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Motivated by observations of enhanced near-inertial currents at the island chain of Palau, the modification of wind-generated near-inertial oscillations (NIOs) by the presence of an island is examined using the analytic solutions of Longuet-Higgins and a linear, inviscid, 1.5-layer reduced-gravity model. The analytic solution for oscillations at the inertial frequency f provides insights into flow adjustment near the island but excludes wave dynamics. To account for wave motion, the numerical model initially is forced by a large-scale wind field rotating at f , where the forcing is increased then decreased to zero. Numerical simulations are carried out over a range of island radii and the ocean response detailed. Near the island, wind energy in the frequency band near f can excite subinertial island-trapped waves and superinertial Poincaré waves. In the small-island limit, both the Poincaré waves and the island-trapped waves are very near f , and their sum resembles the Longuet-Higgins analytic solution but with increased amplitude near the island. The flow field can be viewed as primarily a far-field NIO locally deflected by the island plus an island-trapped contribution, leading to enhanced near-inertial currents near the island, on the scale of the island radius. As the island size is increased, the island-trapped wave frequency deviates further from f and its amplitude depends strongly on the frequency bandwidth and wavenumber structure of the wind forcing. In the large-island limit, the island-trapped wave resembles a Kelvin wave, and the sum of incident and reflected Poincaré waves suppresses the near-inertial current amplitude near the island.

American Samoa is experiencing rapid relative sea level rise due to increases in global sea level and significant post-2009 earthquake land subsidence, endangering homes and critical infrastructure. Wave and water-level observations collected over a fringing reef at Faga‘itua Bay, American Samoa, in 2017 reveal depth-limited shoreline sea-swell wave heights over the range of conditions sampled. Using field data to calibrate a one-dimensional, phase-resolving nonhydrostatic wave model (SWASH), we examine the influence of water level on wave heights over the reef for a range of current and future sea levels. Assuming a fixed reef bathymetry, model results predict rising sea levels will escalate nearshore extreme water levels that are dominated by an increase in nearshore sea-swell wave heights. Model results provide insight into how and at what reef depths rising sea levels reduce reef capacity to dissipate wave energy, compounding shoreline threats. This study aims to bring increased attention to the immediate threats to American Samoa’s way of life, and to demonstrate the utility of SWASH for extrapolating wave transformation to future sea level.

The North Equatorial Current (NEC) transports water westward around numerous islands and over submarine ridges in the western Pacific. As the currents flow over and around this topography, the central question is: how are momentum and energy in the incident flow transferred to finer scales? At the south point of Peleliu Island, Palau, a combination of strong NEC currents and tides flow over a steep, submarine ridge. Energy cascades suddenly from the NEC via the 1 km scale lee waves and wake eddies to turbulence. These submesoscale wake eddies are observed every tidal cycle, and also in model simulations. As the flow in each eddy recirculates and encounters the incident flow again, the associated front contains interleaving temperature (T) structures with 1–10 m horizontal extent. Turbulent dissipation (ε) exceeds 10−5 W kg−1 along this tilted and strongly sheared front. A train of such submesoscale eddies can be seen at least 50 km downstream. Internal lee waves with 1 km wavelengths are also observed over the submarine ridge. The mean form drag exerted by the waves (i.e., upward transport of eastward momentum) of about 1 Pa is sufficient to substantially reduce the westward NEC, if not for other forcing, and is greater than the turbulent bottom drag of about 0.1 Pa. The effect on the incident flow of the form drag from only one submarine ridge may be similar to the bottom drag along the entire coastline of Palau. The observed ε is also consistent with local dissipation of lee wave energy. The circulation, including lee waves and wake eddies, was simulated by a datadriven primitive equation ocean model. The model estimates of the form drags exerted by pressure drops across the submarine ridge and due to wake eddies were found to be about 10 times higher than the lee wave and turbulent bottom drags. The ridge form drag was correlated to both the tidal flow and winds while the submesoscale wake eddy drag was mainly tidal.

The interaction of large-scale oceanic flows with remote island chains can lead to turbulent wakes, enhanced vorticity production, and significant increases in biological productivity. This study showcases the range of flow conditions captured by surface current mapping high-frequency (HF) radar systems deployed around the main island group of Palau in the western Pacific. The radar array captures strong tidal and inertial flows, both near- and offshore, as well as the spatial and temporal variability associated with the synoptic geostrophic flow interacting with the island group. Surface currents measured by HF radar are significantly correlated to currents in the upper 100 m of the ocean water column, as observed with a concurrent mooring, such that the resulting surface spatial maps provide insight on the wake flows of the island across a significant portion of the upper ocean. Composite averages of eastward and westward incident flow show flow-splitting and reconnection 60 km upstream and downstream of the island group, respectively. Surface current variability observed by the radar array includes topographically blocked flow, flow separation and acceleration through passages in the island chain, eddy dipole structure, and coastal eddies with Rossby numbers of 5. The range of variability near the island chain is reflective of the complex incident flow, which encounters Palau from all directions and changes on timescales of hours to weeks. A high-resolution model qualitatively agrees with the HF radar observations and shows vorticity filaments generated downstream of the island passages.

An impulsive or rotary wind stress excites inertial oscillations (IOs) in the ocean mixed layer. In the open ocean, IOs behave as uniform slabs rotating anti-cyclonically. However, IOs can become divergent by features, such as the spatial variability of the winds, latitudinal variations in the Coriolis frequency, f, and coastal topography, pumping the base of the mixed layer and exciting near-inertial waves (NIWs). Studies have found IOs in the mixed layer weaken near coastal topography due to the reflection of NIWs excited by the IOs impinging on a boundary; however, few studies have considered island topography. Here, the encounter between wind-generated near-inertial oscillations (NIOs) and islands, with particular focus on the island chain of Palau, is examined using observational and analytical techniques, and numerical modellingIn the first chapter, mean surface current (0-50 meters) observations from a 10-month field deployment of moored Acoustic Doppler Current Profilers (ADCPs) are used to examine how near-inertial oscillations (NIOs) are modified by coastal boundaries around the island chain of Palau. At moorings farthest from Palau, near-inertial surface currents are intermittent and clockwise rotational, suggestive of wind-generated NIOs. Closer to topography, near-inertial currents become rectilinear, with enhanced energy at the northern and southern tips of the meridionally elongated island. EOF analysis reveals that most of the NIO variance is explained by coherent flow across the breadth of the island (approximately 150 km), suggestive of a slab-like NIO response to local wind stress, which is strongly modulated as the island topography blocks the flow. Vorticity estimates from a cluster of moorings at the northern tip of Palau further reveal that near-inertial vorticity generation increases during bouts of strong near-inertial currents.The second chapter dives deeper into the dynamics of these observations using analytical techniques in conjunction with a linear, inviscid, 1.5-layer reduced gravity model of idealized, circular island topography. Longuet-Higgins (1970) first presented the analytical solutions of IOs around a circular and elliptical island. Here, I revisit this work to understand the limitations and relevance of the solution to the ocean. The reduced gravity model is used to assess the solution under more realistic ocean conditions, most notably when a wind forcing is used to excite IOs and when free waves are allowed to propagate. The numerical model shows the response of IOs around circular islands is composed of three primary components: the radiation of Poincaré waves, a blocking IO response, and an island trapped wave (ITW).Finally, the third chapter presents results from the 1.5-layer reduced gravity model to explain the observed spatial variability of inertial currents around Palau (Chapter 1) and understand their contribution to the generation of vorticity. First, results from Chapter 2 are extended to an elliptical island, which more similarly reflects the geometry of Palau than a circular island. Second, non-linear and viscous terms are considered to examine the generation of vorticity around an ellipse. Third, a Palau shaped land mask is used to understand how features specific to Palau, such as a deep channel separating the main island, Babeldaob, from the northern region, Velasco Reef, impact the behavior of IOs. Finally, because Palau is situated near the equator, the effect of latitudinal variations of f on the behavior of IOs, Poincaré waves, and ITWs around circular islands is explored.

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.

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.