The Use of a Jet Reference Frame to Analyze Drifter Trajectories in the Agulhas Current

Studies based on drifters that are deployed using fixed geographical locations can alias the variability in the Agulhas Current. Numerical model simulations have shown that tracking particles using jet coordinate systems will improve our understanding of the variability in western boundary currents. In this study we use in situ observations to show the potential of quasi-Lagrangian measurements with an investigation into the relationship of the upstream surface velocity configuration and the trajectories surface drifters follow. Additionally, we use these drifters, along with ship-based measurements, to expose biases in satellite-derived geostrophic velocities in the Agulhas Current. Between September 1992 and October 2017, 49 surface drifters crossed the altimeter track #096 in a nonmeandering state. Of the 49 surface drifters, 16 crossed inshore of the surface velocity maxima, 3 of which leaked into the South Atlantic Ocean. Biases between altimetry-derived geostrophic velocities and absolute velocities from ship acoustic Doppler current profiler and drifters measurements have pointed toward surface drifters leaking from inshore of the Agulhas Current core in a region of high shear. However, the bias between these velocities is inconsistent, with the highest range in bias found inshore of the Agulhas Current core. Due to the lack of data in the Agulhas Current and various sources of error, much work remains to be done and results presented here may provide motivation for further targeted drifter deployments in the future.