Cross-Shelf Exchanges in the South African EBC/WBC System
- (Oregon State University)
We propose to better define the circulation over South African continental shelves and the exchanges between those shelves and the adjacent deep ocean. A primary task will be to quantify the mean and eddy transports in the exchange of material and water properties (mass, heat, salt, passive tracers and water parcels) between the shelves and the deep ocean boundary currents, a long-standing challenge in Physical Oceanography. These efforts will continue our improvement of altimeter data use in coastal environments, complementing the altimeter surface heights with other satellite data, in situ data and high-resolution ocean circulation models. Our fundamental approach will be to form control volumes extending from the coast to the deep ocean, defining the mass, heat and passive tracer transports across the boundaries of these control volumes. Although similar approaches have been used in the deep ocean, advances in the retrieval of altimeter height data near land now make this approach possible over continental shelves. We will also utilize improvements in the retrieval of scatterometer wind data closer to land to better define the wind-driven coastal Ekman transports, as well as to drive numerical models of the coastal circulation with realistic wind fields (improving wind stress gradients next to land). Besides our research into physical oceanographic processes, we will participate in the Cal/Val efforts for all of the precision altimeter reference missions (Jason-3, Jason-2, Jason-1 & TOPEX).
Our focus is on the boundary currents and continental shelves next to South Africa. This region includes a unique interaction of a Western Boundary Current (WBC, the Agulhas Current, the AC) and an Eastern Boundary Current (EBC, the Benguela Current System, the BCS). Both systems play important roles in the heat, salt and nutrient budgets of the Southeast Atlantic. Like other EBCs, the BCS is a region of net surface heat flux into the ocean and the fate of that heat has not been well described. In addition, the shelves are sources of nutrients and hypoxic waters that affect the economically important ecosystems and fisheries.
We directly address 6 of the 8 OST objectives stated in the A.11 RFP, as well as the NASA Earth Science climate goal to characterize, understand and predict changes in the Earth System. Basic physical processes (circulation and transports) will be quantified using alongtrack and gridded altimeter data (OST Objectives 1-2). Cal/Val efforts will compare Jason-3 and Jason-2 data during the initial tandem phase within our areas of interest (Objective-6) and compare each pair of the reference missions during their tandem orbits (Objective-3). Improvements in the standard gridded altimeter fields will be developed by blending tide gauge data and original alongtrack data in the near-shore region ("production of the best possible...data sets"). Similar methods can be applied using near-real time data in coastal regions for operational applications (Objective-4). Improvement of gridded SLA fields by adding Sentinel-3 data into the AVISO gridded coastal SLA fields will prepare for combinations of future swath altimeters (SWOT) and improved alongtrack data from delay-Doppler (Jason-CS) altimeters (Objectives-2 & 8).
We have established collaborations with colleagues in the South African oceanographic community and will continue these efforts during the four-year project, helping to organize a short course in Cape Town on advanced satellite analysis during the second year. The proposed work builds on our extensive experience in analyzing satellite altimeter data and high-resolution nested ocean circulation models in other EBCs and WBCs. It applies that experience to the South African coastal ocean, where altimeter data have received limited use. Where it is useful, we will compare our new results off South Africa with results from those other regions.