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. Plain Language Summary In this study we use a combination of surface drifters and satellite-derived surface velocities over the Agulhas Current to investigate the relationship between the structure of the surface velocities and the path the surface drifters follow. Between September 1992 and October 2017, 49 surface drifters crossed the altimeter track #096 in a nonmeandering state of the Agulhas Current. Of the 49 surface drifters, 16 crossed inshore of the surface velocity maxima, 3 of which leaked into the South Atlantic Ocean. In doing this investigation we also found inconsistencies between the surface speeds derived from the satellite and that of the surface drifters and ship-based measurements. These inconsistencies are likely due to processes such as wind or small-scale features, which satellites are unable to measure; other factors may be the unknown shape of the geoid and thus mean state of the ocean in these regions. The Agulhas Current is an under sampled region; thus, this study could be used for motivation to deploy more surface drifters in the region. Studies like this will improve our understanding of how water from the Agulhas Current enters the South Atlantic Ocean.