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2 State-of-the-Art Overview: Physical Oceanographic Processes, Features, and Methods of Potential Importance to the ESP
Pages 25-52

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From page 25...
... Its findings have been grouped for discussion into the following major sections: Transport, Stirring, and Mixing Processes; Numerical Models; Sea Ice; and Sediment Transport. The Problem of Assessing Impacts of Oil Exploratiom A Physical Oceanographic Perspective Before discussing the state of knowledge of the physical oceanography of a region and the adequacy of this information for impact assessments of oil and gas exploration or production, it is appropriate to consider the specific physical information that is needed.
From page 26...
... The first and second processes, advection by currents and wave effects, are of major importance in the immediate translation and dispersal of a spill. Sediment Transport Processes The physical processes responsible for the deposition, mixing, resuspension, and transport of bottom sediments are most closely tied to the long-term effects of petroleum exploration, development, and production.
From page 27...
... Simple Eulerian flow fields varying in time and space with a single frequency and wave number give particle motions with a complex spectrum, containing both harmonics and a zero-frequency component. The latter corresponds to a net drift rate for a particle- a Lagrangian mean flow which is different from the average velocity measured at a point (the Eulerian mean)
From page 28...
... Whether this convergence results from offshelf oceanic processes or simply reflects spatial variations in the wind that forces the coastal ocean is not known. Oil Spill anti Circulation Models The above points regarding mixing and transport have important implications for the models used in oil-spill-risk analysis.
From page 29...
... .1 HANSPORT, STIRRING, AND MIXING PROCESSES Wind Stress Drag Coefficient and Space and Time Resolution Determination of surface-wind stress depends on (1) determination of the wind speed and direction at the appropriate space and time scales and (2)
From page 30...
... 30 ~ ~' W- l W:~m, :\ \3 \ me\ - - ,/ Al · ~ i : Id -~< ado: a: ~ ~ ~ ~ : 0 ~ a.
From page 31...
... The atmospheric boundary-layer measurements made in CODE suggest a simple conceptual model which can explain much of the physiology or structure of the marine layer and associated surface wind field during periods of persistent upwelling-favorable winds. During these periods, which represent the quasi steady state regime during the upwelling season, the inversion base of the marine layer drops eastward towards the coast until it intersects the coastal mountain range at a height of several hundred meters, and the associated thermal wind produces an along-shelf wind jet which has a maximum speed just below the inversion base.
From page 32...
... (1987) reported on an important verification of the classical Ekman theory of wind-driven transport in the ocean surface layer, based on a careful analysis of upper ocean data from the Long Term Upper Ocean Study (LOTUS)
From page 33...
... (Copyright 1988 by the American Geophysical Union.) Mixed Layer Turbulence According to Muller and Garwood (1988~: Because observations of the vertical fluxes of momentum, mass, and heat are still lacking in the upper ocean, observations of the dissipation of turbulent kinetic energy are still the single best evidence of the intensity of turbulent mixing In the mixed layer.
From page 34...
... He found the ratio of surface Stokes drift velocity to wind speed measured at 19.5 m above sea surface to range from 1.6 to 3.6%, which is large enough to make a significant contribution to the overall surface wind drift. Response to Severe Storms Allen et al.
From page 35...
... The occurrence of these extratropical cyclones is manifested by wind forcing over the continental shelf in the 2-10 day synoptic period. Cold air outbreaks that follow the passing of cyclones advect cold, dry continental air across the relatively warm shelf and Gulf Stream waters.
From page 36...
... have presented evidence for the existence of "short" coastal-trapped waves east of Scotland. Despite the observations of truly free coastal-trapped waves, the main value of the wave theory is in its ability to explain the character of wind-forced pressure and alongshore velocity fluctuations over the continental shelf and slope.
From page 37...
... have demonstrated that, when a single first order wave equation (with friction) is driven by a realistic space-time spectrum of wind stress, currents and sea level fluctuations at a given point are always best correlated with wind fluctuations earlier in time and farther to the south.
From page 38...
... stated: Over the last several years, interest has grown considerably in processes which govern time-averaged flow over the continental shelf and slope. Early attempts to explain mean flow patterns centered on wind-driven, frictionally dominated models which ignore alongshore variations.
From page 39...
... The most recently discovered interaction takes place northeast of Cape Hatteras and involves the relation between shifts of the Gulf Stream axis and changes in the transport of slope currents 100-300 km to the north. The dynamics associated with this coupling are unknown and should be examined as a part of a more general study of the physical oceanography of the mid- and upper slope.
From page 40...
... (1983) found evidence from the climatological hydrography that active exchange of water properties occurs across the shelf break: about 20% of the volume of the shelf water is replaced by the Gulf Stream water per month....
From page 41...
... Instead, a well-defined front separates the shelf and slope water masses throughout the year, especially in the lowest half of the water column. During the summer, no density front exists in the upper water column because the lower temperatures of the shelf waters are sufficient to counteract salinity differences.
From page 42...
... (Copyright 1987 by the American Geophysical Union.) A series of fronts is maintained on the Bering Shelf by the varying balance between buoyancy input and turbulent mixing caused by the tidal currents as the depth of the water changes (Coachman, 1986)
From page 43...
... While the alongshore tidal variance is an important signal in many, but by no means all, conditions, its cross-shore variance usually dominates the variance due to other processes. Tidal currents are sufficiently energetic to vertically mix the water column inshore of the 50-m isobath in the Bering Sea shelf (Schumacher et al., 1979)
From page 44...
... However, the difference between Lagrangian and Eulerian mean flows could be real. NUMERICAL MODELS Circulation Modeling Circulation modeling was one of the topics considered at a recent workshop on U.S.
From page 45...
... Testing of Numerical Models There is a need for synthesizing the results of field programs and modeling efforts to achieve the maximum utility from both (a particularly important goal for MMS's OSRA modeling efforts)
From page 47...
... 47 So o _ ~ in a: ., o UJ _ .
From page 48...
... Oil on the sea surface will drift in response to wind and currents. Dispersion of oil into the water column isolates it from the effects of wind, and the dispersed oil plume will drift with the near-surface currents.
From page 49...
... Deterioration of the ice sheet in the spring releases most of the oil in first-year ice back into the water; the rest travels with the broken ice sheet (Logan et al., 1975~. In ice-infested waters (10-80% ice cover)
From page 50...
... Once the ice has melted completely, the oil will reside on the sea surface, possibly being incorporated in a weathered state into the first-year ice of the next winter. Alternatively, such weathered residual oil may sink to the seafloor and become incorporated into the sediments.
From page 51...
... SEDIMENT TRANSPORT Bottom Boundary Layer and Transport of Suspended Materials Brink (1987) wrote: The surface and bottom boundary layers represent the areas where shelf waters absorb the wind and bottom stresses, respectively.
From page 52...
... However, with few exceptions, there appears to be a dearth of specific data on suspended matter concentrations in the water column collected in conjunction with ESP studies conducted in continental margin areas. Bottom-boundary layer and sediment transport studies carried out as part of the ESP have been performed mainly by USGS scientists via a memorandum of understanding with MMS.


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