Inter-Annual Variability of the Along-Shore Lagrangian Transport Success in the Southern Benguela Current Upwelling System

A 3-km resolution regional ocean model is used to investigate the role of wind-driven coastal circulation and mesoscale variability on the inter-annual variability of transport success in the southern Benguela between Cape Point (34 degrees S) and St Helena Bay (32 degrees S) from 1992 to 2011. Lagrangian particles are released within the top 100 m of the water column along an across-shore transect off Cape Point. Transport success is given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released. The analysis of transport success anomalies and their relationship with the local circulation and wind forcing reveal that there is no single driver of the inter-annual variability. The transport success variability of particles released on the shelf (depths <300 m) mainly depends on their capacity to remain embedded within the coastal Benguela Jet. Nevertheless, peaks in offshore Ekman transport and episodic occurrence of a poleward inner-shelf counter-current contribute to negative anomalies. For particles released on the outer shelf edge (depths >500 m), across-shore transports induced by mesoscale eddies are the main contributors to transport success variability. Rare passage of Agulhas rings near the shelf edge can induce strong offshore advection of particles into the open ocean. In contrast, shelf-edge cyclonic eddies favor the onshore transport of particles originating from the outer shelf edge and thus contribute to increasing transport success.

Automated summary

A regional ocean model and Lagrangian particle release experiments were used to investigate the impact of wind-driven coastal circulation and mesoscale variability on transport success in the southern Benguela. The study found that the inter-annual variability of transport success is influenced by both coastal circulation and offshore Ekman transport, as well as mesoscale eddies near the shelf edge.