Review: The Greater Agulhas Current System – Circulation, Variability, Long-term Trends and Impacts on Weather, Climate and Ecosystems

Abstract. The Greater Agulhas Current System (GACS) is a dynamically complex western boundary current system that plays a central role in inter-ocean exchange between the Indian and Atlantic Oceans, regional weather and climate over southern Africa, and marine ecosystem variability along the southeast African margin. Since the publication of The Agulhas Current Book nearly two decades ago, major advances in ocean observing systems, satellite remote sensing, numerical modelling, and interdisciplinary research have substantially expanded understanding of the system. Here we provide an integrated review of progress over the period 2006–2025, synthesising recent advances across all components of the GACS, from its upstream source regions to its downstream outflows and global climate connections. We first summarise key technological and methodological developments that have transformed observation and simulation of the Agulhas system, including sustained moored arrays, autonomous platforms, multi-sensor satellite products, and high-resolution numerical models capable of resolving mesoscale and submesoscale dynamics. We then reassess the state of knowledge for each sub-region of the system: the East Madagascar Current, the Mozambique Channel, the Northern and Southern Agulhas Current, the Agulhas Retroflection and leakage, and the Agulhas Return Current, highlighting how recent studies have addressed uncertainties in circulation pathways, variability, and connectivity. Knowledge developments include improved quantification of transport variability and eddy dynamics in upstream source regions, new observational evidence for eddy dissipation and momentum transfer within the Agulhas Current, refined understanding of the processes governing retroflection and leakage, and growing insight into the role of mesoscale and submesoscale dynamics in air–sea interaction, biogeochemical fluxes, and ecosystem responses. We also review emerging evidence linking Agulhas system variability to Southern Hemisphere wind forcing and to downstream impacts on the Atlantic Meridional Overturning Circulation. We conclude by identifying remaining knowledge gaps and outlining priority directions for future research, emphasising the importance of sustained observations, improved representation of fine-scale processes in models, and stronger integration across physical, biogeochemical, and ecosystem perspectives.