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Ocean-Island Interactions in the Western Pacific


Coastal processes around islands encompass dynamics on a range of scales with physics that can differ from typical continental shelves due to steeper bathymetry, potentially allowing the surrounding basin waters to ‘communicate’ quickly to the shoreline. This thesis work advances our understanding about these interactions, using a unique set of nearshore oceanographic observations from around the island group encompassed by the Republic of Palau in the tropical western Pacific.

The influence of large scale and climatic variability on nearshore island environments can be seen through an empirical model of fore reef temperature structure based on SST and sea level anomaly (SLA) made from nearly two decades of temperature observations (2-90m depth) from three stations around Palau. SLA complements SST by providing a proxy for vertical isotherm displacements driven by local and remote winds on intraseasonal to interannual time scales. Thermal stress on coral ecosystems can now be forecast into the mesophotic zone using this means of predicting subsurface temperatures which are easily accessible for the tropical Pacific.

Baroclinic variability around islands has multiple drivers on a range of time scales. Observations of temperature and currents from around the main island group of Palau exhibit a persistent presence of baroclinic coastally trapped waves and internal tides. The largest amplitude signals of coastally trapped waves in fore reef temperature were concurrent with the passage of Typhoon Haiyan, which crossed the northern most Palauan islands in November of 2013. The sub-inertial signals present after Typhoon Haiyan were tracked propagating around the island group for upwards of a week after the typhoon passed. Internal tides were also deemed to be present, but with varying amplitude and phase modulating in and out of phase with the local surface tide.

Surface currents impinging upon Palau have a direct impact on the local sea level field around the island group. An array of nearshore pressure gauges, in depths of 20-28 m, encircling the island group and a high resolution (1/120° x 1/120°) regional circulation model are used to examine the space-time characteristics of the flow in a channel in the southern extent of Palau in comparison to the large-scale currents near the island group. A balance between the along-channel pressure difference and bottom friction in the channel was inferred based on the current and pressure observations and the high-resolution model simulations. A drag coefficient for the channel, computed using in situ observations, is O(10-3-10-4). Variations in large-scale zonal currents correlate with the pressure difference across the channel as well as the along-channel flow. The model simulations indicate that as the large-scale flow impinges on the island group, topographic blocking results in a pressure difference on either side of the island which causes a pressure gradient along the channel.

The fore reef waters of Palau are shown to be influenced by a range of dynamics across all spatial and temporal scales of our observations. There is an apparent omnipresence of both internal tides and coastally trapped waves throughout the observational window which provide a regular cycling of temperature at depth. These waves have their largest effects at the thermocline, the depth of which can be estimated in this region using only surface variables, as described above. Together, our assessments of these dynamics provide an enhanced perspective on the potential for thermal conditioning of benthic communities living on the outer reef slopes and an advanced perspective of how the large-scale oceanographic field translates to the fore reef environment.

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