Effect of irregular, abrupt topography on near-inertial currents and waves

Abstract

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 modelling In 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.