High Frequency Radar (HFR) observed surface currents at Palmer Deep Canyon in the coastal ocean west of the Antarctic Peninsula in 2020

HFRs use doppler-shifted radio waves backscattered off the ocean surface to observe surface velocity. Signals are transmitted and received by an HFR antenna, and Bragg peaks in the measured Doppler spectra are used to calculate radial components of the surface velocity (Barrick et al., 1977). Measured radial components of the surface ocean velocity are directed towards the HFR antenna with a range resolution of 500 m horizontally and 5 degrees in azimuth. Radial components from the three HFR stations are added together to construct magnitude and direction of surface current velocities using an optimal interpolation algorithm (Kohut et al., 2006) providing hourly maps of surface currents at 1km spatial resolution (Veatch et al., 2022,, preprint: not peer reviewed).

The three-site network included two remote locations on the Wauwermans and Joubin islands operated at a center frequency of 25 MHz and a third site at Palmer Station operated at 13 MHz (Veatch et al., 2024 Figure 1). The two remote sites located beyond existing power grids used Remote Power Modules (RPMs) constructed on site. These RPMs used small-scale micro wind turbines and a photovoltaic array with a 96-hour battery backup to generate the power required by the HFR (Statscewich and Weingartner, 2011; Kohut, 2014). Redundancies were built in to the RPMs, including wind charging/resistive loads, solar energy, and independent battery banks. Redundancies ensured that the system could autonomously adjust power source if one component failed. RPMs consisted of a single water-tight enclosure that housed all power generating equipment and communication gear. HFR and RPMs were assembled at remote sites using shipboard support and zodiacs that lightered materials to shore. Line of sight radio modems (Freewave) were used to communicate between the two remote sites and a central site collocated with the Palmer Station HFR site. Communication equipment enabled remote site diagnostics and maintenance as well as real-time data communication.

The three HFR sites collected hourly radial maps of ocean surface current component vectors over our study area, covering about 1,500 km2 more than 80% of the time (Veatch et al., 2024 Figure 2A). The hourly, two-dimensional surface current maps derived from the radial component vector maps provided by each of the three HFR sites were used to derive our two LCS metrics (Veatch et al., 2024 Figure 2B). Before the Lagrangian coherent structure calculations were done, gaps within the 80% coverage area of the HFR maps were filled using a rigorous HFR-specific method (Fredj et al., 2016). Finally, the \"edges\" outside of the 80% coverage area were added back into the data structure. This was done to increase the residence time of particle release experiments within the data, however results from outside the 80% coverage were not included in final analysis.