Wave-Driven Flows over Coral Reefs and their Effects on Lagoon-Ocean Exchange

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ABSTRACTOCE-0622967Intellectual meritA combined observational and modeling study will be conducted to address barotropic dynamics of wave-driven flows over coral reefs and the concomitant ocean-reef exchange. The work is designed to test how well radiation stress theory explains flows over and through the complex topography of typical reef/lagoon complexes, as well as examine how outflow jets from reef lagoons interact with the wave-driven inflow over a reef and thus determines net exchange. The field observations will be conducted at a micro-tidal lagoon system on Moorea, over a range of wave conditions. In each experiment, an array of Acoustic Doppler Current Profilers (ADCPs) and wave/tide gauges will be deployed along a cross-reef transect to resolve the key terms in the cross-reef momentum balance, and obtain the best estimates to date of bottom stress over coral reefs. A second array will be deployed to measure mass fluxes through the lagoon and out the reef pass, to complete a water and exchange budget for the system. Finally, intensive shipboard profiles of the water column and drifter releases will be used to determine the fate of the pass jet and potential for re-entrainment to the reef, particularly in the presence of persistent alongshore flow. In parallel with the field program, a series of numerical experiments on idealized reef-lagoon-pass systems will be conducted, mapping the parameter space of dynamical balances. The numerical work will use the open source circulation model developed at Stanford University (3D, baroclinic, non-hydrostatic, unstructured grid, finite volume, parallel code) coupled to a wind-wave model. The proposed simulations looks at a class of nearshore flows that has received less attention than has similar flows on beaches and should reveal the extent to which the present model of the interaction of waves and mean flows works for a geometry that is different than that of a beach. The "confrontation" of model and observation may point the way to using new classes of wave models, or even highlight fundamental differences between theory and observation that have no present explanation. From the standpoint of shallow water dynamics, the role of waves in modifying the outflow from a small inlet is also novel. Broader ImpactsThe functioning of coral reefs, some of the most threatened and economically valuable marine ecosystems, are strongly dependent on hydrodynamics. The experiments and computations will significantly improve our understanding of coral reef hydrodynamics, and thus speed progress toward predictive models that could be used to set environmental policies, or in designing marine reserves based on knowledge of "connectivity". From an interdisciplinary standpoint, this research will provide biologists and geochemists working on reefs with means of understanding how remote forcing (swell) changes conditions for organisms of interest or for chemical reactions and transformations. The research team has a record of engaging and collaborating with other disciplines. The physical understanding of the system will greatly aid numerous LTER activities (e.g. zooplankton, fish ecology, nutrient dynamics, etc.) at the field site. The numerical circulation model will provide the essential foundation any future 3D physical biological modeling of this and similar systems. Investment in the Stanford model development will benefit the oceanographic community via our long-standing practice of sharing model source code with other users. In terms of education, this project will involve the training of 2 women PhD students (both NSF fellows), noting that females are underrepresented in physical oceanography. They will both learn about observational physical oceanography, scientific diving and high performance computing. The field work will also involve several other graduate and undergraduate students who will be exposed to and participate in interdisciplinary experiments, as well gaining experience with studies of complex coastal flows.