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Appendix A Glossary of Physical Oceanography Terms Aliasing. The result, in the determination of the spectrum of a data set, that signals with periods shorter than twice the sampling interval have their energy appear at longer periods (lower frequencies) in the spectrum. ARGOS. A commercial satellite service that operates by receiving information from drifters and subsequently relaying the information to ground-based stations. AXBT. Air-dropped expendable bathythermograph. Baroclinic. Baroclinic flow is the portion of the flow due to the additional horizontal pressure gradients resulting from density variations (as opposed to the portion caused by the slope of the free surface). Barotropic. Refers to flow fields that are not affected by the density stratification. BEE. MMS's Branch of Environmental Evaluation. BEM. MMS's Branch of Environmental Modeling. BES. MMS's Branch of Environmental Studies. Beta plane. An approximation in which the variation of the Coriolis paramter is taken to be linear with latitude. BLM. Bureau of Land Management. Brunt-V1istild frequency or buoyancy frequency. The hardest temporal frequency that internal waves can have. Typical oceanic values are 10 s~ . Buoyancy-driven flow. Currents occurring due to thermodynamic ocean forcing. An example is the flow field due to freshwater input from rivers or precipitation. Circulation model. A physical oceanographic mathematical model that calculates estimates of the currents and mass distribution as a function of time and space. Coastal-trapped waves. Waves that propagate along the shore and not out to sea. These can be either surface waves or internal waves. CODE. Coastal Ocean Dynamics Experiment. Convergence zones. Areas where currents come together. Coriolis parameter. The strength of the apparent force that, as a result of the earth's rotation, tends to divert objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. CTD. Current, temperature, and depth. A CTD station will continuously monitor these three quantities. CTM. Characteristic tracing model. Curl. A measure of rotation in the wind-stress field. Cyclogenesis. The processes that create atmospheric or oceanic counterclockwise (clockwise) rotation eddies in the Northern (Southern) Hemisphere. Deep-water wave. A wave whose length is small compared to water depth and whose speed is determined by wave length. Diagnostic model. Steady-state (stationary) ocean circulation model determined from real data. 129
130 Dispersion. The spreading of waves as opposed to the steepening of waves (wave dynamics usage); transport of oil from the sea surface into the water column primarily due to APPENDIX A breaking wave activity and associated near surface turbulence (oil-spilI-modeling useage). Doppler shift. The change in frequency due to relative motion; e.g., the whistle of an approaching train has a higher frequency than the whistle of a departing train. Eddies. In this report, organized horizontal structures that rotate either clockwise or counterclockwise. Gulf Stream rings are examples of eddies. Eddy coefficient. The horizontal or vertical mixing coefficient with units of velocity * e-folding. A factor of 1/e, represents the time/space scale for a change of 1/e if exponential decay/growth is assumed. Ekman layer. The upper few tens of meters of the ocean, which are susceptible to wind-induced transport. Ekman pumping velocity. The vertical velocity in the ocean that is proportional to the curl of the wind stress. Ekman spiral. The flow direction of the Ekman transport. It rotates clockwise with increasing depth. Ekman transport. The horizontal transport of mass by currents in the upper few tens of meters of the ocean by the local wind. It is a function of latitude due to the effect of the Coriolis acceleration (the effect of rotation of the earth) and the wind stress. E1 Nino. The appearance of anomalously warm water off South America in the eastern tropical Pacific. ENSO. E1 Nino-Southern Oscillation. ESP. Environmental Studies Program. Eulerian. See Lagrangian. FACTS. Florida Atlantic Coast Transport Study. Filaments. Typically, narrow mesoscale ocean circulation features in the upper ocean that have temperature and/or density characteristics of nearby prominent features. For example, patches of water originating from the Gulf Stream are found inshore off the Carolinas. FNOC. Fleet Numerical Oceanographic Center. Friction velocity. The square root of the wind or ocean horizontal shear stress divided by the local density, which has the unit of velocity; an important velocity scale for shear- generated turbulence. GALE. Genesis of Atlantic Lows Experiment. OARS. Gulf of Alaska Recirculation Study. GCM. General circulation model. GOES. Geostationary operational environmental satellite. Geostrophic currents. Horizontal currents in which the Coriolis acceleration is balanced by the horizontal pressure gradient. Gulf Stream rings. The Gulf Stream sheds 10-20 strong eddies each year. These are called rings because they have a ring of strong currents (1-3 m/s) at a radius of 50-150 km. The eddies shed to the north of the Gulf Stream have warmer centers and the currents are clockwise (warm-core rings). The rings to the south have relatively cold centers and the currents are counter-clockwise. These rings usually propagate slowly westward and thus the warm-core rings eventually strike the continental shelf/slope region north of the Gulf Stream. Hamiltonian. An integral rather than differential statement of the dynamics of a system. Internal waves. The up-and-down movement of horizontal density surfaces. These structures propagate horizontally and vertically in the ocean. Internal Kelvin waves. (See coastal-trapped waves). These propagate with the coast to the right in the Northern Hemisphere, with the maximum amplitude at the coast. They are named after Lord Kelvin, who discovered the phenomenon mathematically in the late 19th century. Inversion. An increase of temperature with increasing altitude in the atmosphere, or with increasing depth in the ocean. Inviscid. Without friction as a mechanism in the motion of a fluid. ISHTAR. Inner Shelf Transport and Recycling.
GLOSSARY OF PHYSICAL OCEANOGRAPHY TERMS 131 Isopycnal. Line of equal density. Lagrangian. Refers to motion following a specific small parcel of fluid, oil, or buoy. The opposite is Eulerian, which refers to motion at a fixed point. Langmuir cells. Long (1-20 km), organized mesoscale structures in the mixed layer that occur in light winds and transport material in the upper ocean. LFM. Limited-area fine-mesh meteorological model. Loop Current. The clockwise bend in the surface ocean current that flows northward past the Yucatan Peninsula toward Florida and then turns to the southwest toward the Straits of Florida, where the current is called the Florida Current. M constituent. The principal lunar component of semidiurnal tides. MiSAR. Mid-Atlantic Slope and Rise study. Mesoscale. In this report, circulation features on horizontal length scales of 5-100 km. Microstructure. Small (1-20 cm) vertical variations in the oceanic temperature, salinity, or density stucture. Mixed layer. The upper few tens of meters of the upper ocean, which have an almost uniform temperature. The depth at which the ocean temperature decreases by 0.2°C is a common definition of depth of the mixed layer. MMS. Minerals Management Service of the U.S. Department of the Interior. Modes. In this report, vertical structures. Higher mode means smaller vertical structures. Monte Carlo technique. A statistical sampling technique used to create estimates of random processes. NOAA. National Oceanic and Atmospheric Administration. NODC. NOAA's National Oceanographic Data Center. Obukhov scale. A vertical atmospheric or oceanic length-scale that is proportional to the vertical transport of heat and momentum. OCSEAP. OCS Environmental Assessment Program (Alaska, operated by NOAA and MMS). OCSLA. Outer Continental Shelf Lands Act of 1953. OCSLAA. Outer Continental Shelf Lands Act amendments of 1978. OPUS. Organization of Persistent Upwelling Structures program. Orography. Branch of geography dealing with mountains. OSRA(M). Oil Spill Risk Analysis (Model). Polynya. An open area of water surrounded by ice. Usually applied to areas of km size or larger. Positive estuary. Estuary having a higher freshwater input than evaporation. Ppt. Parts per thousand. PROBES. Processes and Resources of the Bering Sea Shelf. Prognostic model. Time-dependent ocean circulation model; a model that can produce forecasts. Pycnocline. The vertical section in the vertical density profile where the density changes most rapidly with depth. Rectification. Generation of time mean flows in an otherwise harmonic flow (i.e., tides) due to nonlinearities in the flow (convective accelerations, frictional dissipation, or violations of shallow water wave assumptions). Reynolds stress. The horizontal shearing stress due to turbulence. Rings. See Gulf Stream rings. RMS. Root Mean Square. Rossby radius of deformation. A horizontal length-scale equal to the gravity-wave speed divided by the Coriolis factor. RTWG. Regional technical working group. SC. Scientific committee. Scatterometry. A remote sensing technique to determine wind speed and direction using active microwave energy refracted from short (5-20 cm) surface waves. Shallow water wave. Wave length is much greater than water depth and wave height is small compared to the wave length. Wave speed determined by water depth. Shallowing mixed layer. A mixed layer whose depth decreases with time.
132 APPENDIX A Shelf break. The position in the ocean where the rate of deepening of the total depth abruptly changes. The inshore region is the continental shelf. The offshore region is the continental slope. The break usually occurs in depths of 100-200 m. Similarity theory. A mathematical technique that asserts the structural shape of a function and not its absolute shape. Slope currents. The currents at the edge of the continental shelf on the continental slope. The slope current is usually in water between 200-400 m deep. Southern Oscillation. A large atmospheric east-west circulation over the tropical Pacific and Indian Oceans that oscillates in intensity every 4-5 years. Squirts. Narrow (0-5 km wide) circulation features in the ocean surface layer that are highly time-dependent and transport mass and constituents onshore and offshore. Stokes drift. The Lagrangian net motion associated with finite amplitude waves. It transports mass in the direction of surface wave propagation. Storm surge. Rise in sea level, particularly noticeable at the coast, due to wind-induced setup and barometric pressure effects. Normally associated with the passage of atmospheric low-pressure systems (storms). Subduction. In this report, the sinking of dense, high-latitude surface water under the lighter surface water at lower latitudes. Subinertial motion. Time-dependent motions with periods less than the period of the earth's rotation. In middle latitudes this period is about 12 hours but it is a function of latitude and decreases toward the poles. Sverdrup. Transport of 1 x 106 m3/s of water. Synoptic scale. A meteorological term indicating the scale of eddies resolved on weather maps which is on the order of 1,000 km. Subsynoptic scale. A meteorological term indicating the scale of eddies smaller than those resolved on weather maps. Terrigenous. Designating or of sea-bottom sediment derived from the erosion of land. Thermal wind. The change of wind or current with depth due to horizontal density variations not a flow, but a rate of change of the flow in amplitude and direction. Thermocline. The vertical section in the vertical temperature profile where the temperature changes most rapidly with depth. Transition probability matrix. A matrix giving the probabilities that a variable will change from each possible state to every other possible state. For wind speed and direction, such a matrix would give the probability that a wind of a given speec! and direction would change to any other speed and direction in a particular time. Undercurrent. A subsurface current that flows in an opposite direction to the surface current. Upwelling. The vertical movement of ocean water toward the surface. VACM. Vector-averaging current meter. Waveguide. Bathymetric or dynamic feature that controls wave propagation. The continental shelf guides waves to propagate in the alongshelf direction. Wind stress. The tangential force per unit area due to the horizontal movement of the wind over the sea. WO. MMS's Washington Office.