Submarine Canyon Modulation of Eddy Kinetic Energy: A Seasonal Regime Shift in the Southern Benguela

Submarine canyons are recognised hotspots of enhanced vertical exchange and complex circulation, yet their role in modulating the seasonal energy cycle between mean and turbulent flows remains poorly understood. This study reveals that Cape Canyon, a major topographic feature in the Southern Benguela Upwelling System (SBUS), exhibits a seasonal reversal in its dominant energy pathway, shifting from a net sink of eddy kinetic energy (EKE) in summer to a net source in winter.

Using a high-resolution (1-km) CROCO simulation, we diagnose instability mechanisms and energy pathways via the eddy-to-mean kinetic energy conversion rate, C. During austral summer (DJF), the strong, stratified Benguela Jet interacts with canyon topography to form coherent, high potential vorticity (PV) vortices. However, the canyon region is a net EKE sink (C < 0). Across 88.3% of the canyon area, energy is transferred from eddies to the mean flow, indicating suppression of eddy growth despite the presence of organised vortices.

In the austral winter (JJA), the sign of C reverses. The canyon becomes a net EKE source (C > 0), with 50.1% of the canyon area exhibiting mean to eddy energy transfer. This transition is consistent with a shift from summer damping of coherent vortices to wintertime barotropic and baroclinic instability of a weakened mean flow. Concurrently, dynamical hotspots shift from surface-intensified (0 to 100 m) in summer to deep-reaching (900 to 1000 m) in winter, co-located with enhanced vertical motion and indications of convective mixing.

Spatially, the response is asymmetric. The northern flank becomes the primary winter instability hotspot (55.5% source area), transitioning from a summer anticyclonic vortex street to a more diffuse eddy generating regime. In contrast, the southern flank remains a persistent, strain-dominated dissipative zone throughout the year, acting as a dynamical barrier.

These seasonal energy pathways likely have biogeochemical consequences. Summer damping may favour retention, whereas wintertime eddy generation and deeper-reaching mixing may enhance nutrient supply from intermediate waters (SAMW, AAIW), potentially preconditioning the system for the spring bloom. Overall, Cape Canyon functions as a seasonal energy switch that modulates cross-shelf exchange and biogeochemical connectivity in the SBUS, with broader implications for the impacts of submarine canyons in Eastern Boundary Current Systems.