HF Radar Observations in Palau

As part of the ONR sponsored Flow Encountering Abrupt Topography (FLEAT) initiative, a 3 station HF Radar observatory was installed along the east coast of Palau to study sea surface current variability. The observatory uses Codar Ocean Sensor's long range Seasonde HF radar operating in the 5MHz band, with radar sites at Kayangel, Melekeok and Angaur, and provides hourly sea surface currents out to 170km with 6km horizontal resolution. Two of the radar sites are located on remote islands (Kayangel and Angaur) and require off-grid solar power and satellite internet connection. All of the Palau sites experience the diurnal noise increase that many HF radar's operating a similar frequency in SE Asia report, and contribute to overall range reduction of the measured sea surface currents. In addition to the diurnal increase in noise typical of ionospheric interference, there are semidiurnal and higher frequency variations in the noise floor. These variations are compared with estimates of the vertical Total Electron Count (TEC) provide by International GNSS Service (IGS). Initial analysis shows hot spots of TEC in the vicinity of the Palau HF radar measurement domain. Current variability is dominated by tidal (dominant K1, M2 constituents) and inertial flows in the near-field and offshore portions of the domain. Observed HF radar currents capture near-field and offshore variability associated with both upstream and wake conditions as the Pacific Equatorial Current and Countercurrent interacts with the island topography. Preliminary analyses highlight the role of internal tides in influencing the total variance of the surface currents, and submesoscale eddies are observed with strong vorticity during wake conditions.