Eddy Wake Generation From Broadband Currents Near Palau

Abstract

Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi‐steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low‐frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small‐scale (∼1 km), vertically sheared wake eddies of different vorticity signs were observed with a shipboard survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low‐frequency currents, is explored here with a simple conceptual model. Application of the model to a 10‐month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography.

Energy and Momentum Lost to Wake Eddies and Lee Waves Generated by the North Equatorial Current and Tidal Flows at Peleliu, Palau

“…The flow downstream of Peleliu exceeds 0.5 m s −1 . This strong westward flow produces wake eddies at the south point of Peleliu, similar to those found at the north point of Velasco (Johnston et al, 2019, in this issue;MacKinnon et al, 2019;Merrifield et al, 2019, in this issue;Rudnick et al, 2019, in this issue;Siegelman et al, 2019, in this issue;Zeiden et al, 2019). The upstream Ro is cyclonic and has a magnitude of about 1 (Figure 1a), which is comparable to the model (Figure 1b).…”

“…With laminar con-ditions, flow rounds the point smoothly without eddies, but with increasing turbulence, a range of processes are noted from the formation of an individual attached eddy to a train of downstream eddies, known as a vortex street. At the north point, the near-bottom turbulence responded to both mean and oscillatory tidal flow speeds (MacKinnon et al, 2019). We expect the same to be true at the south point.…”

“…To the best of our knowledge, wakes arising from such a combination of oscillating and mean flow have been quantified only by MacKinnon et al 2019, which is quite surprising because a combination of mean and oscillating flows occurs at many headlands. Indeed, a broad spectrum of eddying wakes observed at the north point of Palau arises from interannual and monthly variability of the NEC combined with oscillating flows at near-inertial and tidal periods (Johnston et al, 2019, in this issue;MacKinnon et al, 2019;Merrifield et al, 2019, in this issue;Qiu et al, 2019, in this issue;Rudnick et al, 2019, in this issue;Siegelman et al, 2019, in this issue;Simmons et al, 2019, in this issue;Zeiden et al, 2019).…”

“…At the north point of Palau (Velasco; upper green square in Figure 1a), a combination of tidal and mean flows produces a broadband wake that ranges from submesoscale, ~1 km diameter, tidally mediated eddies (MacKinnon et al, 2019) to ~70 km diameter island-scale eddies (Zeiden et al, 2019). These eddies are preferentially cyclonic (counterclockwise) due to the mean westward flow.…”

“…where f is the Coriolis frequency and distance (x, y) and currents (u, v) are positive eastward and northward. Small scale (~1 km diameter) high vorticity (Ro ~ 30) wake eddies generated very near the separation point are strongly influenced by oscillating currents, such as tides (MacKinnon et al, 2019) and inertial motions (Siegelman et al, 2019, in this issue). Farther downstream, a mesoscale wake is observed with a lateral scale of ~70 km, comparable to the size of the topography (in this case, Velasco Reef; Zeiden et al, 2019).…”