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Tidal and low‐frequency flows interact with abrupt topographic features, giving rise to a variety of small‐scale phenomena including lee waves, vortical motions and turbulence. With the goal of identifying the processes that transfer momentum and energy between the larger‐scale and smaller‐scale flows, detailed tidally‐resolving shipboard surveys were conducted near a sharp and steep headland north of Velasco Reef, Palau, in the Western Pacific. We present velocity, microstructure and rapid‐cycling conductivity and temperature measurements, contextualized with a nearby 10‐month moored record, to examine wake and vortex structures. Observed flow near Velasco Reef was layered and multi‐directional in depth such that the "downstream" side of the headland depended on depth. Downstream of the headland, during a period where mean flow dominated over the tide in the top 100 m of the water column, turbulent kinetic energy dissipation rates were enhanced by up to three orders of magnitude relative to those upstream. During a period dominated by tides, the flow reversed periodically and wake eddies were associated with turbulent dissipation rates up to O10−5Wkg−1 $\mathcal{O}\left(1{0}^{-5}\right)\ \mathrm{W}\hspace*{.5em}\mathrm{k}{\mathrm{g}}^{-\mathrm{1}}$. Turbulent dissipation rates in the wake increased with inverse Richardson number for both tidal and mean flows past the headland, and were larger for stronger vorticity flows. Our analysis suggests that the eddies themselves lead to shear instabilities and subsequent turbulence, either through tilting or their limited vertical extent. We associate the enhanced downstream dissipation with a combination of breaking internal tides and either these eddy processes or a multi‐scaled turbulent wake. Plain Language Summary: Tall, steep underwater features such as ridges and headlands interact with tides and large currents to alter the flow downstream. Here, we present observations near Velasco Reef, Palau, an example of a sharp headland in the western Pacific. Turbulence in the wake of the headland is higher than upstream. During a time when the tides were strong the flow changed direction such that eddies were shed from both sides of the headland. These patches of rotating flow were associated with stronger turbulence than elsewhere around the headland. Shearing of the flow in the vertical direction by the eddies and the internal tides are possible mechanisms by which the turbulence is generated. Key Points: Direct observations of turbulent dissipation rates show enhancement downstream of a headland during mean flow and tidal flow regimesRegions of strong vorticity in the wake are associated with the highest turbulent dissipation ratesShear instabilities are a likely mechanism for the generation of turbulence in the wake [ABSTRACT FROM AUTHOR]

An array of moorings deployed off the coast of Palau is used to characterize submesoscale vorticity generated by broadband upper-ocean flows around the island. Palau is a steep-sided archipelago lying in the path of strong zonal geostrophic currents, but tides and inertial oscillations are energetic as well. Vorticity is correspondingly broadband, with both mean and variance O(f) in a surface and subsurface layer (where f is the local Coriolis frequency). However, while subinertial vorticity is linearly related to the incident subinertial current, the relationship between superinertial velocity and superinertial vorticity is weak. Instead, there is a strong nonlinear relationship between subinertial velocity and superinertial vorticity. A key observation of this study is that during periods of strong westward flow, vorticity in the tidal bands increases by an order of magnitude. Empirical orthogonal functions (EOFs) of velocity show this nonstationary, superinertial vorticity variance is due to eddy motion at the scale of the array. Comparison of kinetic energy and vorticity time series suggest that lateral shear against the island varies with the subinertial flow, while tidal currents lead to flow reversals inshore of the recirculating wake and possibly eddy shedding. This is a departure from the idealized analog typically drawn on in island wake studies: a cylinder in a steady flow. In that case, eddy formation occurs at a frequency dependent on the scale of the obstacle and strength of the flow alone. The observed tidal formation frequency likely modulates the strength of submesoscale wake eddies and thus their dynamic relationship to the mesoscale wake downstream of Palau. [ABSTRACT FROM AUTHOR]

Towed shipboard and moored observations show internal gravity waves over a tall, supercritical submarine ridge that reaches to 1000 m below the ocean surface in the tropical western Pacific north of Palau. The lee-wave or topographic Froude number, Nh0/U0 (where N is the buoyancy frequency, h0 the ridge height, and U0 the farfield velocity), ranged between 25 and 140. The waves were generated by a superposition of tidal and low-frequency flows and thus had two distinct energy sources with combined amplitudes of up to 0.2 m s−1. Local breaking of the waves led to enhanced rates of dissipation of turbulent kinetic energy reaching above 10−6 W kg−1 in the lee of the ridge near topography. Turbulence observations showed a stark contrast between conditions at spring and neap tide. During spring tide, when the tidal flow dominated, turbulence was approximately equally distributed around both sides of the ridge. During neap tide, when the mean flow dominated over tidal oscillations, turbulence was mostly observed on the downstream side of the ridge relative to the mean flow. The drag exerted by the ridge on the flow, estimated to O ⁡ ( 10 4)   N   m − 1 for individual ridge crossings, and the associated power loss, thus provide an energy sink both for the low-frequency ocean circulation and the tidal flow. [ABSTRACT FROM AUTHOR]

An impulsive or rotary wind stress can excite near-inertial oscillations (NIOs) in the surface mixed layer. Although previous work shows that coastal boundaries modify NIOs, few studies have explored their behavior near island topography. Understanding how topography modifies NIOs provides insight into physical processes that contribute to local mixing, which enhances biological richness near islands. Here, encounters between NIOs and the island chain of Palau are examined using moored current meter measurements from a 10-month field study. Near-inertial currents are surface intensified, where typical speeds of 0.15 m s-1 in the surface layer are twice those below 50 m. At moorings farthest from Palau topography, nearinertial surface currents are intermittent and clockwise rotational, suggestive of wind-generated NIOs. Closer to the topography, near-inertial currents become more rectilinear, with enhanced energy at the northern and southern tips compared to along the north-south oriented coastline of the elongated island chain. The first empirical orthogonal function of near-inertial vector surface currents (62% of the total variance in the near-inertial band) reveals a broadly uniform flow that spans the northern and southern extents of the island, suggestive of a slab-like NIO response to wind stress, modified as the island topography blocks the flow. To further characterize the impact of the topography on near-inertial currents, a cluster of moorings at the northern tip of Palau is used to estimate vorticity, which increases as near-inertial current speeds increase. Near-inertial vorticity is attributed to frictional torque caused by the topographically enhanced near-inertial currents brushing against the northern tip of Palau. [ABSTRACT FROM AUTHOR]

Palau, an island group in the tropical western North Pacific at the southern end of Kyushu-Palau Ridge, sits near the boundary between the westwardflowing North Equatorial Current (NEC) and the eastward-flowing North Equatorial Countercurrent. Combining remote-sensing observations of the sea surface with an unprecedented in situ set of subsurface measurements, we examine the flow near Palau with a particular focus on the abyssal circulation and on the deep expression of mesoscale eddies in the region. We find that the deep currents time-averaged over 10 months are generally very weak north of Palau and not aligned with the NEC in the upper ocean. This weak abyssal flow is punctuated by the passing of mesoscale eddies, evident as sea surface height anomalies, that disrupt the mean flow from the surface to the seafloor. Eddy influence is observed to depths exceeding 4,200 m. These deep- reaching mesoscale eddies typically propagate westward past Palau, and as they do, any associated deep flows must contend with the topography of the Kyushu-Palau Ridge. This interaction leads to vertical structure far below the main thermocline. Observations examined here for one particularly strong and well-sampled eddy suggest that the flow was equivalent barotropic in the far field east and west of the ridge, with a more complicated vertical structure in the immediate vicinity of the ridge by the tip of Velasco Reef. [ABSTRACT FROM AUTHOR]

Using a combination of models and observations, the US Office of Naval Research Flow Encountering Abrupt Topography (FLEAT) initiative examines how island chains and submerged ridges affect open ocean current systems, from the hundreds of kilometer scale of large current features to the millimeter scale of turbulence. FLEAT focuses on the western Pacific, mainly on equatorial currents that encounter steep topography near the island nation of Palau. Wake eddies and lee waves as small as 1 km were observed to form as these currents flowed around or over the steep topography. The direction and vertical structure of the incident flow varied over tidal, inertial, seasonal, and interannual timescales, with implications for downstream flow. Models incorporated tides and had grids with resolutions of hundreds of meters to enable predictions of flow transformations as waters encountered and passed around Palau's islands. In addition to making scientific advances, FLEAT had a positive impact on the local Palauan community by bringing new technology to explore local waters, expanding the country's scientific infrastructure, maintaining collaborations with Palauan partners, and conducting outreach activities aimed at elementary and high school students, US embassy personnel, and Palauan government officials. [ABSTRACT FROM AUTHOR]

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. Key Point: Island wake generation is influenced by both low‐frequency and tidal flows [ABSTRACT FROM AUTHOR]