Impact of regional and temporal changes in the Agulhas Current’s course on coastal and shelf regions.

Marjolaine Krug, Department of Environmental Affairs Oceans and Coasts, Cape Town 8012, South Africa and Laura Braby, University of Cape Town, Department of Oceanography, Cape Town, South Africa
The Agulhas Current, which flows along the eastern shores of South Africa is a major driver of variability for the coastal and shelf regions. Mesoscale and sub-mesoscale variability within the Agulhas Current drive changes in the shelf regions through processes such as frontal eddies, current intrusions, current driven upwelling or filament and plume propagation onto the shelf. The intrinsic variability of the Agulhas Current is also region dependent. In regions where the shelf is narrow and steep, topographic steering generally leads to a more stable Agulhas Current. In contrast, in the southern Agulhas and in the Natal Bight where the shelf is wider, more meanders are observed. While a number of studies have provided significant insights into the different dynamics which characterise the Agulhas Current across its latitudinal extent, more research is needed to determine where the Agulhas originates, how its meandering nature has changed in time and the impact of such changes on the shelf and coast. Here we use in-situ current observations, satellite observations of Sea Surface Temperature (SST) and Sea Surface Height (SSH) to better define the different dynamical provinces within the Agulhas Current. An eddy tracking algorithm and an Agulhas Current core algorithm are applied to the altimetry dataset to quantify changes in the Agulhas Current’s mesoscale variability. The impact of such changes on the coast are assessed through the use of SST observations. Preliminary analyses of the in-situ current dataset and the SSTs show a transition in the current’s variability south of 27.0oS, where the current stabilises following the sudden narrowing of the shelf. This sharp stabilisation in the current is associated with a marked increase in temperatures near the coast and is thought be the region where the Agulhas Current originates. Further south, in the Natal Bight, waters along the coast and shelf regions once again become colder than the waters located further offshore, and this temperature gradient is maintained downstream until the Agulhas Current reaches the Retroflection. Further analyses are underway to relate the Agulhas Current’s mesoscale variability to changes observed over the shelf regions over the last two decades.