the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Feb 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6573', Lisa Beal, 26 Mar 2026
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RC2: 'Comment on egusphere-2025-6573', Anonymous Referee #2, 30 Mar 2026
Review of the MS “The Greater Agulhas Current System – Circulation, 1 Variability, Long-Term Trends and Impacts on Weather, Climate and Ecosystems” by Halo and colleagues
The MS is a review paper about the Greater Agulhas Current System, which comprises the Agulhas Current, the Agulhas Return Current, the East Madagascar Current, and the Mozambique Channel circulations and their undercurrents. The purpose of the MS was to update the famous Lutjeharms (2006) book on the region.
I recognize that writing a review paper is a difficult task, particularly given the considerable research in this area since the publication of Lutjeharms’ book. To complicate matters, there have been a couple of extensive review papers on the Indian Ocean (which includes the Greater Agulhas System) since 2006, including Schott et al. (2009) and Phillips et al. (2021). There were also updates through books such as Talley and colleagues in 2011 (Descriptive physical oceanography: an introduction). More recently, there was a dedicated book edited by Ummenhofer and Hood (2024) (The Indian Ocean and its Role in the Global Climate System) that includes sections providing a modern view of the Greater Agulhas System. So, we are not in a situation where no updates have been made in the literature since Lutjeharms (2006).
Given that, I was expecting to see a review paper that goes deeper and more detailed than the above-cited references. But the MS is poorly organized and without direction. There is some information repeated over and over, in circular arguments.
I don’t know what the authors wanted to accomplish with Section 2, for instance, (“Observational and modelling advances over the past two decades”). The advantages of in-situ global observational systems, such as Argo, or of satellite-based systems (e.g., SWOT), cited in the MS are valid for any area of the global ocean, not just the Southwest Indian Ocean.
For example, L182-187: “Over the past two decades (2006–2025), the capacity and capability to observe and model the Greater Agulhas Current System (GACS) have expanded substantially, leading to significant advances in understanding its circulation, variability, and impacts. This progress has been driven by sustained developments in global and regional ocean observing systems, including the expansion of in situ measurement networks, the emergence of autonomous observing platforms, and improvements in satellite remote sensing coverage and resolution…”. We can replace GACS in this statement with any other region of the global ocean, and it will still be valid.
The text of Section 2 feels quite bland and non-specific. After reading this section, I asked myself whether this is a review of observation and satellite systems (although it would also have failed if that was the direction).
One of the things that bothered me most throughout the reading was that the authors refer to the open questions posed by Lutjeharms (2006) many times but never explicitly state which ones, what has been found, or what remains open.
Not only in Section 2, but overall, statements throughout the MS are quite generic; there is no quantitative description. For example, L82 says “.. 84 Sv at the latitude of the South African east coast..”, well, the South African east coast spans from 30.5S to about 35S, so which latitude exactly? This is just a small example, but the text is crowded with these ‘generic statements’, which are not acceptable in a review paper.
I also missed many studies that have changed the view of the region in the last two decades, which were not cited (it seems there was a preference to cite papers by the authors involved in the MS writing). The authors need to do a better job here. Also, many cited references are missing from the reference lists, which is the core of a review paper. For instance, several references to Ullgren et al. (2012) relate to the East Madagascar Current and its undercurrent (e.g., L224, L713). This paper is not cited in the references. The only paper of Ullgren et al. (2012) that I am aware of is on the Mozambique Channel, not on the East Madagascar Current domain. There are so many papers not appearing in the reference list (Vousden et al., 2012, Halo et al., 2017, Roberts and Termon, 2020, Ullgren et al., 2012, etc) that I stopped taking note.
Upon reaching the end of the MS (Summary and Conclusions), I found the review didn’t accomplish any of its purposes. We don’t know what the open questions were in 2006, what has been accomplished since, or what remains to be done. So, the review failed. It doesn’t inspire new research in the area.
My complete surprise, however, happened in the end. This is a review paper about the science, but it concludes with this: “The next two decades represent a pivotal opportunity: with adequate investment, integrated observing and modelling frameworks, and a well-trained interdisciplinary workforce, South Africa is uniquely positioned to lead globally in Agulhas Current research”. Nothing against South Africa, but this was not a report to the South African government or audience. Until that point, there had been no mention of any country in policy terms. There are many countries surrounding the Greater Agulhas System, and they could and should all be involved in research in the area. This led me to think that part of the text may have been generated by AI (it reads like a typical AI-generated statement). The tools I have looked at identify AI-mixed text (not only in this statement but also in other passages). The problem is that there is no acknowledgment of AI on the MS.
Given the MS's many problems, I decided not to type the line-by-line comments. In my opinion, unfortunately, the MS does not deserve publication at this stage. I would advise the authors to rewrite the entire MS.
Citation: https://doi.org/10.5194/egusphere-2025-6573-RC2
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Major Comments
The expectation set up by the authors, for a synthesis review of our understanding of the Agulhas System over 20 years of research, is not met by this manuscript. Regretfully, I recommend the manuscript is rejected.
The manuscript appears to have been written with AI. The sentences are smooth and peppered with appropriate terminology, but there is ultimately no scientific content. At least, I was not able to grasp it. A quick read of the summary can affirm this. An acknowledgement of the use of AI is missing.
The title and abstract are written like a table of contents, promising an interesting review of "circulation, variability, trends, and impacts on weather, climate and ecosystems", but the scientific insights and conclusions themselves are missing from the front matter. This must be rectified.
From my careful reading and note-taking over many hours with the entire manuscript, I could not find reviews and insights delivered on these topics at all. For example, on variability, the amplitude and phase of the seasonal cycle of the Agulhas Current and how it is thought to come about is not covered. What season has a tendency for more upwelling, more productivity? Why? The frequency of meandering and ring-shedding is not covered. The trend in the strength of the Agulhas Current is not covered. And there are no coherent explanations of how and why variability comes about in the system, of the processes and dynamics and teleconnections.
There are a few themes that the manuscript circles around: Is the Mozambique Channel dominated by eddies or mean flow? Does the East Madagascar Current retroflect? How do incoming eddies impact the variability of the Agulhas Current? How does the ARC interact with bathymetry? These themes are largely focussed on the surface geography of the system and do not align with the title and abstract. Ultimately, the review delivers an updated schematic showing streamlines and fronts--data reference needed--with eddies and regions of high productivity drawn on top as rings and ovals (see figure 16). It is not clear how these elements of the system are related or how they are varying/trending, or what information is new.
There are a few snippets of pointed scientific information related to some of the recent work of the many co-authors. These snippets are disconnected with the content around them and there is no synthesis of what all the research together adds up to.
The section on modelling and observational advances is generic and vague. Most sentences contain a list of observing platforms or projects or simulations joined somehow with a list of things they can measure joined somehow with terms like coverage, critical, increasingly, extensive, pervasive, revealed. Many sentences could be written about any other energetic part of the ocean. There is little here that is unique to the Agulhas system. For instance, ground-truthing of ocean colour is mentioned "Major advances have since been achieved through the development of regional bio-optical algorithms tailored to the optically complex waters of the--fill in the blank--allowing quantitative investigation of phytoplankton variability, bloom dynamics, and physical-biological coupling." And on models, "A range of studies demonstrated that addressing--fill in the blank--requires more than increased horizontal resolution alone, prompting efforts to improve numerical precision...refine bathymetry and dissipation schemes, and better represent key dynamical feedbacks".
The section on scientific advances is split into geographic regions. This results in a lot of repetition and circular prose because all regions are connected and are influenced by the same processes. For example, as the authors highlight, eddies in the Mozambique Channel can instigate anomalies that propagate through the entire Agulhas system. Do these eddies belong to the channel, the northern or southern Agulhas, the retroflection, leakage or return current? Another example, lines 849-854 repeat the information of lines 756-762. An organisation in terms of the topics from the title would be better.
There are many paragraphs on what was known pre-2006 using Lutjeharms comprehensive book. This re-iteration seems unnecessary. Can the authors get straight to the point of the review, using context from the book only to highlight the advances? Twenty years is a lot of research. Is it appropriate to refer everything back to 2006? Research papers often build one on another over time leaving long threads of "giant shoulders".
For the topics I am most familiar with I found some serious mismatches between the statements made and the references given. For example, "Transport estimates derived from ship-based sections indicated that the NAC carries the majority of the volume transport within the GACS, with values increasing downstream as additional source waters are incorporated (Beal and Bryden, 1999)." This reference uses data from a single section from which it is not possible to quantify a downstream increase. Or line 929 which is about path stability and meandering but again the references contain only a snapshot section which cannot reveal variability.
In summary, this manuscript falls far short of being acceptable for publication. I am sorry I cannot be more encouraging at this time. The authors will need to engage far more rigorously with the bibliography they reference in order to synthesize the state-of-the-science into a learned and useful review that advances understanding. I hope they do the work. It would be really interesting to read.