Particle-Based Lagrangian Filtering for Locating Wave-Generated Thermal Refugia for Coral Reefs

Tides and tidally generated waves play a key role in modulating the heat budget of the nearshore environment, which can significantly affect the ecology of the coastal and reef zones. The high spatial resolution required to resolve such waves in numerical models means that diagnosing wave-generated heat fluxes (WHFs) has so far only been possible over very limited areas. Because many marine ecosystems that are affected by WHFs, such as coral reefs, are patchily distributed within domains of hundreds of kilometers, an alternative to fully wave-resolving models is needed to identify thermal refugia and guide conservation efforts over larger regions. In this study, a one-way nested series of regional ocean simulations has been conducted to study the role of waves in driving high-frequency temperature variations in the Coral Triangle and to develop a method for identifying WHF in coarser-resolution models. A filtering method using Lagrangian particles is used to separate the wave component of the flow, which is used to diagnose the wave energy and to identify locations experiencing large, high-frequency temperature variability. These locations are shown to possess three key characteristics: large time-mean wave kinetic energy, shallow depth, and a steep bathymetric gradient that gives access to a nearby source of cold, subthermocline water. A function of the wave kinetic energy and bathymetric slope is found to closely correlate with areas of maximal temperature variance, suggesting its use as a convenient and readily calculable metric to locate thermal refugia in observations or numerical experiments over large spatial domains.