EXEBUS: Ecological and Economic Impacts of the intensification of extreme events in the Benguela Upwelling System

The Benguela Upwelling System (BUS) of South Africa, Namibia, and Angola is one of four Eastern Boundary Upwelling Systems globally. It is bordered by the warm tropical water of the Angola Benguela Front Zone in the north and the Agulhas Current in the south. The EXEBUS project aims to understand the drivers of change in the BUS and its contribution to the changing variability of the system, emphasizing extreme events. IMR and University of Cape Town (UTC) co-lead Work Package 1 – Baseline and Variability in the BUS. In early 2023, EXEBUS Consortium suffered the unfortunate and untimely passing of one of the WP-1 leads, Prof Mathieu Rouault, based at the University of Cape Town. Consequently, in 2023, IMR stepped in to supervise Prof Rouault's graduate students working on EXEBUS-related studies and contribute to their progress. The first study on the recent warming along the Angolan and Namibian coast revealed a pronounced long-term warming trend in Angolan coastal waters during the austral summer (November-January) and autumn (March-May), associated with an intensification of the seasonal poleward flow in the Angola current. In contrast, the long-term warming trend observed in the modelled sea surface temperature (SST) during the austral winter (June-August) was significantly less pronounced. The second study was focused on the low salinity intrusions accompanying the two most extreme marine heatwave events of the last three decades: the 1995 and 2011 Benguela Niños. This study revealed contrasting strengths of these intrusions – a strong intrusion in 1995 and a weaker one in 2011. It identified two independent mechanisms responsible for this difference: the strength of the coastally trapped waves (CTW) that drive the intrusion process and the rate of the Congo River discharge prior to the CTW event, which regulates the volume of low-salinity water advected with it. The third study focused on the evolution of marine heatwaves in the Agulhas Bank, a region important for South Africa's socio-economic activities. It aimed to catalogue the area's extreme sea surface temperature (SST) events. Four distinct regions, each characterized by different long-term variabilities, were identified. Extensive analyses were conducted to assess SST extremes' spatial and temporal variability. The findings revealed a significantly higher frequency of marine heatwaves (MHWs) during the austral summer compared to the winter. Furthermore, the study identified different driving mechanisms for these events across seasons: atmospheric forcing during summers and advection associated with meanders and eddies of the Agulhas Current during winters. Through 2023, apart from contributing to the UCT-placed EXEBUS themes, IMR continued, in collaboration with the Angolan Institute of Fisheries and Marine Research, the work on evaluating the impacts of marine heatwaves on the availability of sardinella – the key small pelagic fish for food security of the coastal population of Angola. A reanalysis of acoustic survey results, 1994-2023, was carried out using newly developed algorithms to investigate evolution in the distributions of sardinella by species and age groups. Two sardinella species dominate Angolan waters S. maderensis, and S.aurita. The results identified a low response of S. maderensis to climatic variability. Conversely, S. aurita responded strongly to decadal changes in upwelling conditions. The Angolan upwelling is a windless process, occurring seasonally between June and August, controlled remotely by coastally trapped waves emanating from the equator. During the 2004-2014 decade, this upwelling was characterized by a stable temperature range of 21-22°C. The recent oceanographic findings attributed this stable temperature range and high coastal productivity to the vertical mixing induced by the breaking of baroclinic tides and long-range swells. Our investigation revealed a general colocation of sardinella aggregations and the zones of enhanced mixing alongshore, identifying these zones as principal nursery areas providing retention to early life stages and food security for sardinella pre-recruits during 6-8 months of their life. Stemming from these observations, we concluded that during 2004-2014, the Angolan upwelling season maintained a favourable habitat for S. aurita breeding and survival in their early stages. The favourable conditions terminated in 2016 with the intensification of MHWs, which was not observed before. Particularly severe MHW episodes associated with the Atlantic Niños hit the Angolan waters during the 2020 and 2021 upwelling seasons. The most recent fisheries data, unavailable before late 2023, evidenced a collapse of S. aurita in Angolan waters consistent with our predictions, derived from analyses of satellite data using the conceptual frameworks that link physical processes to favourable recruitment habitats for small pelagic fish.

The Benguela Upwelling System (BUS) of South Africa, Namibia, and Angola is one four Eastern Boundary Upwelling Systems globally. It is bordered with warm tropical water of the Angola Benguela Front Zone in the north and by the Agulhas Current in the south. Changes in the coupled atmosphere-marine climate, natural or anthropogenic, both within the BUS and beyond, affect ecological and socio-economic important sub-systems, potentially affecting millions of people—residents of the coastline and those who derive their livelihoods and resources from the BUS.Our aim in the EXEBUS project is to understand the drivers of change in the BUS and its contribution to the changing variability of the system, with an emphasis on extreme events. Given that both natural- and human-induced changes to the functioning of the BUS occur at a range of time and space scales (and are also interdependent), we seek to understand the changing envelope of variability, extremes of this variability, and their impact. EXEBUS undertakes an Integrated Ecosystem Assessment (IEA) to establish the roles, trends, and range of variability and the extremities of natural and anthropogenic geophysical, biological, governance, socio- economic features and phenomena, and assess their impact on ecological, sociological, governance, and macroeconomic systems and processes in the BCLME. The goal is to strengthen the rational basis for management on relevant spatial and temporal scales.The work is undertaken by transdisciplinary consortium of international researchers, practitioners, and representatives of the private sector and civil society proposed here brings unique and complementary perspectives for both understanding the changes in this dynamic system, applying, and communicating the importance of these findings. This will enable the development of an Ecosystem Based Management framework for application in the region.