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# Oceanography Glossary

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A

Absolute Humidity
Measurement of atmospheric humidity. Absolute humidity is the mass of water vapor in a given volume of air (this measurement is not influenced by the mass of the air). Normally expressed in grams of water vapor per cubic meter of atmosphere at a specific temperature.

Absolute zero
The theoretical temperature at which molecular motion vanishes and a body would have no heat energy; the zero point of the Kelvin and Rankine temperature scales. Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold source that would render a Carnot cycle 100 percent efficient. The value of absolute zero is not estimated to be -273.15° Celsius, -459.67° Fahrenheit, 0° Kelvin, and 0° Rankine.

Absorption
The process by which radiant energy is absorbed and converted into other forms of energy. A substance that absorbs energy may also be a medium of refraction, diffraction, or scattering; these processes, however, involve no energy retention or transformation and are to be clearly differentiated from absorption.

Abyssal Fan
Fan shaped accumulation of sediment from rivers that is deposited at the base of a submarine canyon within an ocean basin.

Abyssal Plain
Another name for ocean floor.

Acid
(1) Substance having a pH less than 7.
(2) Substance that releases hydrogen ions (H+).

Acid Deposition
Atmospheric deposition of acids in solid or liquid form on the Earth's surface. Also see acid precipitation

Acidic
Any substance with a pH below 7.

Acidic Solution
Any water solution that is acidic (pH less than 7) or has more hydrogen ions (H+) than hydroxide ions (OH-). Also see basic solution and neutral solution.

Acid Precipitation
Atmospheric precipitation with a pH less than 5.6. Normal pH of precipitation is 5.6.

Acid Rain
Rain with a pH less than 5.6. Normal pH of precipitation is 5.6.

Acid Shock
A sudden acidification of runoff waters from the spring melting of accumulated snow in the middle latitudes because of the winter deposition of acidic precipitation.

Acoustic Ground Discrimination Systems (AGDS)
Acoustic Ground Discrimination Systems detect the acoustic reflectance properties of seabed substrata that can be linked to the physical, or occasionally biological nature of the substrata, that can be useful in habitat mapping.

Actual Evapotranspiration
Is the amount of water that is actually removed from a surface due to the processes of evaporation and transpiration.

1. The process of transport of an atmospheric property solely by the mass motion of the atmosphere; also, the rate of change of the value of the advected property at a given point.
2. Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere whereas convection describes the predominantly vertical, locally induced motions.
3. To transport or carry. In air quality, the rate at which particulate matter is transported.

Fog generated when winds flow over a surface with a different temperature. Two types of advection fog exist. When warm air flows over a cold surface it can produce fog through contact cooling. Cold air blowing over a warm moist surface produces a form of advection fog know as evaporation fog.

Aerosols
Particles, other than water or ice, suspended in the atmosphere ranging in radius from one-hundredth to one-ten-millionth of a centimeter -- or 102 to 10-3 microns (µ). Aerosols are important as nuclei for the condensation of water droplets and ice crystals, and as participants in various atmospheric chemical reactions. Perhaps most significantly, they absorb solar radiation, then emit and scatter it, influencing the radiation budget of the Earth-atmosphere system, which in turn influences the climate on Earth's surface. Aerosols from volcanic eruptions can lead to a cooling at the surface, which may delay greenhouse warming for a few years following a major eruption.

Aggregate
This is the collective term for sand, gravel and crushed rock. They can be compacted to firmly fill a space and are often bound together with cement (to make concrete) or bitumen (for road surfacing).

Agulhas current
Also called Agulhas stream, a generally southwestward-flowing ocean current of the Indian Ocean; one of the swiftest of ocean currents. Throughout the year, part of the south equatorial current turns south along the east coast of Africa and feeds the strong Agulhas current. To the south of latitude 30=AE$=AFS, the Agulhas current is a well-defined and narrow current that extends less than 100 km from the coast. To the south of South Africa the greatest volume of its waters bends sharply to the south and then toward the east, thus returning to the Indian Ocean by joining the flow from South Africa toward Australia across the southern part of that ocean. = However, a small portion of the Agulhas current water appears to round the Cape of Good Hope from the Indian Ocean and continue into the Atlantic Ocean. Alaska current An ocean current, the northward flowing division of the Aleutian current . It circulates cyclonically around the Gulf of Alaska; part of the water passes between the Aleutian Islands into the Bering Sea from which it emerges as the Oyashio , and part rejoins the Aleutian current. It enters the Gulf of Alaska along the American west coast and, since it comes from the south, it has the character of a warm current in spite of the fact that it carries subarctic water. It therefore exercises an influence on climatic conditions similar, on a small scale, to that which the North Atlantic current and Norwegian current exercise on the climates of northwestern Europe. Aleutian current - also called subarctic current. An eastward flowing ocean current which lies north of the North Pacific current; it is the northern branch of the Kuroshio extension which moves northeast then east between 40=AE$=AFN and 50=AE=AFN. As it approaches the cost of North America it divides to form the northward-flowing Alaska current and the southward-flowing California current. Amphidromic point A point on a chart of cotidal lines from which the cotidal lines radiate. Amphidromic region An oceanic region whose cotidal lines radiate from one amphidromic point. Antarctic circumpolar current Also called west wind drift, the ocean current with the largest volume transport (approximately 110 x 10[s] m[s]/sec), and the swiftest current; it flows from west to east through all the oceans around the Antarctic Continent. It is locally deflected from its course, partly by the distribution of land and sea and partly by the submarine topography. Beside the bends that are associated with the bottom topography, the effects of the distribution of land and sea and of the currents in the adjacent oceans are also evident. On its northern edge it is continuous with the South Atlantic current , the South Pacific current and the eastward-flowing extension of the Agulhas current in the Indian Ocean. A flow to the west near the Antarctic Continent is evident only in the Weddell Sea area, where an extensive cyclonic motion occurs to the south of the circumpolar current. antarctic convergence. Antarctic Circle Latitude of 66.5° South. The northern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year. Antarctic High A region of high pressure that occupies central Antarctic throughout the year. This pressure system is responsible for very cold temperatures and extremely low humidity. Anticyclone An atmospheric pressure system consisting of an area of high pressure and outward circular surface wind flow. In the Northern Hemisphere winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise and undefined at the Equator. Anticyclone gyre A closed circulation system, ranging in size from a few hundred meters to thousands of kilometers; associated with the concentrated center of highest pressure in an anticyclone. Antilles current An ocean current, the northern branch of the north equatorial current flowing along the northern side of the Great Antilles carrying water that is identical with that of the Sargasso Sea. The Antilles current eventually joins the Florida current (after the latter emerges from the Straits of Florida) to form the Gulf. Anomalous Not usual or regular; abnormal. Difficult to explain or classify. Antarctic circumpolar current A current that flows eastward completely around Antarctica. It is caused by the west wind surface drift. Arctic Circle Latitude of 66.5° North. The southern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year. Archipelago An archipelagos or island group is a chain or cluster of islands that are formed tectonically. Atmospheric window The spectral region between 8.5 and 11.0 microns where the atmosphere is essentially transparent to longwave radiation. Attenuation 1. Reduction in intensity. 2. The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals. Atoll A ring shaped reef composed largely of coral. These features are quite common in the tropical waters of the Pacific Ocean. Augmentation The apparent increase in the semidiameter of a celestial body as its altitude increases. Autumnal Equinox One of two days during the year when the declination of the Sun is at the equator. The autumnal equinox denotes the first day of the fall season. For the Northern Hemisphere, the date of autumnal equinox on either September 22 or 23 (changes yearly). March 20 or 21 is the date of the autumnal equinox in the Southern Hemisphere. During the autumnal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night. AVHRR (Advanced Very High Resolution Radiometer) A five-channel scanning instrument that quantitatively measures electromagnetic radiation, flown on NOAA environmental satellites. AVHRR remotely determines cloud cover and surface temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice. B Backshore Area behind the shore. This coastal feature is located between the beach berm and the backshore slope. Backshore slope Sloping bank landward of the shore. This coastal feature is composed of relatively non-mobile sediments. Backswamp Marshy low lying area in a stream's floodplain. Commonly found behind levees. Backwash The return water flow of swash. This sheet of water flows back to ocean because of gravity. Bar (1) Coarse grained deposit of sediment from a stream or ocean currents. (2) A unit of measurement for quantifying force. Equivalent to 1,000,000 dynes per square centimeter. Baroclinic instability An instability associated with flows with vertical shear and meridional temperature gradients that grows by conversion of potential energy in the mean flow. Baroclinic wave Describes the synoptic scale disturbance that grows in mid-latitudes due to baroclinic instability. Barotropic instability An instability associated with shear and jet flows that grows by extracting kinetic energy from the mean flow. Barrier Beach A long and narrow beach of sand and/or gravel that runs parallel to the coastline and is not submerged by the tide. Barrier Island Long, narrow islands of sand and/or gravel that are usually aligned parallel to the shore of some coasts. Bathymetric chart A map delineating the form of the bottom of a body of water, usually be means of depth contours ( isobaths ). Bathymetry The study of water depths. Bay A body of sheltered water found in a crescent shaped coastal configuration of land. Bayhead Beach An extensive deposit of sand and/or gravel in the form of a beach at the back of a bay. Bay-Mouth Bar A narrow deposit of sand and/or gravel found across the mouth of a bay. Beach The terrestrial interface area in between land and a water body where there are accumulations of unconsolidated sediments like sand and gravel. These deposits are laid down by the action of breaking waves. Beach Drift The lateral movement of sediments on a beach when the angles of swash and backwash differ. Beach drawdown The transport of sediment down a beach profile into natural or dredging-related depressions offshore. Beach profile A cross-sectional plot of beach survey data, which can be used to illustrate seasonal and long-terms changes in beach volume. Beach replenishment Replacement of beach sand removed by ocean waters. Sediment from other areas is supplied by mechanical means to supplement sand on an existing beach or to build up an eroded beach. Bearing A system that measures in reference to the cardinal points of a compass in 90 degree quadrants. Beaufort Wind Scale Descriptive system that determines wind speed by noting the effect of the wind on the environment. Originally developed for use at sea by Admiral Beaufort of the British Navy in 1806. Bedload Sediment transported in contact with the bed. Benguela current The northward flowing current along the west coast of Africa; it is one of the swiftest of ocean currents, the strongest current in the South Atlantic. It is a continuation of the South Atlantic current . Proceeding toward the equator, the Benguela current gradually leaves the coast and continues as the northern portion of the south equatorial current. Berm Low hill of sand that forms along coastal beaches. Bermuda High High pressure system that develops over the western subtropical North Atlantic. Also called Azores High. Beta effect Denotes how fluid motion is affected by spatial changes of the Coriolis parameter, for example, due to the Earth's curvature. The term takes its name from the symbol \beta, representing the meridional gradient of the Coriolis parameter at a fixed latitude. A linearly sloping lower boundary to fluid in a rotating system also experiences the beta effect. Beta plane An approximation, useful for the study of equatorial and midlatitude flow, whereby the Coriolis parameter is taken to vary linearly with latitude. Explicitly, the Coriolis parameter is given approximately by f \simeq f_0 + \beta y in which y is the meridional distance from some fixed latitude where the Coriolis parameter is f_0, and \beta (from which the beta plane'' gets its name) is the meridional gradient of f at that fixed latitude. Bittern region The liquid remaining after sea water has been concentrated by evaporation until salt has crystallized. Blackbody 1. An ideal emitter that radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. A blackbody also absorbs all the radiant energy in the near visible spectrum incident upon it. No actual substance behaves as a true blackbody, although platinum black and other soots rather closely approximate this ideal. However, one does speak of a blackbody with respect to a particular wavelength interval. This concept is fundamental to all the radiation laws, and is to be compared with the similarly idealized concepts of the whitebody and the graybody. In accordance with Kirchhoff's law, a blackbody not only absorbs all wavelengths but emits at all wavelengths and does so with maximum possible intensity for any given temperature. 2. A laboratory device that simulates the characteristics of a blackbody. See blackbody radiator. Blackbody radiation 1. The electromagnetic radiation emitted by an ideal blackbody; it is the theoretical maximum amount of radiant energy of all wavelengths that can be emitted by a body at a given temperature. The spectral distribution of blackbody radiation is described by Planck law and the related radiation laws. If a tiny opening is made into an otherwise completely enclosed space (hohlraum), the radiation passing out through this hole when the walls of the enclosure have come to thermal equilibrium at some temperature will closely approximate ideal blackbody radiation for that temperature. 2. Any physical body absorbs and emits electromagnetic radiation when its temperature is above absolute zero. Planck's law determines the radiant flux of a body at a specific wavelength. In atmospheric chemistry, the calculation involving Earth's blackbody radiation shows that Earth's surface temperature would be below the freezing point of water if it did not have an atmosphere that absorbed some of the outgoing radiation. Blackbody radiator A hypothetical, ideal radiator that totally absorbs and reemits all energy incident upon it. Actual objects only approach this ideal. Blind roller Long high swells which have increased in height, almost to the breaking point, as they pass over shoals or run in shallow water. Bottomset Bed Horizontal deltaic deposit of alluvial sediment composed of fine silt and clay. Boundary current system The series of currents that form the eastern and western segments of the oceanic gyres in the three tropical oceans, carrying water poleward in the stronger western boundary currents and toward the equator in the weaker eastern currents. Boussinesq approximation An approximation to the dynamical equations of motion whereby density is assumed to be constant except in the buoyancy term, -g\rho', of the vertical velocity equation. The approximation is reasonable if the vertical extent of the dynamics being considered is much smaller than the density scale height - the height over which the density changes by a factor e. It is generally applicable to most oceanographic circumstances. If a system does not satisfy the Boussinesq approximation it is said to be non-Boussinesq. Brackish Environment that is influenced by seawater with a salinity less than 35 parts per thousand (usually caused by the presence of an inflow of fresh water). Brave west winds A nautical term for the strong and rather persistent westerly winds over the oceans in temperate latitudes. They occur between latitudes 40 and 65 in the northern hemisphere and 35 to 65 in the southern hemisphere, where they are more regular and are strongest between 40 and 50 (roaring forties). They are associated with the strong pressure gradient on the equatorial side of the frequent depressions passing eastward in sub-polar temperate latitudes; hence they fluctuate mainly between southwest and northwest. Brazil current The warm ocean current flowing southward along the Brazilian coast. Its origin is in the westward flowing south equatorial current , part of which turns south and flows along the South American coast as the Brazil current, a tongue of water of high temperature and high salinity. At about 35=AE=AFS it meets the Falkland (Malvinas) current , the two turning east and crossing the ocean as the South Atlantic current .

Breaker
A sea-surface wave which has become too steep to be stable. Waves in shoaling water become higher and shorter (hence steeper) as the water becomes shallower. When the steepness (ratio of wave height to wave length) exceeds 1/7, the laws which govern surface-wave motion can no longer be satisfied and the crest of the wave outraces the body of the wave to form a foaming white turbulent mass of water called a breaker. Roughly, three kinds of breakers can be distinguished, depending primarily on the gradient of the bottom: ( a) spilling breakers = (over nearly flat bottom) which form a foamy path at the crest and break gradually over a considerable distance; ( b) plunging breakers (over fairly steep bottom gradient) which peak up, curl over with a tremendous overhanging mass, and then break with a crash; ( c) surging breakers (over very steep bottom gradients) which do not spill or plunge but surge up the beach face. Waves also break in deep water if they build too high while being generated by the wind, but these are usually short-crested and are termed whitecaps .

Brightness temperature
A measure of the intensity of radiation thermally emitted by an object, given in units of temperature because there is a proportional correlation between the intensity of the radiation emitted and physical temperature of the radiating body.

Brine
Seawater with a salinity greater than 35 parts per thousand. Usually occurs in isolated bodies of seawater that have high amounts of water loss due to evaporation.

Brunt-Vaisala frequency
See buoyancy frequency.

Buoyancy force
In a fluid with vertical density variation, the buoyancy force is the difference between the weight of an initial infinitessimal volume of fluid with the weight of a a fluid parcel of the same volume displaced to same location.

Buoyancy frequency
Also called the Brunt-Va'isa'la' frequency, or Va'isa'la' frequency. In a continuously stratified fluid, the buoyancy frequency is the natural frequency of vertical oscillation of fluid parcels. Explicitly the squared buoyancy frequency is N^2 = -(g/\rho) d\rho/dz, in which g is the acceleration due to gravity and \rho(z) is density as a function of height z.

Burger number
A dimensionless number comparing the buoyancy with Coriolis forces. Explicitly, the Burger number is Bu = N^2 D^2/(f^2 L^2) in which N is the buoyancy frequency, f is the Coriolis parameter, and D and L are characteristic vertical and horizontal length scales, respectively.

Burgers vortex
Exact solution of the Navier-Stokes equation for a steady vortex in which the diffusion of vorticity is balanced by vortex stretching in an external strain field. The vortex has a Gaussian shape. For example, for a vertical axisymmetric vortex in an external velocity field given by (u,v,w)=\gamma (-x/2,-y/2,z), the vorticity is \omega = \omega_0 exp(-\gamma (x^2 + y^2)/4\nu) in which \nu is the kinematic viscosity.

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C

cable
A nautical unit of horizontal distance, equal to 600 feet (100 fathoms ) and approximately one-tenth of a nautical mile .

California Current
The ocean current flowing southward along the west coast of the United States from approximately Washington to northern Baja California. It is the major branch of the Aleutian current . As a whole, the current represents a wide body of water that moves sluggishly toward the southeast. Off Central America, the California current turns toward the west and becomes the north equatorial current .

Calving
The breaking away of a mass of ice from a glacier, an ice shelf, or an iceberg.

Canaries current
The southern branch of the North Atlantic current (which divides on the eastern side of the ocean); it moves south past Spain and North Africa to join the north equatorial current .

Carbon Dioxide
Common gas found in the atmosphere. Has the ability to selectively absorb radiation in the longwave band. This absorption causes the greenhouse effect. The concentration of this gas has been steadily increasing in the atmosphere over the last three centuries due to the burning of fossil fuels, deforestation, and land-use change. Some scientists believe higher concentrations of carbon dioxide and other greenhouse gases will result in an enhancement of the greenhouse effect and global warming. The chemical formula for carbon dioxide is CO2.

Carbon Pump
The organic carbon that forms the biological pump is transported primarily by sinking particulate material, for example dead organisms (including algal mats) or faecal pellets. However, some carbon reaches the deep ocean as dissolved organic carbon (DOC) by physical transport processes such as downwelling rather than sinking. Carbon pump.

Cardinal Points
The four main navigational directions (North, East, South, and West) found on a compass or a map.

Caribbean current
An ocean current flowing westward through the Caribbean Sea. It is formed by the commingling of part of the waters of the north equatorial current with those of the Guiana current . It flows through the Caribbean Sea as a strong current and continues with increased speed through the Yucatan Channel; there it bends sharply to the right and flows eastward with great speed out through the Straits of Florida to form the Florida current .

Catchment area
A land area where precipitation runs off into streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the River Severn contain thousands of smaller drainage basins.

Circulation
The water current flow within a large area.

Cirrus clouds
A type of cloud composed of ice crystals and shaped in the form of hair-like filaments. It is formed at an altitude of approximately 9700 m.

Clapotis
A standing wave phenomenon associated with the reflection of an ocean-wave train from a vertical surface, such as a breakwater or pier.

Coastal Dune
Sand dune that forms in coastal areas. The sand for its formation is supplied from a beach.

Coastal morphology
The study of the interaction of waves and currents with the coast.

Coastal Wetland
habitat found along a coastline and is covered with ocean salt water for all or part of the year. Examples of this type of habitat include tidal marshes, bays, lagoons, tidal flats, and mangrove swamps.

Coastal Zone
Relatively nutrient-rich, shallow part of the ocean that extends from the high-tide mark on land to the edge of the continental shelf.

Coastline
The line that separates a land surface from an ocean or sea.

Cold wall
The steep water-temperature gradient between the Gulf Stream and (a) the slope water inshore of the Gulf Stream or (b) the Labrador current.

Comber
A large ocean wave with high, breaking crest.

Condensation
Change of a substance to a denser form such as gas to a liquid. The opposite of evaporation.

Conduction
The transfer of energy within and through a conductor by means of internal particle or molecular activity and without any net external motion. Conduction is to be distinguished from convection (of heat) and radiation (of all electromagnetic energy).

Conductivity meter
Instrument which measures ionic conductivity, which can be used to determine chloride levels within marine aggregates.

Continental borderland
A submarine plateau or irregular area adjacent to a continent , with depths greatly exceeding those on the continental shelf , but not as great as in the deep oceans.

Continental platform
The zone that includes both the continental shelf or continental borderland and the continental slope .

Continental shelf
The zone around the continents extending from the low-water mark seaward to where there is a marked increase in slope to greater depths.

Continental Shelf Break
Boundary zone between the continental shelf and slope.

Continental slope
The declivity from the outer edge of the continental shelf or continental borderland into greater depths.

Convection
Fluid motion which results from the action of unbalanced buoyancy forces.

Convective instability
An instability due to the buoyancy force of heavy fluid over light fluid overcoming the stabilizing influence of viscous forces.

Countercurrent
A current flowing adjacent to another current both in the opposite direction.

Coral
Simple marine animals that live symbiotically with algae. In the symbiotic relationship, the algae provides the coral with nutrients, while the coral provide the algae with a structure to live in. Coral animals secrete calcium carbonate to produce a hard external skeleton.

Coral Bleaching
Situation where coral lose their colorful symbiotic algae. Thought to be caused by unusually warm water, changes in salinity of ocean seawater, or excessive exposure to ultraviolet radiation.

Coral Reef
Ridge of limestone found generally below the ocean surface. This marine feature is produced by numerous colonies of tiny coral animals, called polyps, that create calcium carbonate structures around themselves for protection. When the corals die, their vacant exterior skeletons form layers that cause the reef to grow. Coral reefs are found in the coastal zones of warm tropical and subtropical oceans.

Coriolis effect
The tendency for linear motion to be deflected in a rotating (non-inertial) reference frame. In most geophysical circumstance, the horizontal deflection of horizontal motion is most significant. Zonal motion experiences an acceleration -f v and meridional motion experiences an accleration f u in which u and v are the zonal and meridional components of velocity, respectively, and f is the Coriolis parameter.

Coriolis force
An apparent force arising from the fact that Earth turns on its axis. It is an apparent force that makes sense only because Earth is a noninertial frame of reference. Earth's spinning creates a constant centrifugal acceleration in which objects appear to curve because Earth is spherical, with different points on the surface spinning at different speeds. If, instead of being a spherical, rotating planet, Earth were flat, there would be no Coriolis force because all points would spin at the same speed.

Coriolis parameter
A measure of planetary rotation as a function of latitude. The parameter denoted by f varies with latitude \phi according to f= 2\Omega\sin\phi, in which \Omega is the frequency of planetary rotation.

Cretaceous
Geological period, covering 136 to 65 Million Years ago, representing the last part of the Mesozoic era.

Current
A horizontal movement of water in a well-defined, established pattern, as in a river or stream. The movement of a definite body of air in a certain direction.

Cuspate Foreland
Is a triangular accumulation of sand and/or gravel located along the coastline. This feature is formed by the joining of two.

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D

Davidson current
A countercurrent of the Pacific Ocean running north along the west coast of the United States (from northern California to Washington to at least latitude 48 degrees) during the winter months.

Decay
In ocean wave studies, the loss of energy from wind generated ocean waves after they have ceased to be acted on by the wind; this process is accompanied by an increase in length and a decrease in height of the wave.

Devensian
Last geological stage of the Pleistocene epoch, covering 50,000 to 10,000 years ago, representing the last main advance of glaciers.

Direct tide
A gravitational solar or lunar tide in the ocean or atmosphere which is in phase with the apparent motions of the attracting body, and consequently has its local maxima directly under the tide-producing body and on the opposite side of the earth. A gravitational tide which is in opposite phase to the apparent motions of the sun or moon is called a reversed tide .

Diurnal tide
A tide in which there is only one high water and one low water each lunar day .

Divergence
A horizontal flow of water outward from a common center or zone, often associated with upwelling.

Double diffusive convection
Fluid motion that results from the release of potential energy from one of two or more factors that determine a fluid's density (for example, heat and salinity). Even if the density is statically stable, convection may result if one of the factors is statically unstable. There are three major types of double diffusive convection relevant to heat and mass transport in the ocean. Finger modes may occur when hot salty fluid overlays cold fresh fluid so that convection results in the form of narrow cells carrying salty water downwards and fresh water upwards. Diffusive modes occur when a stable salinity field is heated from below so that convection results in the form of a series of well mixed layers separated by sharp density gradients. Intrusive modes occur when there are horizontal density gradients in one of the components determining the fluid's density even if the fluid density as a whole is horizontally uniform. This instability develops in the form of interleaving intrusions.

Downwelling
The process of accumulation and sinking of warm surface waters along a coastline. A change of air flow of the atmosphere can result in the sinking or downwelling of warm surface water. The resulting reduced nutrient supply near the surface affects the ocean productivity and meteorological conditions of the coastal regions in the downwelling area.

Drainage basin (catchment area, watershed)
A land area where precipitation runs off into streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the River Severn contain thousands of smaller drainage basins.

Dynamic-height anomaly
In oceanography, the excess of the actual geopotential difference, between two given isobaric surfaces , over the geopotential difference in a homogeneous water column of salinity 35 per mille (%) and temperature 0 degrees. Also called anomaly of geopotential difference. The dynamic-height anomaly between two isobaric surfaces is the product of the mean specific-volume anomaly and the difference in pressure (in decibars); the latter is assumed to equal the difference in depth in meters.

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E

East Australia current
The ocean current flowing southward along the east coast of Australia. It is formed by the division of part of the south equatorial current as it approaches the coast of Australia. Part of the east Australia current joins the west wind drift flowing eastward toward South America. In the summer (Southern Hemisphere) part of this water flows westward along the south coast of Australia into the Indian Ocean.

East Greenland current
An ocean current flowing south along the east coast of Greenland, carrying water of low salinity and low temperature. The east Greenland current is joined by most of the water of the Irminger current . The greater part of the current continues through Denmark Strait between Iceland and Greenland, but one branch turns to the east and forms a portion of the counterclockwise circulation in the southern part of the Norwegian Sea. Some of the east Greenland current curves to the right around the tip of Greenland, flowing northward into Davis Strait as the west Greenland current . The main discharge of the Arctic Ocean is via the east Greenland current.

Easterly Wave
Atmospheric disturbance in the tropical trade winds. Occasionally these systems intensify into hurricanes.

Easting
First measurement of a grid reference used to specific the location of a point on a rectangular coordinate system. The distance measured eastward from the origin of a rectangular coordinate system.

Ebb current
The movement of a tidal current away from the coast or down an estuary or tidal waterway; the opposite of flood current. Nontechnically called ebb tide.

Ebb Tide
Time during the tidal period when the tide is falling. Compare with flood tide.

Echosounder
Depth measuring device using sound waves.
Echo-sounding
A method of determining water depths by measuring the time lapse between the generation of an initial sound pulse on the ship and the return of the echo. With knowledge of the speed of sound in water, the depth of water can be calculated.

Eddy
A localized chaotic movement of air or liquid in a generally uniform larger flow.

Eddy Diffusion
Mixing of the atmosphere by chaotic air currents.

Eddy viscosity
An approximation to turbulent flow whereby the net effect of molecular diffusion enhanced by strain flows between eddies is parametrized by an eddy viscosity acting on large scale motion. Eddy viscosity may be taken as constant or dependent on the length scale of motion. The former case is equivalent to assuming that the Reynolds stresses are proportional to the gradients of the large scale flow velocity.

Edge wave
An ocean wave traveling parallel to a coast, with crests normal to the coast line. Such a wave has a height that diminishes rapidly seaward and is negligible at a distance of one wave length offshore.

Ekman convergence
A zone of convergence of warm surface water caused by Ekman transport, creating a marked depression of the ocean's thermocline in the affected area.

Ekman layer
One of the main layers of the troposphere, it is the layer of transition between the surface boundary layer, where shearing stress is constant, and the free atmosphere, where the atmosphere is treated as an ideal fluid in approximate geostrophic balance. Also called spiral layer. Together with the surface boundary layer, it makes up the planetary boundary layer.

Ekman spiral
A theoretical representation that a wind blowing steadily over an ocean of unlimited depth and extent and uniform viscosity would cause, in the Northern Hemisphere, the immediate surface water to drift at an angle of 45° to the right of the wind direction, and the water beneath to drift further to the right, and with slower and slower speeds, as one goes to greater depths.

Ekman theory
See Ekman spiral.

Ekman transport
The net mass displacement of water from one place to another, caused by wind blowing steadily over the surface; the net mass transport is 90° to tie right (in the Northern Hemisphere) of the wind's direction.

El Niño events
An irregular variation of ocean current that from January to March flows off the west coast of South America carrying warm, low salinity, nutrient poor water to the south. It does not usually extend farther than a few degrees south of the equator, but occasionally it does penetrate beyond 12° S displacing the relatively cold Peru Current. The effects of this phenomenon are generally short-lived, and fishing is only slightly disrupted. Occasionally (in 1891, 1925, 1941, 1957-58, 1965, 1972-73, 1976, and 1982-83), the effects are major and prolonged. Under these conditions, sea surface temperatures rise along the coast of Peru and in the equatorial eastern Pacific Ocean and may remain high for more than a year, having disastrous effects on marine life and fishing. Excessive rainfall and flooding occur in the normally dry coastal area of western tropical South America during these events. Some oceanographers and meteorologists consider only the major, prolonged events as El Niño phenomena rather than the annually occurring weaker and short-lived ones. The name was originally applied to the latter events because of their occurrence at Christmas time.

El Niño Southern Oscillation (ENSO)

Interacting parts of a single global system of climate fluctuations, ENSO is the most prominent known source of interannual variability in weather and climate around the world, though not all areas are affected. The Southern Oscillation (SO) is a global scale seesaw in atmospheric pressure between Indonesia and North Australia and the southeast Pacific. In major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SO pattern. Many of the countries most affected by ENSO events are developing countries that depend on their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events can have a global impact. While ENSO is a natural part of Earth's climate system, whether its intensity or frequency may change as a result of global warming is an important concern. See also El Niño events and Southern Oscillation.

Energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields existing in space or in the media. The term radiation alone is used commonly for this type of energy, although it actually has a broader meaning. Also called electromagnetic energy or simply radiation. See electromagnetic spectrum.

Electromagnetic spectrum
The ordered array of known electromagnetic radiations, extending from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other wavelengths of radio energy. See absorption spectrum. The division of this continuum of wavelengths (or frequencies) into a number of named subportions is rather arbitrary and, with one or two exceptions, the boundaries of the several subportions are only vaguely defined. Nevertheless, to each of the commonly identified subportions there correspond characteristic types of physical systems capable of emitting radiation of those wavelengths. Thus gamma rays are emitted from the nuclei of atoms as they undergo any of several types of nuclear rearrangements; visible light is emitted, for the most part, by atoms whose planetary electrons are undergoing transitions to lower energy states; infrared radiations are associated with characteristic molecular vibrations and rotations; and radio waves, broadly speaking, are emitted by virtue of the accelerations of free electrons (the moving electrons in a radio antenna wire).

Emission spectrum
The array of wavelengths and relative intensities of electromagnetic radiation emitted by a given radiator. Each radiating substance has a unique, characteristic emission spectrum, just as every medium of transmission has its individual absorption spectrum.

Emissivity
A property of a material, measured as the emittance of a specimen of the material that is thick enough to be completely opaque and has an optically smooth surface.

Equatorial Kelvin wave
Eastward propagating wave centered about the equator with zonal but no meridional velocity. The zonal velocity has a Gaussian meridional structure centered about the equator with standard deviation equal to the equatorial Rossby deformation radius. In the shallow water approximation the waves are non-dispersive with frequency \omega = +/- c k, in which k is the zonal wavenumber and the phase speed c= (gH)^(1/2) with g the acceleration due to gravity and H the mean fluid depth. The equatorial Rossby deformation radius in this case is R=(\beta/c)^(1/2) in which \beta is the meridional gradient of the Coriolis parameter at the equator.

Equatorial Poincare wave
By analogy with Poincare waves in a channel, these are inertio-gravity waves confined to a region about the equator. The meridional velocity of the n'th mode of these waves has meridional structure of the form exp(-y^2/2R^2) H_n(y/R), in which y is the meridional distance from the equator, R is the equatorial Rossby deformation radius, and H_n is the n'th Hermite polynomial. In the shallow water approximation R^2=(gH)^(1/2)/\beta in which, \beta is the meridional gradient of the Coriolis parameter at the equator, g is the acceleration of gravity, and H is the mean fluid depth. The dispersion relation for the n'th mode (n>0) with zonal wavenumber k is given by those roots of the equation, cubic in frequency \omega, ((gH)^(1/2)/\beta) [-k\beta/\omega - k^2 + \omega^2/(gH)] =2n+1 for which the frequency exceeds R\beta.

Equatorial Rossby wave
An equatorially trapped westward propagating wave that moves due to isentropic gradients of potential vorticity. The meridional velocity of the n'th mode of these waves has the form exp(-y^2/2R^2) H_n(y/R), in which y is the meridional distance from the equator, R is the equatorial Rossby deformation radius, and H_n is the n'th Hermite polynomial. In the shallow water approximation R^2=(gH)^(1/2)/\beta in which, \beta is the meridional gradient of the Coriolis parameter at the equator, g is the acceleration of gravity, and H is the mean fluid depth. The dispersion relation for the n'th mode (n>0) with zonal wavenumber k is given by those roots of the equation, cubic in the frequency \omega, ((gH)^(1/2)/\beta) [-k\beta/\omega - k^2 + \omega^2/(gH)] =2n+1 for which the frequency is less than R\beta.

Equatorial undercurrent
An ocean current flowing eastward (counter to and between the westward-flowing north equatorial current and south equatorial current ) through all the oceans. In the Atlantic Ocean, it flows east across the ocean between the north and south equatorial currents across the full width of the ocean in northern summer, and across the eastern half of the ocean in northern winter. It eventually becomes the Guinea current . In the Pacific Ocean, it is one of the swiftest ocean currents; it flows east across the ocean between the latitudes 3=AE$=AFN and 10=AE$=AF= N. = East of the Philippines it is joined by the southern part of the north equatorial current. In the Indian Ocean, it flows between the north and south equatorial currents, to the east; unlike the equatorial countercurrents of the Atlantic and Pacific Oceans, it lies south of the equator. In northern summer, when the southwest monsoon forms a continuation of the southeast trade winds, the countercurrent, along with the north equatorial current, is replaced by an easterly flowing monsoon current .

Equatorial tide
Tide occurring when the moon is near the equator; diurnal inequality is at a minimum.

Equilibrium solar tide
A theoretical concept in analogy to Laplace's oceanic equilibrium tide; roughly, the form of the atmosphere determined solely by gravitational forces in the absence of any rotation of the earth relative to the sun.

Estuary
Somewhat enclosed coastal area at the mouth of a river where nutrient rich fresh water meets with salty ocean water.

Eustatic sea-level change
Refers to a global or worldwide change in sea level and is unrelated to local/regional effects. The most common cause of eustatic sea-level variation is change in the ocean water volume.

Evanescent level
A theoretical boundary between a region in the fluid where waves of some frequency are propagating and a region in the fluid where waves of the same frequency do not propagate (where they are evanescent). This is also called a critical level but is distinct from the critical level where the background flow has the same speed as the phase speed of the waves.

Evaporation
Evaporation can be defined as the process by which liquid water is converted into a gaseous state. Evaporation can only occur when water is available. It also requires that the humidity of the atmosphere be less than the evaporating surface (at 100% relative humidity there is no more evaporation). The evaporation process requires large amounts of energy. For example, the evaporation of one gram of water at a temperature of 100° Celsius requires 540 calories of heat energy (600 calories at 0° Celsius).

Evaporation Fog
A type of fog produced from the advection of cold air over warm water or warm or moist land. This type of fog is sometimes called steam fog or sea smoke.

Exponent
The part of a floating-point number specifying the power of ten by which the mantissa should be multiplied. In the common notation, e.g., 3.1E8, the exponent is 8.

Exponentially
Expressible or approximately expressible by an exponential function; especially characterized by or being an extremely rapid increase (as in size or extent); an exponential growth rate.

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F

Falkland current
In the Atlantic Ocean, an ocean current flowing northward along the Argentine coast. The Falkland current originates as a branch of the antarctic circumpolar current . At about 35 degrees it is joined by the Brazil current , both flowing east across the ocean as the South Atlantic current .

Falling tide
The portion of the tide cycle between high water and the following low water. Sometimes called ebb tide.

Fathom
The common unit of depth in the ocean, equal to six feet. It is also sometimes used in expressing horizontal distances, in which case 100 fathoms make one cable or very nearly one-tenth nautical mile .

Fathom curve
Same as isobath.

Feeling bottom
The action of a deep-water wave on running into shoal water and beginning to be influenced by the bottom.

Fen
A habitat composed of woodland and swamp

Fetch
The area in which ocean waves are generated by the wind. It is generally delineated by coast lines, fronts , or areas of wind curvature or divergence.

Flandrian
Geological stage representing the Post-Glacial, Recent Epoch.

Floodplain
A strip of relatively flat and normally dry land alongside a stream, river, or lake that is covered by water during a flood.

Florida current
All of the northward-moving water from the Straits of Florida to a point off Cape Hatteras where the current ceases to follow the continental slope. It is one of the swiftest of ocean currents (flowing at a rate of 2 to 5 knots). The Florida current can be traced directly back to the Yucatan Channel because the water flowing through the channel continues on the shortest route to the Straits of Florida and only a small amount sweeps into the Gulf of Mexico, later to join the Florida current. After passing the Straits of Florida the current is reinforced by the Antilles current , but the name Florida Current is retained as far as Cape Hatteras. The Florida current is part of the Gulf Stream system .

Fluvial
Involving running water, usually relating to river or stream processes.

Fluvio-glacial
Pertaining to glacial rivers.

Footprint
The amount of geographical space covered by an object. E.g. The sediment footprint is the area over which suspended sediment settles out of the sediment plume.

Free atmosphere
That portion of Earth's atmosphere above the planetary boundary layer in which the effect of Earth's surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid. The base of the free atmosphere is usually taken as the geostrophic wind level. Also called free air.

Freezing point
The temperature at which a substance in liquid form freezes, equal to the temperature at which its solid form melts; this represents equilibrium between the liquid and solid phases.

Friction
A force that opposes the relative motion of two material surfaces that are in contact with one another; the direction of the force on each body is opposite to the direction of its motion relative to the other body.

Froude number
A dimensionless number relating the ratio of inertial to buoyancy forces applicable, in particular, to homogeneous shallow water flow, or two layer flow. Explicitly, in the shallow water approximation the Froude number is Fr=U^2/(gH) in which U is the characteristic velocity, H the characteristic fluid depth, and g the acceleration due to gravity.

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