Hydrodynamics of a Pacific Atoll System – Mechanisms for Flow and Ecological Implications

We present characterization of the waves and hydrodynamics of the Palmyra Atoll system based on field measurements from 2012 to 2014, and modeling simulations using a coupled waves and three-dimensional hydrodynamic model (COAWST). At the scale of the atoll itself, strong regional flows create flow separation and a well-defined wake, similar to the classical fluid mechanics problem of flow past a cylinder. Circulation within the atoll is primarily governed by tides and waves, and secondarily by wind and regional currents. Tidally driven flow is important at all field sites, and tidal phase delay occurs between the forereef and interior lagoons. Wave driven flow is significant at most of the field sites, and is a strong function of the dominant wave direction. The wave climate is characterized by strong northerly waves which drive across the atoll to the south and through the lagoon system to the west and out the channel to offshore. Wind driven flow is generally weak, except on the shallow terraces. Bottom stress is significant for depths less than about 10 m. Based on Lagrangian float tracks, the mean age appears to clearly differentiate the geomorphic structures. The biological cover shows significant trends with mean flow, age and temperature. While high mean flow appears to differentiate productive coral regions, low water age and low temperature appear to be the most important variables for distinguishing between biological cover types at this site. The resulting connectivity within the atoll system shows that the general trends follow the mean flow paths; however, some connectivity exists between all ten regions of the atoll system.