24^oS-30^oS. Here, most of the current separates from the coast as it bends toward the northwest across the Southern Atlantic. However, two branches of the current do continue along the coast, and one of them joins the Angola Current at the Angola-Benguela front near 16^oS.

The Benguela Current has a well-defined mean flow that is mostly confined near the continent and a more variable transient flow on its western side. It is unique among eastern-boundary upwelling systems in that it is bordered at both the northern and southern ends by warm water systems - the Angola Current and the Agulhas Current respectively.

Fish stocks

The Benguela Current is an important centre of plankton production and is rich in fish such as sardine (pilchard), anchovy, hake and horse mackerel, which are all heavily exploited by humans. In common with other coastal upwelling areas, the Benguela ecosystem exhibits large interannual and decadal variability caused by large variations in environmental forcing. This natural forcing is compounded by the impact of fishing, resulting in large fluctuations in the abundance of some fish stocks with associated collapses, recoveries and rapid switches in species dominance.

Figure 1. The Benguela System (Shannon V. 2006. A Plan comes Together. In: Large Marine Ecosystems, Vol 14. V. Shannon, G. Hempel, P. Malanotte-Rizzoli, C. Moloney and J. Woods (Editors), pages 3-10

Pulsing of the boundaries impacts on the ecosystem as a whole and its harvested resources. Shared fish stocks are a feature of the Benguela ecosystem, with some species moving between countries, so it is important that national management strategies account for resource dynamics and management actions in neighbouring countries.

The Benguela Niño

A particular phenomenon that can be encountered in the Benguela Current system is the Benguela Niño. Like Pacific El Niños, Benguela Niños are thought to be a result of anomalous atmospheric conditions in the western tropical Atlantic.

Every year there is a southward intrusion of warm Angolan water into the northern Benguela, but during a Benguela Niño the Angola-Benguela front is displaced south, causing the advection of warm, highly saline water as far as 25^oS. Benguela Niños are less intense and less frequent than Pacific El Niños. Three Benguela Niños have been recorded (1934, 1963 and 1984), during which time temperatures off the coast of Namibia were 2-4^oC higher than normal.

Angola/South African project

Whilst the seasonal cycle and surface features of the Angola Benguela Frontal Zone (ABFZ) are known, the inter-annual variability has not been well studied, in particular with reference to the sub-surface processes which are important in the advection of oxygen-depleted water and the transmission of sub-surface Benguela Niño signals into the northern Benguela. A better understanding of these processes is necessary to be able to understand and even predict their implication on fisheries in southern Benguela, and potential climate variability.

The objective of a joint research project between South Africa and Angola has been to investigate the productivity and oceanographic processes and interactions within this zone (ABFZ), aimed at improved management of the biotic and water resources of the northern Benguela. Variability in the ABFZ has been suggested to control local biology and thus fish stocks, as well as rainfall variability. Hence the monitoring and investigation of the ABFZ should be regarded as a major fisheries and water resources management tool.

Knowledge sharing and respective capacity building is essential in such a research project to develop not only the skills and expertise, but also the local knowledge and data. This project also aimed to increase collaboration between the two countries.

The research project focused on the spatial and temporal variability of both surface and subsurface water masses within the ABFZ influence and its direct impact on the distribution and abundance of key living and water resources. There is considerable expertise in South Africa with regards to the oceanography of this region, as well as data analysis methods and modelling the area. In Angola there is considerable local knowledge of the resources as well as a large amount of cruise data.

Figure 2. Model-derived snapshot of Sea Surface Temperature of the bio-physical coupled model at 1/4 spatial grid resolution for the February 3, of the first year of the simulation. Along the African coast, a clear distinction between the warmer and colder waters is evident near 15oS, establishing the ABF zone. Colder waters are from the Benguela upwelling system, while warmer waters are from the Angola Current, which appear to be from the equatorial regions.

Furthermore, this study fits neatly into the objectives of the 10 Global Change Grand Challenges Research Plan of the DST, which endeavours to guide the South African research community towards probing and addressing key issues that affect people's future through observing and monitoring a changing planet.

During the course of the project, two workshops have been arranged in Angola (final workshop still to happen) and a modelling lab has been set up there. A realistic high-resolution numerical model has been validated in the region, and a coupled biogeochemical model is being finalised. The Angolans have participated in extended visits to Cape Town, as well as participating in some modelling workshops at the University of Cape Town (UCT).

Input around data sharing between South Africa and Angola has begun and links have been made with SADCO. Two Angolans are working on a PhD and MSc at UCT and the University Agostinho Neto respectively, and two students have graduated (Hons and MSc) from UCT.

Project researchers have attended a CLIVAR conference and a Benguela Current Commission workshop where they presented their results. Through PIRATA (Research Moored Array in the Tropical Atlantic), the Kizomba array is measuring oceanographic variables off the Angolan coast.

SAEON would like to thank the National Research Foundation (NRF) for funding the project and the co-investigators of this project: Dr Jenny Jackson-Veith, Mr Tiago Queiroz, Prof. Chris Reason and Prof. Mathieu Rouault.