Properties of the Agulhas Current's Inshore Front During The Shelf Agulhas Glider Experiment (SAGE)

The response of coastal and shelf regions to changes in the Agulhas Current remains poorly studied. This is partly due to observational challenges associated with sampling western boundary currents. Cross-shelf exchange in such energetic current systems occurs through a range of meso- (~50-200 km) and sub-meso (<10 km) scale processes which are difficult to observe using moored current arrays or Lagrangian platforms. Profiling gliders offer a revolutionary technology to continuously sample the energetic inshore regions of the Agulhas Current at a high spatial (100’s of meters to 3km – well within the sub-mesoscale range) and temporal (0.5-4 hourly) resolution. In April 2015, two SeaGliders were deployed off Port Elizabeth (34S) at the inshore edge of the Agulhas Current as part of the Shelf Agulhas Glider Experiment (SAGE), testing for the very 1^st time the feasibility of operating autonomous platforms in this highly turbulent and energetic western boundary current system. For a period of approximately two months, the Seagliders provided continuous observations at the inshore boundary of the Agulhas Current at an unprecedented spatial resolution. Observations from the Seagliders showed that at the inshore edge of the Agulhas Current, both surface and depth averaged currents are aligned in a south-west / north- east direction, with stronger flows encountered over deeper regions of the shelf, when the gliders are closer to the Agulhas Current. In the absence of large meanders, the mean flow at the inshore boundary of the Agulhas Current is characterised by strong shear with a counter current flowing in opposite direction to the mean current field. Instances of counter currents occur 45% of the time in the surface flow and 54% of the time in the depth-averaged record. More than 80% of return flow occurrences occur when glider is in water depth of less than 200m.