On the retroflection of upwelling jets near coastal bends

Bays in coastal upwelling regions often serve as larvae retention zones underpinning functioning of marine ecosystems. Using a three-dimensional hydrodynamic model, this process-oriented study explores the ocean dynamics that follow from the relaxation of a spatially-uniform, upwelling-favourable wind field behind a headland. Findings reveal that wind relaxation leads to the appearance of a swift coastal countercurrent (CCC) as the inshore retroflection of the coastal upwelling jet. The analysis reveals that the CCC starts to form along the downwind coast of the headland where the upwelling-induced onshore barotropic pressure gradient opposes the wind stress. Here, wind relaxation indirectly induces a flow convergence that reverses the barotropic pressure gradient within ∼5 km from the coast which geostrophically drives the CCC. Once generated, this inshore barotropic pressure anomaly propagates along the coast as a coastal Kelvin wave to form an inshore retroflection of the upwelling jet. On the other hand, the upwelling creates a plume of denser water on the shelf behind the headland. After wind relaxation, the rotational-gravitational adjustment of this dense-water plume marks the front of the retroflection zone as a cyclonic baroclinic feature with a diameter of the baroclinic deformation radius (∼10 km). Findings show that this rotational-gravitational adjustment intensifies the pressure-gradient forcing of the CCC. Overall, the findings demonstrate that the retroflecting coastal current can traps both upwelled water and particles in vicinity of the headland, which is clearly of relevance to marine productivity.