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Sanctuary Rocky
Intertidal Open
Ocean & Endangered
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Observations
of Physical Ocean Processes by the Wave and current forcing over the continental shelf and surf zone have profound effects on all life forms within the coastal ocean, as they interact strongly with the shallowing ocean bed towards the shore. This is most obviously seen during extreme storm events when waves reshape the inner shore bed, removing massive volumes of sand to offshore bars, lowering beach levels, and increasing undercutting of sand dunes and cliffs. However even during times of low wind and wave forcing, waves and currents shape and modify the ocean bed, which provides habitats for a wide range of species. The complex interactions between the moveable sandy bed and fluid motions resulting from surface waves, wind-forced currents, internal waves, and tidal currents are still poorly understood. These are important research topics for physical oceanographers interested in understanding and modeling these processes. In August 1999 the Monterey Inner Shelf Observatory (MISO) was established as a component of the Naval Postgraduate School (NPS) Oceanography Department's Rapid Environmental Assessment Laboratory (REAL). The facility consists of a long-term cabled instrument frame deployed near the southern end of Monterey Bay, offshore from the NPS property. The cable end node, which can power and provide data links for up to eight instrument systems, is deployed in twelve meters of water in a sandy area, about 600 meters from the shoreline. A multi-conductor/fiberoptic cable connected to the shorefront Marine Operations Laboratory at NPS provides power and high-bandwidth data links, including multiple video channels, to data acquisition and processing computers onshore. Two additions were made to the REAL observation program early in 2000. A complete meteorological station was established on the sand dunes inshore from the MISO array, and a directional wave rider buoy was deployed ten kilometers offshore from NPS. Further information on these systems and real time graphical data from all the sensor systems can be found at: http://www.oc.nps.navy.mil/~stanton/miso/ During the first year of operation, nearly continuous observations have been made of the ocean current velocity. These include measurements of cross- and along-shore currents every 0.5 meters to the surface, high resolution water pressure time series, continuous digital video and structured light observations of the sandy bed, and acoustic altimeter mappings of the bed. The scientific emphasis has been to measure the changes in bed ripples in response to wave and current changes, since the bed ripples greatly affect the rate of wave energy loss as large swell waves travel across the continental shelf. The long-term observations at the MISO site are being used to improve wave propagation models in coastal areas so that the effects of large storm systems on the coast can be predicted more accurately. Figure 1 illustrates the application of wave and current observations to the understanding of the inner shelf ecosystem. A two-day period of current velocity profiles has been integrated in time to infer the lateral displacement of particles in the water column at different heights above the bed. Each particle trajectory starts from the (0,0) coordinate in the figure, and the different displacement traces in the water column result from the differing, depth-dependent, time-evolving currents. For example, at 2.9 meters' height the currents are dominated by four small tidal displacement loops with a small net offshore displacement of one kilometer, whereas 12.5 meters above the bed the tidal component is much smaller than the wind-driven on-shore and long-shore currents resulting from the summer-time afternoon sea breeze typically seen across Monterey Bay. The strong vertical gradients in currents that result in these depth-dependent particle tracks are largely the result of density gradients in the water column (caused by small temperature and salinity changes with depth), which act to isolate surface wind stress from the deeper water column.
Understanding the physical oceanography of the water column and
bottom boundary
Timothy Stanton |
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For comments or question please refer to the Webmaster Last modified on: Jan 15, 2000 |
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