Shelf-edge jet currents in the southern Benguela: A modelling approach
Résumé
The dynamics and seasonal variability of jet currents on the southern Benguela shelf-edge are investigated using a climatologically forced Regional Ocean Modelling System (ROMS) model. The jet is primarily forced by the intense horizontal gradients that exist across the southern Benguela shelf. These gradients are set up by nearshore cooling via the strongly seasonal upwelling regime and variable offshore warming by the advection of Agulhas waters. While the nearshore cooling is prevalent only during the spring and summer upwelling season, the offshore warming exists throughout the year. As a result intensified geostrophically adjusted currents exist throughout the year, particularly off the Cape Peninsula and Cape Columbine. However, the distinct shelf-edge jet features are most intense during upwelling seasons and extend, more or less continuously, from Cape Agulhas, the southern-tip of the continent, to Cape Columbine. The spring and summer jet off the Cape Peninsula reaches speeds of at least 0.7 m.s-1, bifurcates as it moves northward. The branch that continues northward over the shelf goes on to feed the offshore branch of the Cape Columbine jet (over the 500 m isobath) and to a less extent the nearshore branch (over the 200 m isobath) that is locally enhanced by upwelling processes. During winter, the Cape Peninsula jet is more confined to the shelf region and goes on to feed the whole outer-shelf (200-500 m) region off and beyond Cape Columbine. An ageostrophic component associated with offshore non-linearities related to Agulhas influx causes the mean manifestation of the Cape Peninsula jet to broaden slightly (60 km) compared to its 40 km-wide geostrophic core which is situated over the 350 m isobath. The ageostrophic component is related to the generation of eddies that cause the isopycnals to flatten out.