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CRES: Component: Physical Oceanography
Overview
One of the most integral factors across the whole of the CRES program is a detailed understanding of basic current patterns around Puerto Rico and the US Virgin Islands. Currents play a fundamental role in coupling the input of terrestrial materials (sediments, nutrients, pollutants) and the final deposition on the bottom or uptake into the benthic coral reef community, and are thus an integrator of terrestrial and marine processes. Currents flows are also the primary determinant of the extent of larval connectivity among areas and therefore key to understanding the large scale function of reef communities and an important consideration in designing networks of marine reserves and understanding their impact.
OBJECTIVES
- To develop predictive, high resolution models of current flow around Puerto Rico and the US Virgin Islands.
- To adopt this model to predict the transport of substances (e.g., sediment, eggs, larvae) by currents in the region.
- To build a user-friendly interface to these models so that they can readily be used by managers and other technical staff.
- To specifically use these models to predict the expected larval connectivity between existing and proposed marine reserves and known and suspected reef fish spawning aggregations.
DESCRIPTION
The primary approach for this component is the application of the Regional Ocean Model System (ROMS)separately to Puerto Rico and the Virgin Islands. ROMS is a free-surface, hydrostatic, primitive equation ocean model that uses stretched, terrain-following coordinates in the vertical and orthogonal curvilinear coordinates in the horizontal. This model accommodates a variety of features that makes it ideal for this type of work, including high-order advection schemes, accurate pressure gradient algorithms, several subgrid-scale parameterizations, atmospheric, oceanic and benthic boundary layers, biological modules, radiation boundary conditions, and data assimilation. These features allow the model to run in shallow water (unlike other currently available models of dispersal processes), and thus to incorporate the effects of shelf topography and the shelf-edge drop-off thought to be important factors controlling nearshore circulation and the potential for dispersal/retention of water-borne materials.
ROMS requires input of (i) wind data to provide the surface boundary stresses driving water circulation and wave action, and which will be obtained from local weather stations and various sources (e.g., ECMWF, NASA JPL Scatter-winds), (ii) high resolution bathymetry from NOAA-generated data and local surveys, and (iii) climatological variables (e.g., temperature, salinity) from NOAA sponsored databases such as COADS and Levitus (1998). The implementation of ROMS will include a high resolution grid (target at 500m), with a yearly identified (initially 2003 – 2004) climatology and winds (local and satellite). The simulation forcing includes a stability run and may include initial, boundary and forcing conditions by nesting with a larger scale model.
Figures 1 and 2, although from an earlier model (SCRUM) and using a coarser (2km) grid and monthly averaged wind stress, illustrate on a large scale the type of output expected. These suggest that an almost perennial (November-August) westward flow (2.5cm/s) exists along the nearshore waters off southwestern Puerto Rico (Figure 1), with a return circulation further offshore to the east. During September-October the flow was eastward (Figure 2). Eddy structures were also noted in relation to three topological features in the Mona Passage west of Puerto Rico: Mona Island, El Pichincho seamount, and the western insular shelf.
To track particles (e.g. sediments, larvae), an advective model with programmed particle behavior will be developed based on Lagrangian movements (taken from the output of the general circulation model). This will be used to test patterns of advection and dispersal.
RELATION TO OTHER RESEARCH COMPONENTS
Major anticipated results of these modeling efforts include descriptions of the (i) main features of circulation on the shelf and slope regions at horizontal scales of 500m or less, roughly approximating the spatial scale relevant to many coral reef processes, and the (ii) transport/advection of dissolved substances, shelf sediments, and meroplankton. These efforts will help to integrate and further understand the processes affecting patterns of water borne transport of sediments and nutrients as determined in the components on Transport of Watershed-based Sediments and Water Column Transport Processes, and ultimately to the impact of these materials on the Coral Reef Communities.
The potential to predict the transport of eggs and larvae is a critical component of the MPA and Marine Reserve studies. Model output will be important for designing networks of reserves as it will indicate the necessary spacing to maintain connectivity while identifying the areas most likely to benefit from seeding of reserved-spawned larvae. The modeling efforts included in the component on Ecological and Socioeconomic Coupling will incorporate the output of the flow model for understanding large scale linkages and ecological change.
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