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152 Cards in this Set
- Front
- Back
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countercurrent
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A current flowing adjacent to another current both in the opposite direction.
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amphidromic point
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A point on a chart of cotidal lines from which the cotidal lines radiate.
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amphidromic region
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An oceanic region whose cotidal lines radiate from one amphidromic point.
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baroclinic wave
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Describes the synoptic scale disturbance that grows in mid-latitudes due to baroclinic instability.
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bathymetric chart
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A map delineating the form of the bottom of a body of water, usually be means of depth contours ( isobaths ).
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bathymetry
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The study of water depths.
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bittern region
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The liquid remaining after sea water has been concentrated by evaporation until salt has crystallized.
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blind roller
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Long high swells which have increased in height, almost to the breaking point, as they pass over shoals or run in shallow water.
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brave west winds
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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.
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breaker
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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 .
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cable
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A nautical unit of horizontal distance, equal to 600 feet (100 fathoms ) and approximately one-tenth of a nautical mile .
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clapotis
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A standing wave phenomenon associated with the reflection of an ocean-wave train from a vertical surface, such as a breakwater or pier.
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cold wall
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The steep water-temperature gradient between the Gulf Stream and (a) the slope water inshore of the Gulf Stream or (b) the Labrador current.
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comber
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A large ocean wave with high, breaking crest.
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continental borderland
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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.
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continental platform
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The zone that includes both the continental shelf or continental borderland and the continental slope .
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continental shelf
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The zone around the continents extending from the low-water mark seaward to where there is a marked increase in slope to greater depths.
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continental slope
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The declivity from the outer edge of the continental shelf or continental borderland into greater depths.
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direct tide
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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 .
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diurnal tide
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A tide in which there is only one high water and one low water each lunar day .
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dynamic-height anomaly
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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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ebb current
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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.
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edge wave
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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.
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Ekman spiral
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As originally applied by {Ekman} to ocean currents, a graphic representation of the way in which the theoretical wind-driven currents in the surface layers of the sea vary with depth. In an ocean which is assumed to be homogeneous, infinitely deep, unbounded and having a constant eddy viscosity , over which a uniform steady wind blows, Ekman has computed that the current induced in the surface layers by the wind will have the following characteristics: ( a) At the very surface the water will move at an angle of 45 degrees cum sole from the wind direction. ( b) In successively deeper layers the movement will be deflected farther and farther cum sole from the wind direction, and the speed will decrease. ( c) A hodograph of the velocity vectors would form a spiral descending into the water and decreasing in amplitude exponentially with depth. The depth at which the vector first points 180=AE$=AF from the wind vector is called the depth of frictional influence (or depth of frictional resistance ). At this depth the speed is [s] times that at the surface. The layer from the surface to the depth of frictional influence is called the layer of frictional influence =2E. If the velocity vectors from the surface to the depth of frictional influence are integrated, the resultant motion is 90=AE$=AF =
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El Nino
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A warm ocean current setting south along the coast of Ecuador, so called because it generally develops just after Christmas. In exceptional years, concurrently with a southerly shift in the tropical rain belt, the current may extend along the coast of Peru to 12=AE$=AFS. When this occurs, plankton and fish are killed in the coastal waters and a phenomenon somewhat like the red tide of Florida results.
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equatorial Kelvin wave
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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.
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equatorial Poincare wave
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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.
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equatorial Rossby wave
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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.
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equatorial tide
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Tide occurring when the moon is near the equator; diurnal inequality is at a minimum.
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equilibrium solar tide
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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.
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falling tide
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The portion of the tide cycle between high water and the following low water. Sometimes called ebb tide.
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fathom
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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 .
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fathom curve
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Same as isobath.
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feeling bottom
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The action of a deep-water wave on running into shoal water and beginning to be influenced by the bottom.
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fetch
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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.
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gradient current
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In oceanography, a current associated with horizontal pressure gradients in the ocean and determined by the condition that the pressure force due to the distribution of mass balances the coriolis force due to the earth's rotation. The gradient current corresponds to the geostrophic wind in meteorology. In practice, the distribution of density is determined by measurements of salinity and temperature at a series of depths in a number of positions. From this the geopotential topography of any isobaric surface relative to any other isobaric surface may be computed and the horizontal pressure gradient may be expressed by the geopotential slope of the isobaric surface. In this way = relative gradient currents are obtained, corresponding to thermal wind in meteorology. If one isobaric surface is known to be level, the absolute geopotential topography of any other surface may be computed by reference to this, and hence absolute gradient currents are obtained. Where no isobaric surface is level, the total gradient current will consist of the relative gradient current , due to the distribution of density, and the slope current , due to that portion of the inclination of the isobaric surfaces which is not the result of the distribution of density.
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gravity wave
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Waves that propagate under the influence of buoyancy forces. Gravity waves include surface gravity waves, such as waves on the surface of the ocean, interfacial gravity waves, such as waves at the interface between fresh surface water and underlying salty ocean water, and internal gravity waves which propagate vertically as well as horizontally in density stratified fluid.
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halocline
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A vertical salinity gradient in some layer of a body of water, which is appreciably greater than the gradients above and below it; also a layer in which such a gradient occurs. The principal haloclines in the ocean are either seasonal, due to fresh water inputs, or permanent.
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ice shelf
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A thick ice formation with a fairly level surface, formed along a polar coast and in shallow bays and inlets, where it is fastened to the shore and often reaches bottom. It may grow hundreds of miles out to sea. It is usually an extension of land ice , and the seaward edge floats freely in deep water. The calving of an ice shelf forms tabular icebergs and ice islands. Also called shelf ice.
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inertio-gravity wave
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An internal gravity wave propagating under the influence of both buoyancy and Coriolis forces. The dispersion relation is given by frequency \omega = +/- (N k_h + f m)/| _k| in which N is the buoyancy frequency, f is the Coriolis parameter, and k_h and m are the the horizontal and vertical components, respectively, of the wavenumber vector _k. For all wavenumbers, inertio-gravity waves have frequency smaller than N and greater than f. Their group velocity is perpendicular to the phase velocity such that the vertical component of the group velocity is opposite in sign to the vertical component of the phase velocity. For an upward propagating inertio-gravity wave in the northern hemisphere, the perturbation wind vector turns anticyclonically with height.
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internal gravity wave
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Also called "internal wave". A wave that propagates in density stratified fluid under the influence of buoyancy forces. Internal gravity waves are also called internal waves and gravity waves. The dispersion relation is given by frequency \omega = +/- N k_h/| _k | in which N is the buoyancy frequency, and k_h is the horizontal component of the wavenumber vector _k. For all wavenumbers, internal gravity waves have frequency smaller than N. Their group velocity is perpendicular to the phase velocity such that the vertical component of the group velocity is opposite in sign to the vertical component of the phase velocity. Internal gravity waves are like surface waves on the ocean, they both propagate under the restoring force of gravity. The restoring force is felt when there is a vertical variation in density such as the density difference between water and air. When the density difference is very large (at the interface between two fluids, like water and air) waves are trapped at the interface and you have surface waves. When the density varies gradually as in a stratified fluid internal gravity waves may exist. Such waves may move vertically upward as well as horizontally. In the atmosphere, the effect of internal gravity waves can be seen on partly cloudy days. The rise and fall of the waves at just the right altitude can make wave condense and evaporate, respectively, and this leads to a banded cloud pattern.
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internal wave
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See internal gravity wave.
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isobath
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A contour of equal depth in a body of water, represented on a bathymetric chart.
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isobathytherm
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A line or surface showing the depths in oceans or lakes at which points have the same temperature. Isobathytherms are usually drawn to show cross sections of the water-mass.
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isohaline
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Of equal or constant salinity. A line on a chart connecting all points of equal salinity; an isopleth of salinity.
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Kelvin wave
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Near a boundary in a rotating system, a Kelvin wave propagates with wave crests perpendicular to the side wall and wave height greatest at the side wall to the right of an observer looking in the direction of wave propagation. The wave height decreases exponentially from the side wall with e-folding length scale equal to the Rossby deformation radius c/f, in which f is the Coriolis parameter and c is the phase speed of the wave in the along boundary direction. In the shallow water approximation the waves are non-dispersive with frequency \omega = +/- c k, in which k is the along boundary wavenumber and the phase speed c = (gH)^(1/2) with g the acceleration of gravity and H the mean fluid depth. Related to Kelvin waves in a channel are Poincare' waves.
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longshore current
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The resultant current produced by waves being deflected at an angle by the shore. Also called littoral current. In this case the current runs roughly parallel to the shoreline. The longshore current is capable of carrying a certain amount of material as long as its velocity remains fairly constant; however, any obstruction, such as a submarine rock ridge or a land point cutting across the path of the current will cause loss of velocity and consequent loss of carrying power.
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meridional
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Refers to motion or distance along lines of longitude. See also zonal.
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meteorological tide
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Annual or semi-annual changes in sea level due to shifts in prevailing winds or seasonal changes in water temperature; distinguish from atmospheric tide.
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mixed layer
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In oceanography, the surface layer of virtually isothermal water, which frequently exists above the thermocline .
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mixed tide
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A tide in which the diurnal and semidiurnal components are both prominent. Diurnal inequality is present in high waters , low waters , or in both.
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neap tide
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A tide of decreased amplitude, occurring semimonthly one or two days after quadrature .
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occlusion
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In meteorology, the process of formation of an occluded front . Some persons restrict the use of this time to the usual case where the process begins at the apex of a wave cyclone ; when the process begins at some distance from the apex, they call it seclusion .
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ocean current
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A movement of ocean water characterized by regularity, either of a cylic nature, or more commonly as a continuous stream flowing along a definable path. Three general classes, by cause may be distinguished: ( a) currents related to sea water density gradients, comprising the various types of gradient current ; ( b) wind-driven currents , which are those directly produced by the stress exerted by the wind upon the ocean surface; ( c) currents produced by long-wave motions. The latter are principally tidal currents , but may include currents associated with internal waves , tsunamis , and seiches . The major ocean currents are of continuous, stream-flow character, and are of first-order importance in the maintenance of the earth's thermodynamic balance.
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oceanography
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The study of the sea , embracing and integrating all knowledge pertaining to the sea's physical boundaries, the chemistry and physics of sea water, and marine biology.
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perigean tide
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Tide of increased range occurring when the moon is near perigee.
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planetary wave
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See Rossby wave.
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Poincare wave
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Within a channel in a rotating system, a Poincare' wave has sinusoidally varying cross-channel velocity with an integral or half integral number of cross-channel waves spanning the channel. In the shallow water approximation the waves have dispersion relationship with squared frequency \omega^2 = f^2 + c^2 (k^2 + n^2 \pi^2/L^2), in which f is the Coriolis parameter, k is the along channel wavenumber, L is the width of the channel, n is any positive integer, and c is the phase speed for shallow water gravity waves: c=(gH)^(1/2), in which g is the acceleration due to gravity and H is the mean depth of the fluid. Related to Poincare' waves are Kelvin waves which take the role of the mode with n=0.
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potential temperature
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In oceanography, the temperature that a water sample would attain if raised adiabatically to the sea surface. For the deepest points of the ocean, which are just over 10,000 meters, the adiabatic cooling would be less than 1.5=AE$=AFC.
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pycnocline
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A vertical density gradient (as determined by the vertical temperature and salinity gradients and equation of state) in some layer of a body of water, which is appreciably greater than the gradients above and below it; also a layer in which such a gradient occurs. The principal pycnoclines in the ocean are either seasonal, due to heating of the surface water in summer or fresh water inputs, or permanent.
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red tide
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A growth of dinoflagellates (single-celled plant-like animals) in surface waters in such quantities as to color the sea red and kill fish.
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reversing current
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A tidal current which flows alternately in approximately opposite directions, with periods of slack water at each reversal. Such currents occur chiefly in restricted channels; open sea areas generally have rotary currents .
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rip current
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A strong water-surface current of short duration flowing seaward from the shore; the return movement of water piled up on the shore by incoming waves and wind. It usually appears as a visible band of agitated water; and, with the outward movement concentrated in a limited band, its velocity is somewhat accentuated. A rip current is often miscalled a ``rip tide.'' To swimmers, the phenomenon is known as ``undertow.''
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rip tide
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Also called rip current.
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raoring forties
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A popular nautical term for the stormy ocean regions between 40 degreesand 50 degreeslatitude. It nearly always refers to the southern hemisphere, where there is an almost completely uninterrupted belt of ocean with strong prevailing westerly winds.
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rollers
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Swells coming from a great distance and forming large breakers on exposed coasts. They are best known on the islands of St. Helena and Ascension in the South Atlantic Ocean during the months from December to April, when they come from the northwest. = They arrive, often in calm weather, with practically no warning, and are dangerous to shipping. Rollers also occur at Fernando do Noronha, Tristan da Cunha, and on the coasts of West Africa, Peru, and the East Indies.
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Rossby-gravity wave
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An equatorial wave whose dispersion relation is asymptotic to that for equatorial Kelvin waves for large positive (eastward) zonal wavenumbers and asymptotic to equatorial Rossby waves for large negative (westward) zonal wavenumbers. Rossby-gravity waves are also called Yanai waves and ``mixed'' Rossby-gravity waves. In the shallow water approximation, the dispersion relationship is given by frequency \omega= k c [1-(1+4\beta/(k^2 c))^(1/2)]/2, in which k is the zonal wavenumber, \beta is the meridional gradient of the Coriolis parameter at the equator, and c=(gH)^(1/2) for which g is the acceleration of gravity and H is the mean fluid depth.
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Rossby wave
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A large scale, westward propagating wave that moves due to isentropic gradients of potential vorticity. Rossby waves are also called planetary waves. In particular, in the shallow water approximation, the waves move due to the variation of the Coriolis parameter with latitude, what is known as the ``beta''-effect. The dispersion relation in this approximation has frequency \omega = -\beta k/(k^2+l^2) in which \beta is the meridional gradient of the Coriolis parameter, and k and l are the zonal and meridional wavenumbers, respectively.
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rotary_current
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A tidal current that changes direction progressively through 360 degreesduring a tidal cycle.
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salinity
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A measure of the quantity of dissolved salts in sea water. It is formally defined as the total amount of dissolved solids in sea water in parts per thousand ([s]) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized. These qualifications result from the chemical difficulty in drying in salts in sea water. In practice, salinity is not determined directly but is computed from chlorinity, electrical conductivity, refractive index, or some other property whose relationship to salinity is well established. The relationship between chlorinity Cl and salinity S as set forth in { Knudsen } 's tables isIFI. In 1940, however, {Lyman} and {Fleming} found that the relationship between total dissolved salts [s] and chlorinity was better expressed by IFI.
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seiche
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An oscillation of a fluid body in response to a disturbing force having the same frequency as the natural frequency of the fluid system. Tides are now considered to be seiches induced primarily by the periodic forces caused by the sun and moon.
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semidiurnal tide
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A tide having two high waters and two low waters each lunar day , with little or no diurnal inequality . This applies equally to solar tides and to atmospheric tides .
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shelf ice
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Same as ice shelf.
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solitary wave
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A hump-shaped wave of permanent form. Solitary waves are well known for their tendency to travel long distances without dispersion and, despite being large amplitude (so that linear superposition principles do not apply) colliding waves effectively pass through each other.
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specific-volume anomaly
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In oceanography, the excess of the actual specific volume of the sea water at any point in the ocean over the specific volume of sea water of salinity 35 per mile ([s]) and temperature 0 degrees at the same pressure. The integral of specific-volume anomaly with depth is the dynamic-height anomaly .
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stratified fluid
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In a stratified fluid the effective density varies with depth. Such circumstances occur naturally, for example, due to variations in temperature in the atmosphere, and due to temperature and salinity variations in the ocean.
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sea surface temperature
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In oceanography, the temperature of the layer of sea water nearest the atmosphere. It is generally determined either as bucket temperature or injection temperature .
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surface gravity wave
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A wave that propagates, typically, on the surface of water under the influence of buoyancy forces. In water of uniform depth H, the dispersion relationship is given by squared frequency \omega^2 = gk tanh(kH), in which k is the wavenumber and g is the acceleration of gravity. In shallow water, when the wavelength is much larger than the fluid depth, the waves are nondispersive: the dispersion relationship is \omega = c k, in which phase speed c=(gH)^(1/2). In deep water the waves have dispersion relationship \omega = +/- (gk)^(1/2).
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surge
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Water transported up a beach by breaking waves.
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swash
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Intermittent landward flow of water across a beach where surf is breaking.
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synthetic schlieren
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A new technique for visualising and measuring small density variations by digitally recording variations in an image observed through a stratified fluid.
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T-S diagram
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Abbreviation for temperature-salinity diagram.
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temperature salinity diagram
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A graph with temperature as ordinate and salinity as abscissa, on which the points observed at a single oceanographic serial station are joined by a curve (the T-S curve ).
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thermocline
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A vertical temperature gradient in some layer of a body of water, which is appreciably greater than the gradients above and below it; also a layer in which such a gradient occurs. = The principal thermoclines in the ocean are either seasonal, due to heating of the surface water in summer, or permanent. thermohaline circulation - Circulation in water caused by changes in density brought about by the combined effect of variations in temperature and salinity.
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thermosteric anomally
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The specific-volume anomaly ( steric anomaly ) that the sea water at any point would attain if the sea water were brought isothermally to a pressure of one standard atmosphere =2E. In other words, thermosteric anomaly is the specific-volume anomaly calculated for the given salinity and temperature but for a standard pressure.
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tidal current
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The horizontal movement of water associated with the rise and fall of the tide. Also called a tidal stream. In relatively open positions, the direction of tidal currents rotates continuously through 360 degrees diurnally or semi-diurnally. = In coastal regions, the nature of tidal currents will be determined by local topography as well.
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tropic tide
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Tide occurring when the moon is near maximum declination; the diurnal inequality is then at a maximum.
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tsunami
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An ocean wave produced by a submarine earthquake, landslide, or volcanic eruption. These waves may reach enormous dimensions and have sufficient energy to travel across entire oceans. They proceed as ordinary gravity waves with a period between 15 and 60 minutes. Tsunamis steepen and increase in height on approaching shallow water, inundating low-lying areas; and where local submarine topography causes extreme steepening, they may break and cause great damage. Tsunamis have no connection with tides ; the popular name is entirely misleading.
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undercurrent
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In oceanography, a water current flowing beneath a surface current at a different speed or in a different direction.
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upwelling
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The rising of water toward the surface from subsurface layers of a body of water. Upwelling is most prominent where persistent wind blows parallel to a coastline so that the resultant wind-driven current sets away from the coast ( see Ekman spiral ). It constitutes a distinct climatogenetic influence by bringing colder water to the surface. = Over the ocean, upwelling occurs wherever the wind circulation is cyclonic , but is appreciable only in areas where that circulation is relatively permanent. It is also observable when the southern trade winds cross the equator. The upwelled water, besides being cooler, is richer in plant nutrients, so that regions of upwelling are generally also areas of rich fisheries.
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west wind drift
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Same as antarctic circumpolar current.
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wind-driven current
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A current produced in a body of water by blowing over the water surface. It flows to the right of the wind. Sometimes called wind drift; sometimes called drift current. Considerable evidence now exists to show that the Gulf Stream is such a current. Which of the ocean currents are wind driven and which thermally driven is a subject now under intensive investigation.
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Yanai wave
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See Rossby-gravity wave.
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zonal
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Refers to motion or distance along lines of latitude. See also meridional.
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hydrodynamic instability
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The development of an infinitessimal disturbance in a mean flow field that grows by drawing energy from the mean flow.
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barotropic instability
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An instability associated with shear and jet flows that grows by extracting kinetic energy from the mean flow.
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baroclinic instability
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An instability associated with flows with vertical shear and meridional temperature gradients that grows by conversion of potential energy in the mean flow.
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Kelvin-Helmholtz instability
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Also called "shear instability". An instability of an unbounded parallel shear flow to the growth and nonlinear development of waves with phase speed in the along flow direction approximately equal to the speed of the inflection point of the shear. This is also referred to as shear instability and is a specific example of barotropic instability. The instability, whether in homogeneous or stratified fluid, occurs due to a resonant coupling between wave-like disturbances on either flank of the shear flow where the gradient of the shear is non-zero. Increasing stratification acts to inhibit the growth of the instability.
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shear instability
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See Kelvin-Helmholtz instability.
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Holmboe instability
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An instability of an unbounded stratified parallel shear flow to the development of cusp-like waves that propagate with phase speed in the along flow direction significantly different from the speed of the inflection point of the shear. The propagating phase speed of Holmboe instability distinguishes it from Kelvin-Helmholtz instability. The instability occurs only in stratified fluid due to a resonant coupling between an internal gravity wave and a wave-like disturbance where the background shear varies vertically.
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convective instability
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An instability due to the buoyancy force of heavy fluid over light fluid overcoming the stabilizing influence of viscous forces.
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inertial instability
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An instability of geostrophic (cyclostrophic) motion due to imbalance between pressure gradient and inertial forces for an infinitessimal disturbance that meridionally (radially) displaces fluid. For geostrophic motion in the Northern (Southern) Hemisphere, inertially instability may occur only if the absolute vorticity (f- \partial u_g/\partial y) is negative (positive). Here f is the Coriolis parameter, u_g the zonal component of geostrophic velocity, and y is distance in the meridional direction.
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symmetric instability
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Similar to inertial instability but due to imbalance between pressure gradient and inertial forces for infinitessimal disturbances that meridionally displaces fluid along isentropes (in atmosphere) or isopycnals (in ocean). For geostrophic motion in the Northern Hemisphere, symmetric instability may occur only if the potential vorticity is negative.
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line vortex
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Idealized vortex in which vorticity is zero everywhere except along a line in space where it is infinite. The strength of a line vortex is \Gamma, the circulation along any circuit around the line.
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point vortex
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Idealized vortex in two dimensions with zero vorticity everywhere except at a point where it is infinite. Specifically, a point vortex of strength \Gamma has vorticity \omega = \Gamma \delta( _r- _r_0}), in which \delta( _x) is the Kronecker delta and \Gamma is the circulation around the point vortex.
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Burgers vortex
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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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Kirchhoff vortex
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An idealized vortex in unbounded fluid with uniform vorticity inside an elliptical patch and zero vorticity outside. For an ellipse with semi-axes a and b and vorticity \omega in its interior, it rotates steadily with angular velocity \omega ab/(a+b)^2.
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Rankine vortex
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An idealized vortex in unbounded fluid with uniform vorticity inside a circular patch and zero vorticity outside.
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Kelvin-Helmholtz billows
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Vortical structures that result from the growth and nonlinear development of unstable waves in a shear flow. The billows get their name from the instability responsible for the growth of the unstable waves: Kelvin-Helmholtz instability.
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Mediterranian lenses
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A coherent mass of anticyclonically rotating, warm salty water in the Atlantic Ocean originating from the Mediterranian Sea. Also called ``Meddies'', these mesoscale lenses have been observed to persist for up to many months.
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meddy
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See Mediterranian lenses.
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gravity current
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A flow driven by horizontal pressure gradients, due typically to the flow of a fluid of one density intruding into an ambient fluid of different density. In non-rotating fluid, the head of the current travels with uniform horizontal speed C (g'H)^(1/2) in which g' is the reduced gravity, H is the current depth, and C is a constant of order unity. Sea breezes, landslides, and some instances of cold water formation are common manifestations of gravity currents in geophysical circumstances.
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convection
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Fluid motion which results from the action of unbalanced buoyancy forces.
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double diffusive convection
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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.
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Rossby deformation radius
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Horizontal length scale of a rotating system measuring the distance over which the gravitational tendency to render a free surface flat is balanced by a tendency of the Coriolis acceleration to deform the surface. Also called the deformation radius or Rossby radius. In the shallow water approximation, the Rossby deformation radius is R = c/f in which c=(gH)^(1/2) with g the acceleration due to gravity and H the mean depth of the fluid. In stratified fluid, the appropriate length scale is called the internal Rossby deformation radius, and is given by R = ND/f in which N is the buoyancy frequency and D is a characteristic vertical length scale. At the equator, the effect of \beta (the meridional gradient of the Coriolis parameter) determines the equatorial Rossby deformation radius R=(c/\beta)^(1/2), and the equatorial internal Rossby deformation radius R=(ND/\beta)^(1/2).
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deformation radius
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See Rossby deformation radius.
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Rossby radius
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See Rossby deformation radius.
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Coriolis effect
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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.
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Coriolis parameter
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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.
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beta plane
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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.
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beta effect
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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.
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hydrostatic balance
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Describes a balance between vertical pressure gradient and buoyancy forces. Explicitly dp/dz = -\rho g. When this balance does not hold the fluid is non-hydrostatic.
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geostrophic balance
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Describes a balance between Coriolis and horizontal pressure gradient forces. Explicitly, f v = (1/\rho) \partial p/\partial x and f u = -(1/\rho) \partial p/\partial y, in which f is the Coriolis parameter, u and v the zonal and meridional components of velocity, x and y the zonal and meridional co-ordinates, p the pressure, and \rho the density.
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buoyancy force
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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.
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reduced gravity
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The effective change in the acceleration of gravity acting on one fluid in contact with a fluid of different density due to buoyancy forces. Explicitly, the reduced gravity is g'=g \Delta\rho/\rho_0, in which g is the acceleration of gravity, \rho_0 is the reference density, and \Delta\rho is the difference in density between the two fluids.
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buoyancy frequency
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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.
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Brunt-Vaisala frequency
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See buoyancy frequency.
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Vaisala frequency
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See buoyancy frequency.
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Boussinesq approximation
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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.
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shallow water approximation
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An approximation to the equations of motion whereby it is assumed the fluid is homogeneous and horizontal scales of interest are much larger than the depth of the fluid. Waves in this approximation are non-dispersive and have phase speed c=(gH)^(1/2) in which g is the acceleration due to gravity and H is the mean depth of the fluid.
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potential vorticity
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A materially conserved quantity in adiabatic, frictionless flow that accounts for changes of vorticity due to vortex line stretching and Coriolis effects. In the shallow water approximation, the potential vorticity is q=(f+\zeta)/(H+\eta), in which f is the Coriolis parameter, \zeta is the relative vorticity, H is the mean fluid depth, and \eta is the surface elevation above the mean.
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wave packet
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A collection of waves the amplitudes of which are largest for waves with frequency and wavelength in a range about some central frequency and wavelength.
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Gaussian wave packet
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A collection of waves the amplitudes of which have a Gaussian dependence on wavenumber about some central wavenumber.
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evanescent level
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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.
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power spectrum
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The Fourier transform of the kinetic energy field. The power spectrum provides a useful diagnostic to measure a what length scales energy is concentrated and, in turbulent flow, over what length scales energy is transferred and dissipated.
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inertial range
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The range of length scales over which energy is transferred and dissipation due to molecular viscosity is negligible. The power spectrum has power law behavior over the inertial range. In two dimensional turbulence the power spectrum is theoretically proportional to k^{-3} in which k is the wavenumber, and in three dimensional turbulence the power spectrum is theoretically proportional to k^{-5/3}. The latter is known as the Kolmogorov minus 5/3 law.
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Ozmidov scale
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The horizontal extent of internal gravity waves, below which vertical overturning of waves may occur and above which overturning is inhibited by stratification. The length scale is proportional to \epsilon^(1/2)/N^(3/2) in which \epsilon is the flux of energy from large to small scales and N is the buoyancy frequency.
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Kolmogorov scale
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Length scale of turbulent motion below which the effects of molecular viscosity are non-negligible. In three dimensional turbulence, the Kolmogorov scale is (\nu^2/\epsilon)^(1/4) in which \nu is the kinematic viscosity and \epsilon is the energy dissipation rate per unit mass.
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Garrett-Munk spectrum
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An approximation to the internal gravity wave spectrum (for waves with large horizontal extent compared with the vertical extent) which is observed to have the same structure throughout the deep ocean. The energy density spectrum is proportional to Ef/\omega (\omega^2-f^2)^(-1/2) ~= Ef/\omega^2 (for \omega>>f) for waves of frequency \omega in which f is the Coriolis parameter and E is a nondimensional parameter empirically determined to be E ~= 6*10^(-5). For a wide range observations E has been found to vary by no more than a factor 2.
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eddy viscosity
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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.
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turbulence
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Describes fluid motion disturbed from its average behavior by random fluctuations over a range of temporal and spatial scales.
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Burger number
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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.
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Richardson number
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A dimensionless number relating the ratio of buoyancy to inertial forces. In a stratified fluid with characteristic squared buoyancy frequency J,the bulk Richardson number is Ri = J/(U/L)^2 in which U and L are characteristic velocity and length scales. The gradient Richardson number is defined as a function of height for stratified parallel flows by Ri_g (z) = N^2(z)/(dU/dz)^2 in which U(z) is the mean horizontal velocity and N^2(z) is the squared buoyancy frequency as a function of height. The flux Richardson number is defined for stratified turbulent flows with perturbation horizontal and vertical velocity fields u' and w', respectively, by Ri_f = -(g/\rho_0)<\rho'w'>/[(dU/dz) ] which is the ratio of the rate of conversion to potential energy by the vertical flux of perturbation density \rho' to the rate of extraction of mean kinetic energy from the background horizontal velocity U(z). The flux Richardson number is also called the mixing efficiency.
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mixing efficiency
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See Richardson number.
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Froude number
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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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Reynolds number
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Dimensionless number relating the ratio of inertial to viscous forces. Explicitly, the Reynolds number is Re=UL/\nu in which U and L are characteristic velocity and length scales, respectively, and \nu is the kinematic viscosity.
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Prandtl number
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Dimensionless number relating the ratio of a fluid's capacity to diffuse momentum to its capacity to diffuse heat. Explicitly, the Prandtl number is Pr=\nu/\kappa in which \nu is the kinematic viscosity and \kappa is the thermal diffusivity.
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Pe'cle't number
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Dimensionless number relating the ratio of inertial forces to thermal diffusion. Explicitly, the Pe'cle't number is Pe=UL/\kappa in which U and L are characteristic velocity and length scales, respectively, and \kappa is the thermal diffusivity.
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Schmidt number
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A dimensionless number relating the ratio of inertial to molecular diffusive forces. Explicitly, Sc = UL/\kappa_D in which U and L are characteristic velocity and length scales, respectively, and \kappa_D is the diffusion constant of a solute in solution, such as salt in water.
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Rossby number
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A dimensionless number relating the ratio of inertial to Coriolis forces. Explicitly, the Rossby number is Ro=U/(fL) in which U and L are characteristic velocity and horizontal length scales, respectively, and f is the Coriolis parameter at a fixed latitude. Flows with sufficiently small Rossby number are in geostrophic balance.
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Ekman number
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A dimensionless number relating the ratio of eddy viscous forces to Coriolis forces. Explicitly the vertical Ekman number is E_v = 2 K_v/(fD^2), in which K_v is the vertical eddy viscosity, f the Coriolis parameter, and D the characteristic vertical length scale. The horizontal Ekman number is E_h = 2 K_h/(fL^2), in which K_h is the horizontal eddy viscosity, and L is the characteristic horizontal length scale. The Ekman number gives a measure of the rate at which stresses at a boundary (for example, wind induced stresses at the ocean surface) are communicated to the fluid interior.
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Rayleigh number
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Expresses the ratio of the destabilizing effects of buoyancy to the stabilizing effects of diffusion of heat and momentum. Explicitly, Ra=g\beta \Delta T H^3/(\nu\kappa) in which g is the acceleration due to gravity, \beta is the thermal expansion coefficient, \Delta T is the temperature difference, h is the vertical length scale, \nu is the kinematic viscosity, and \kappa is the thermal diffusivity. The Rayleigh number is usually used to classify the stability of a fluid column heated from below, convection resulting when some relatively small value is exceeded.
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