The Broadband, Multi-scale Vorticity Wake Generated by flow past Palau
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The Broadband, Multi-scale Vorticity Wake Generated by flow past Palau

  • Author(s): Zeiden, Kristin
  • Advisor(s): Rudnick, Daniel L;
  • MacKinnon, Jennifer A
  • et al.
Abstract

Island wakes often have a significant influence on regional current variability and theycan be a strong source of vorticity far from the boundaries of major ocean basins. Historically they have been difficult to characterize due to the wide range of flow scales which contribute to their variance. In this thesis work, observations from gliders, moorings, and surface drifters are combined to investigate the vorticity wake behind Palau, a ~200 km long island in the deep tropical North Pacific. First, the island-scale (>30 km) flow is characterized using velocity data obtained with underwater gliders which profiled along two parallel meridional sections upstream and downstream of the island. On average, westward flow is topographically blocked, accelerates around the island, and there is recirculation in the lee with vertical vorticity up to 0.3 f . Here f is the local Coriolis frequency. However, vorticity variance is high and instantaneous values can exceed f when the incident flow is strongest. Then, a triangular array of moorings (7 km) deployed within a few km of the north end of the island is used to capture submesoscale vorticity as it is injected into the wake. At this scale, vorticity can exceed 6 f during strong westward flow. This vorticity is associated with the formation of strong wake eddies. We find that tidal and inertial currents heavily modulate the frequency of eddy formation, even during strong low-frequency flow. Finally, vorticity downstream of Palau is investigated with clusters of surface drifters deployed in the same location as the mooring array. Drifters were often entrained in submesoscale wake eddies with vorticity up to 6 f , consistent with observations from the moorings. Outside the eddy cores, vorticity is inversely proportional to cluster scale and we conclude the growth of wake eddies is controlled by large-scale shear in the regional currents.

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