eNews

#03 2025

Eddies on the edge: What happens when the Agulhas Current meets the coast?

By Dr Gustav Rautenbach, Operational Ocean Modeller, Egagasini Node, NRF-SAEON

Along South Africa’s east coast, just offshore from Durban, the powerful Agulhas Current meets the more tranquil waters of the KwaZulu-Natal Bight. This interaction creates swirling eddies – ocean vortexes that can profoundly influence coastal circulation.  

My PhD research followed the life cycle of these eddies and uncovered how they trap, transport, mix and even reshape the waters nearshore.  

What was the research about?

My study focused on the KwaZulu-Natal Bight, a semi-sheltered coastal bay that stretches between Richards Bay and Durban. This area is directly influenced by the poleward flowing Agulhas Current – one of the fastest and strongest ocean currents in the world.

When this current reaches the KwaZulu-Natal Bight it becomes unstable. The unstable Agulhas Current generates sine-scale turbulence at the inshore front that grows into “Durban eddies”, cyclonic circulations (clockwise rotating) at the ocean’s surface. My thesis examined how these eddies form, move and interact with the coastal environment.

How did I study this?

I used a high-resolution (1 km) numerical ocean circulation model, run over an eight-year period, to simulate the formation and behaviour of these cyclonic frontal eddies. The model was assessed by comparing it to satellite and in-situ observations. I adapted an eddy-tracking algorithm (AMEDA – Angular Momentum Eddy Detection and tracking Algorithm) to identify and follow the paths of eddies as they generated and evolved. In addition, I used Lagrangian particle tracking (Pyticles) and the rescaled Potential Vorticity to investigate the fine-scaled dynamics associated with the eddies.

Figure 1: Snapshot of the modelled sea-surface temperature (colour) of the east coast of South Africa. The outer domain represents the low-resolution model (3 km) from which the high-resolution model (1 km), indicated with the black box, gained its boundary conditions. The black and grey contours show the sea-surface height and topography respectively. The white arrow indicates the warm poleward flowing Agulhas Current. Yellow arrows show the occurrence of cyclonic frontal eddies inshore of the Agulhas Current. The green boxes show the location of four Marine Protected Areas that the frontal eddies can influence during their life cycle. The magenta transect shows the location of the in-situ transect (Agulhas Current Time-series array – ACT; and the Agulhas System Climate Array – ASCA) that was used to validate the model.

Key findings

  • Eddies are formed frequently (roughly every six days) and drive significant vertical mixing and cross-shore transport, influencing deeper water layers (~ 1000 m) and coastal circulation patterns.
  • Two types of eddies were identified:
    • Stationary eddies are chaotic in their generation and forms and dissipated anywhere within the KwaZulu-Natal Bight. Their smaller spatial scales (radius = 11– 18 km) and weaker circulation lead to a shorter lifespan (< 12 days) and slower propagation speed (1.5 m.s-1).
    • Propagating eddies generate offshore of Durban, grow into larger cyclonic eddies (radius = 19–28 km) that are associated with stronger circulation, travelled downstream (1.8 m.s-1) along the coast and had a significant impact on the coastal environment.
  • Propagating eddies gain their source from the frictional boundary layer of the Agulhas Current. They trap the relatively cool and nutrient-laden coastal waters and transport the waters downstream where valuable Marine Protected Areas are located.
  • As the eddy propagates downstream, it is torn apart by the land boundary and the Agulhas Current and redistributes the trapped water, which can stimulate primary production.

Figure 2: Mean stationary (a.) and propagating (b.) eddy trajectory as a function of the eddy lifespan in days (colour). Gray lines indicate trajectories of all the eddies. The 100-m, 500-m, 1000-m and 3000-m isobaths are shown in black contours and the green boxes indicate positions of Marine Protected Areas.

Why does this matter?

Frontal eddies play a critical role in shaping coastal ocean conditions. They can uplift deeper, nutrient-rich waters, support marine ecosystems and influence local fisheries. Understanding how and when these eddies form, helps scientists better predict changes in coastal dynamics and improves ocean circulation models that inform climate and marine resource management.

What’s next?

This research lays the groundwork for more targeted studies on the ecological and biogeochemical impacts of eddies. It also raises interesting questions about the role of coastal topography and current shear in forming nearshore currents such as the Natal Bight Coastal Counter Current (NBC3). I hope to continue exploring these processes in future work, possibly extending the research into coupled physical-biological models.

Gustav Rautenbach recently completed a PhD in Oceanography at Nelson Mandela University under the supervision of Prof. M. Roberts, Prof. S. Herbette, Dr J. Veitch, Dr P. Penven and Dr G. Cambon. The work was supported by the Bayworld Centre for Research and Education. Gustav currently works for the SOMISANA Team (Sustainable Ocean Modelling Initiative: a South AfricaN Approach) at SAEON’s Egagasini Node, where he develops operational ocean forecast systems that are used for decision-making.