Observations of Broad-band Vorticity Generation around the Island of Palau

Kristin Zeiden1, Jennifer A MacKinnon1, Daniel L Rudnick1, Matthew Alford1, Gunnar Voet1 and Hemantha W Wijesekera2, (1)Scripps Institution of Oceanography, La Jolla, CA, United States, (2)Naval Research Laboratory, Ocean Sciences, Stennis Space Center, MS, United States
Current observations from moorings and autonomous underwater Spray gliders are used to characterize the broad-band content of vorticity generated around Palau, a steep-sided island in the tropical North Pacific. Repeat meridional glider sections up- and down-stream of the island provide mean currents, as well as variability on time scales of weeks to months and spatial scales greater than 30 km. An array of 14 moorings deployed closer to the island provide concurrent high-resolution velocity time series as well as vorticity estimates from sub-sampled mooring triangles ranging in scale from 6 - 20 km. We find that on scales greater than 30 km and 14 days, the mean incident current accelerates to circumvent the island and separates, leading to recirculation in the lee. The resultant mean wake vorticity at this scale can reach up to 0.3f near the surface, i.e. a Rossby number = O(10-1). However, current variability is high and instantaneous vorticity can exceed f for exceptionally strong surface currents. Closer to the topography and on smaller scales, current variability in the inertial and tidal bands is similar to the mean flow, and high-frequency vorticity can exceed 5f on scales as fine as 6 km. This range of Rossby numbers suggests that ageostrophic effects are likely important in the Palau wake. We further observe significant increases in vorticity variability in the tidal and inertial bands during prolonged periods of strong low-frequency flow (i.e. weeks and greater), suggestive of a non-linear, multi-scale vorticity generation process. Finally, estimates of vorticity flux are used to investigate the evolution of vorticity away from topography.