|Ocean Current Glossary, Physical Oceanography Jargon, and a Good Place to Start Reading|
Physical oceanography, like all sciences, has its own jargon or special terms. This glossary contains short definitions of the oceanographic jargon used to describe ocean surface currents on this web-site and in the cited books and papers. Phrases highlighted in capitol red letters are linked to Wikipedia. * implies multiplications; m2 is squared meters, a unit of area; and m3 is cubic meters, a unit of volume.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Absolute vorticity is a measure of spin for fluid in the ocean. The absolute vorticity is the sum of the planetary and relative vorticity. Planetary vorticity is due to the spin imparted onto the ocean by the earth's rotation and is equal to f, the Coriolis parameter given by twice the earth's angular speed multiplied by the sine of the latitude. Relative vorticiy is spin imparted by the fluid flow field.
Advection vs Convection
Historically, these terms were used interchangeably and denote the macroscopic transport (or movement) of a fluid and its properties (temperature, salinity, oxygen, etc.) by the fluid's organized velocity field. Convection involes a transport of both mass and the property. common usage now is that advection is the horizontal transport of a fluid and its properties and convection is the vertical transport of a fluid and its properties. For many important ocean and atmospheric phenomena, convection is driven by density differences in the fluid, e.g. the sinking of cold, dense water in polar regions of the world's oceans; and the rising of warm, less-dense air during the formation of cumulonimbus clouds and hurricanes. thermal convection, conduction, and radition are the three primary physical processes, as well as chemical processes, that redistributes or transfers heat in a fluid. thermal radition is an electromagnetic wave process due to the random motion of water molecules generating charged particles, while conduction is a diffusive process due to random fluid motion that transfers kinetic energy between fluid molecules. conduction is very important for heat transfer in solids. In ocean and atmospheric fluids, diffusion is dominated by turbulent process, that are faster and more efficient than molecular processes.
Altimeters are instruments for measuring elevation. Space-borne altimeters onboard satellites measure sea surface height relative to the marine geoid (a gravitational equipotential surface) or a mean sea surface (MSS) constructed from years of historical altimetry data. The fundamental principle of altimetry is simple: send a radar pulse from the satellite to the sea surface, and record when the reflected signal returns to the satellite. The measured round trip travel time is converted to range (distance between the altimeter and the sea surface) by dividing by two (to convert to one-way travel time) and dividing by the speed of light (to convert time to distance). Transforming range into height of the sea surface above the geoid (or MSS) requires knowledge of the satellite's orbit from independent Precision Orbit Determination measurements from GPS, laser, and/or DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) tracking. Geophysical corrections for path delays through the atmosphere, due to air molecules and water vapor, and through the ionosphere are also applied. Finally, for studies of large scale circulation and ocean surface currents additional corrections are applied to remove sea surface signals which aren't relevant to these processes: tides, inverse barometer effects, and sea state biases between wave crests and troughs. Recent advances in determination of the marine geoid have made it possible to estimate absolute sea surface height, which is comparable to traditional hydrographic dynamic height. By contrast, measurements of sea surface height anomaly (relative to a MSS) do not contain the mean height signal. An example of this, analyzing seasonal changes in the Gulf Stream, can be found in the recent article by Lillibridge & Mariano in Deep-Sea Research II: http://dx.doi.org/10.1016/j.dsr2.2012.07.034. To date there have been many historical satellite altimetry missions: SeaSat, Geosat, ERS-1 & -2, Topex/Poseidon, Geosat Follow-On, Envisat, Jason-1 & -2, Cryosat-2, HY-2A, and SARAL. In the coming years we will add Sentinal-3 (A & B), Jason-3, Jason-CS (A & B) and the first wide-swath ocean altimeter: SWOT. Beginning with Cryosat-2, and continuing on the Sentinel-3 and Jason-CS missions, a fundamentally new measurement technique is being exploited known as 'Delay-Doppler Altimetry' or 'SAR mode' which improves along-track spatial resolution and reduces height noise as a function of surface wave height. We are on the cusp of a revolution in the way altimetry is used to measure small scale processes, for example in coastal regimes, which will mature as we gain experience from these new missions. Although the primary measurement from altimetry is sea surface height, the shape of the return radar echo and its overall power level can be used to estimate significant wave height and wind speed, respectively. These parameters are extremely useful for operational maritime applications. To learn more about satellite altimetry go to http://www.altimetry.info (with thanks to the French & European Space Agencies, CNES & ESA). An example of the critical application of altimetry measurements to the study of global sea level rise can be found at NOAA's Laboratory for Satellite Altimetry: Click here for an overview of satellite measurements of sea surface height. A big thanks to John L. Lillibridge, III (NOAA/NESDIS Laboratory for Satellite Altimetry) for this entry.
Advanced Very-High Resolution Radiometers (AVHRR) are sensors aboard the NOAA polar orbiting satellites that measure energy at different frequencies in the visible and infra-red bands. these "skin" (upper few mm of the ocean near-surface) measurements are calibrated with in-situ ocean temperature measurements, mostly from the oceanic mixed-layer, to produce estimates of sea surface temperature. There are local area coverage (lac) and global area coverage (gac) data with 1 km and 4 km, horizontal resolution, respectively. AVHRR data have been routinely collected from 1978 and are also used for producing images of hurricanes. An animation describing AVHRR data is figure 7 on this page, and further information, including data resources, at the JPL physical oceanography distributed active archive center.
Barotropic vs Baroclinic
Barotropic fluid is a fluid whose density is only a function of pressure. A barotropic flow is the same from the top of the ocean to to the bottom-the flow is said to be uniform with depth or depth-independent. In classic wind-driven ocean circulation theory, barotropic flow is the flow that is in dynamical balance with the sea surface slope. The barotropic component of a flow can be defined as the depth-averaged flow. Baroclinic denotes the depth-dependent part of the flow. The baroclinic component of the flow results from the density distribution of the fluid that varies due to different temperature and salinity waters. A simple approximation is to decompose the flow into a barotropic and a baroclinic mode based on data or theory. The barotropic flow is the flow at the zero-crossing depth of the baroclinic mode. It is the component of flow that acts to cancel the sea surface flow. An example can be seen in this figure here. The large sea surface velocity is due to the pressure gradient from the slope of the sea surface. The velocity decreases with depth because the pressure gradient due to the temperature changes across the front produces a flow in an opposite direction.
Barotropic instability is the process in which mesoscale turbulence uses the kinetic energy of the mean flow to enhance the growth of small scale features and form energetic ocean eddies. This process occurs in regions of strong ocean currents like western boundaries and along the equator. Baroclinic instability is the process in which mesoscale turbulence uses the available potential energy contained in stratified fluids. This process occurs in regions with large vertical gradients in buoyancy due to temperature and salinity differences.
Beta, denoted by β, is the Greek letter used by oceanographers to represent the linear change in the Coriolis parameter with latitude. One assumes that the Coriolis parameter f (see below) changes linearly with latitude y, f(y)=f0 +β(y-y0), about a center latitude y0 that has a Coriolis value of f0. Beta is responsible for western boundary intensification, westward propagation of eddies, and is the restoring force for a class of large-scale planetary waves known as Rossby Waves (see below).
Bottom water is the water found at the deepest depths of the ocean. Antarctic Bottom Water (AABW), formed by deep convection primarily in the Weddell sea, is characterized by an average temperature of -0.4°C and a (temperature, salinity) of (-2 to 0.3°C, 34.66) at its source and (1.8-1.9°C, 34.91) at its northern extent. North Atlantic Deep Water is found at depth in the northern Atlantic (see below).
Brackish water is water that is not fresh, but is not salty like ocean water. The salinity of brackish water is between 0.5 and 30.0 parts per thousand. It is often characterized by a dirty color, small amounts of salts, and is heavily influenced by the tides.
Brine is water containing relatively large amounts of salts, > 50 parts per thousand. Brine water is created in areas of strong evaporation, like the Dead Sea, and in polar regions, when salt is ejected from water during freezing.
Celsius or centigrade is a scale and unit for temperature. The Celsius scale is named after the Swedish Astronomer, Anders Celsius (1701-1744), who constructed a temperature scale based on the boiling and freezing point of pure water.
Cold-core vs Warm-core rings
Rings are oceanic vortices formed when a current with a large meander wraps around on itself and detaches. Cold-core rings are large vortices that have a core of cold water. Cold-core rings are found south of the Gulf Stream. Warm-core rings are large vortices that have a core of warm water. Warm-core rings are found north of the Gulf Stream. The radius of the rings are around 50 to 100 km with maximum swirl speeds of 1-2 m/s. Examples of Gulf Stream rings can be seen in the the animation on figures 9 and 12 here.
Cold surface currents come from polar and temperate latitudes, and they tend to flow towards the equator. Similar to warm surface currents, cold surface currents are driven mainly by atmospheric forces and are influenced by the earth's rotation. the E. Greenland Current, Labrador Current, Malvinas Current, and Benguela current are important cold surface currents in the Atlantic Ocean.
Conduction vs Convection
Both convection and conduction transfer heat in fluids, while conduction is the most important physical mechansim for transferring heat in solids. Convection by the fluid velocity involves both heat and mass transfer, while conduction of heat is due to the random motion of fluid particles and does not involve a transfer of mass. Convection of heat by both the mean fluid flow and turbulent fluid flow is much greater than conduction of heat by molecular processes. See advection vs convection in this glossary for more information and links.
Convergence vs Divergence
Mathematically, divergence is the net gain or loss of fluid per unit volume per unit time and is given by the dot product of the gradient operator and the velocity vector of the fluid. An incompressible fluid, which is an excellent approximation for the study of ocean currents, has zero divergence. Common usage for the surface of the ocean is that convergence is the accumulation of water particles, and water sinks or is down-welled. Weed lines, that are great for fishing, and oceanic surface drifters are usually found in convergence zones. Divergence of surface waters is indicated by surface drifters leaving an area. Upwelled water, that is usually colder and nutrient-rich, from below replaces the diverging surface flow.
Coriolis force is the deflection of moving objects when the objects are observed in a rotating coordinate system. The CORIOLIS FORCE is extremely important for large-scale motion on the rotating earth and is named in the honor of the French scientist Gaspard-Gustave Coriolis. Water parcels are deflected to the right (left) of their motion in the N. (S.) Hemisphere. The amount of deflection increases from the Equator to polar seas because the component of the earth's rotation important for ocean motion is largest at the poles. The most important component is denoted by f that equals twice the earth's rotation speed multiplied by the sine of the latitude. At mid-latitudes, the value of f is 10-4.
CTD is the acronym for an instrument that measures Conductivity-Temperature-Depth of sea water. Conductivity is used to estimate salinity, temperature is measured by a thermo-ressitor, and depth is measured by a pressure sensor as the instrument falls through the water column. This set of hydrographic data is used to calculate vertical profiles of density.
Currents are the coherent horizontal movement of water. Density currents are driven by gravity. Density differences in a fluid in a gravitational field leads to pressure differences known as pressure gradients that force fluid flow. Their is a large density difference between air and water and when the sea surface is sloped, a presure gradient results that forces ocean currents. Density-driven currents, due to differences in temperature and salinity, are flows of dense water over sills called turbidity currents, the flow of river water into the ocean, frontal currents, and the large-scale thermohaline circulation. Geostrophic currents are controlled by a balance between a pressure gradient force and the Coriolis deflection. Geostrophic currents flow along isobars, in contrast, to our everday experience of fluids flowing from high pressure to low pressure. Large-scale mid-latitude ocean (and atmospheric) flow are in approximate geostrophic balance. The other significant component of large-scale ocean circulation flow is wind-driven and is known as Ekman (see below) flow.
Density is a fundamental dynamical property of sea water and is defined to by the ratio of mass divided by volume. The density of seawater is between 1.02 and 1.04 g/cm3 over most of the ocean. Smaller values are near the surface and in the tropics. Larger values of density are found at great depths in the ocean, and in the polar regions. Density is a function of temperature, salinity, and pressure. An empirical relation called the equation of state for seawater is used to determine density.
Diapycnal mixing occurs between density surface. The majority of mixing takes along density surfaces but the slower, diapycnal mixing is also important for exchanging fluid between the surface water and the deep ocean.
Diffusion is either characterized as molecular or turbulent. molecular diffusion results from the random motion of molecules and it leads to a homogenization of fluid properties if given enough time. turbulent diffusion of fluid properties such as heat, salinity, momentum, and chemical concentrations is due to fluctuating fluid velocities. the velocity fluctuations are, in general, very energetic across a wide range of space and time scales and are efficient stirrers. Thus, turbulent ocean and atmospheric mixing rates are much larger than mixing rates from molecular diffusion. non-local turbulent diffusion is due to mesoscale/eddy ocean variability and include ocean rings, current meandering, and planetary waves. Local diffusion depends on the random motion of small-scale coherent features such as submesoscale vortices, instability waves, and inertial(-gravity) waves. The modeling of turbulent diffusion is one of the outstanding problems in applied physics.
Downwelling regions are due to the convergence of surface waters and are areas where water sinks. These subduction regions are an important fluid pathway for exchanging properties between the air-sea interface and the deeper ocean. Less dense particles such as plastics and sea weed, that can not sink, will convergence on the surface, forming weed- and debris patches/lines in downwelling regions.
Diapycnal mixing occurs between density surface. The majority of mixing takes along density surfaces but the slower, diapycnal mixing is also important for exchanging
Drifters are a class of instruments at or just below the sea surface that are tracked by ship, shore, or satellite in order to study ocean and coastal currents. A lot of good information from Dr. Rick Lumpkin and his colleagues can be found here.
Eddies is a term that has been used to denote all time-varying mesoscale ocean phenomena such as planetary waves and oceanic vortices. Some authors reserve the use of this term for closed circulation features like Gulf Stream rings and other mesoscale vortices.
Eddy Kinetic Energy (EKE) is a measure of the varibility about the average horizontal flow (U,V). EKE per unit volume is defined to be one-half the average of the density multiplied by (u-U)^2 + (v-V)^2.
Ekman flow is named after the pioneering work of the Swedish oceanographer, V.W. Ekman, who laid the foundation for dynamical theories of wind-driven ocean circulation while under the guidance of Vilhelm Bjerknes and Fridtjof Nansen from Norway. He derived the steady-state solution for the problem of a constant wind blowing on an infinite slab of water. This solution, known as the EKMAN SPIRAL, has a flow that begins 45 degrees to the right (left) of the wind in the N. (S.) hemisphere and rotates (counter-) clockwise with depth. Ekman transport is the total average flow, integrated over depth, and is 90 degrees to the right (left) of the wind in the N. (S.) hemisphere. It is the convergence and divergence of Ekman transport and the resulting flow that set up the large-scale SCALE OCEAN GYRES.
ENSO is an acronym for the El-Nino Southern Oscillation, a significant coupled ocean-atmospheric event that has both warm and cold phases for tropical Pacific ocean temperatures leading to anamolous global precipitation and weather patterns.
f see Coriolis parameter.
Fahrenheit is a temperature scale named in the honor of Daniel Gabriel Fahrenheit. It is only officially used in a few countries. Degrees fahrenheit equal to 9/5 times degrees Celsius + 32°F.
Filaments are oceanographic features that are formed by turbulent stirring. Filaments are evident in satellite images of sea surface temperature and ocean color. These features are elgonated in the direction of the primary flow. Their widths are usually less than 10 km or so and their widths are less than 100 km, but larger features have been seen in satellite imagery.
Flux is the rate of flow of a property (mass, momentum, heat, salinity, chemical concentration, energy, ....) per unit area. Oceanographers study the advective flux due to transfer by the fluid velocity and the diffusive flux due to the random movement of molecules. FLUX is a vector quantity with both magnitude and direction. The divergence of a flux is the accumulation (or loss if negative) of a property at a point.
Freezing point is the temperature that water becomes ice. Its value is 0°C (32°F) for fresh water and is about -2°C or 28°F for salty ocean water.
The Fram Strait is the deepest sill, approximately 2,600 m deep and 500 km wide, that connects the Artic Ocean to the rest of the world's ocean via the polar Greeland and Norwegian seas. The Fram Strait is located betwwen the northeastern coast of Greenland and the Svalbard Archipelago to the east. The transport through the Fram Strait is important for exchanging the colder, fresher Arctic Ocean water with the warmer, saltier Atlantic Ocean water. Part of this transport includes sea ice.
The geostrophic balance is a balance between the horizontal pressure gradient force and the Coriolis acceleration. The geostrophic balance is the dominant force balance for large-scale ocean and atmospheric flows. This balance leads to the counter-intuitive result that water (and air) flow along lines of constant pressure and not from high to low pressure.
Gradient is the rate of change of some property (temperature, pressure, salinity, velocity, density, ...) with respect to another property usually distance or time. Spatial gradients in pressure results in fluid motion. Pressure gradients are due to changes in density. Ocean fronts are regions where gradients have a large value. For example, the Gulf Stream Current there are strong gradients in temperature, salinity, specie composition of plankton, potential vorticity, oxygen, nitrates and other chemicals.
Gravity is one of the fundamental forces of nature. The great Sir Issac Newton law of gravitation states that any two masses attracts each other. The large mass of the earth is responsible for earth's gravity. The average value of gravity on the surface of the earth is 9.81 kg/m2 with a slightly larger (lower) value at the poles (equator) because the shape of the eath is an oblate spheroid. Gravity decreases with distance away from the center of the earth. Gravity is the restoring force for any vertical movement of a water parcel, for surface waves, and for internal waves.
Gyres are large-scale, closed ocean flow patterns that result from wind forcing, buoyancy forcing, and the Coriolis acceleration. Since the Coriolis acceleration changes with latitude, gyre circulations are not symmetric and the flow on the western boundaries is stronger. Subtropical gyres are found in all the world's oceans at mid-latitudes and they have a clock-wise circulation in the northern hemisphere and counter clock-wise circulation in the southern hemisphere. Subpolar gyres have the opposite circulation and are found poleward of subtropical gyres. Recirculation gyres are flows associated with major ocean currents and consists of water that recirculates in a closed pattern around most of the ocean basin. Large-scale recirculation gyres are associated with fast western boundary currents (worthington fig ). mesoscale recirculations are associated with meandering currents ( maybe sat image).
Heat is a thermodynamical quantity that represents the amount of energy transferred by thermal interactions between molecules. It is usually denoted by Q. Heat flows from warmer to colder bodies such that Q=mass * specific heat capacity * temperature change. The SPECIFIC HEAT capacity is the amount of heat per unit mass required to change a substance one degree Kelvin. The specific heat capacity for water is 4.1855 J/(gK).
Heat flux is the rate of heat transfer through a given surface area. It is given by Joules/(second*m^2) or Watts/m^2. It is a vector quantity with each component calculated as the product of the heat Q multiplied by the velocity components of the flow field. If the fluid velocity is given by (u,v,w), then the heat flux is given by (Qu,Qv,Qw). If the heat flux entering a region is greater than the heat flux leaving the region, the temperature in the region will increase.
Hydrography is the study and mapping of the physical properties of bodies of waters using measurements. The analyzed properties include water depth, temperature, salinity, density, oxygen, tides, and currents. In some applications, hydrographic data implies vertical profiles of temperature or salinity or both.
Hydrostatic pressure is the pressure at some point in a fluid that results from the weight of the fluid above. The hydrostatic balance for a fluid column of height h, at rest, is,
p= rho * g* h or pressure = density * gravity * height
where p is pressure, g is the magnitude of the gravitational acceleration (=9.81 m/s^2), and rho is the mean density of the fluid volume. This is an excellent approximation for mesoscale and large-scale ocean flows. Nonhydrostatic flows contain significant vertical velocities that vary in space and time.
Intermediate water is found at great depths in the world's oceans. Intermediate water lies above the deepest bottom waters and is the result of sinking water that does not have sufficient density to make it all the way to the bottom. Examples include Antarctic Intermediate Water (AAIW) with a salinity of 34.2 and temperatures between 2 and 4 C. AAIW is a subpolar mode water found above NADW in the tropical Atlantic and S. Atlantic. AAIW mixes with AABW near its formation site and mixes with salty, S > 36.5, Meditterian waters at depths between 1500 and 2000 m near its northern extent. Meditterian water is an intermediate water mass formed by strong evaporation in the Meditterian Sea. Water with salinities greater than 38.4 flow down the still at Gibraltar to depths greater than 1000 m and mixes with the Atlantic Ocean. Its average temperature is 12.8. Vortices with cores of Meditterian water are known as Meddies.
An isobar is a contour (or line) of constant pressure. Meteorologist study maps of isobars to predict the weather.
An isohaline is a contour (or line) of constant salinity. One approach to analyze ocean data is to the plot the value of salinity, for example, as a function of either longitude, latitude, depth, distance from coast, or along your transect. Oceanographers then connect all locations where salinity is constant, say 35 parts per thousand, by drawing a line or surface through all of the points whose salinity value is 35.
An isopycnal is a contour (or surface) of constant density. Most ocean mixing takes place along isopycnal surfaces.
An isotherm is a contour (or surface) of constant temperature. Fronts are found where different isotherms are close to each other indicating a large temperature change.
Joule is a unit of energy, work, and heat. Joule is defined as the amount of work required to produce one Watt of power for one second or the amount of work expanded to apply one Newton of force over one meter of distance. This unit equal to kgm^2/s^2=Watt*second is named in honor of English physicist James Prescott Joule (1818-1899).
Lord Kelvin was a famous British scientist who studied fluid dynamics, electricity, and thermpdyanmics. A unit of temperature degrees Kelvin was named in his honor for determining the value of absolute zero, where molecular motion stops at -273.15°Celsius. The Kelvin scale is set by absolute zero and the triple point of pure water.
Kelvin waves are boundary trapped waves that propagate with the coast to its right (left) in the N. (S.) Hemisphere. Both the wave height and particle velocity decreases quickly (exponentially) in the offshore direction. The length scale of this decay is the baroclinic Rossby radius of deformation (see below). Along the equator, double Kelvin waves exist that use each other as a boundary and propagate toward the east. Large amplitude Kelvin waves are responsible for initiating the warm phase of ENSO in the equatorial Pacific.
Latent heat is the heat that is required to evaporate water to change its state to a gas. Heat is released whan the water vapor condenses back into liquid water as it does during rain. The process of water evaporating in the (sub-)tropics and poleward transport of the resulting water vapor that cools the air mass resulting in rain is a net latent heat flux pole. Winds, ocean currents, and this latent heat flux are important for redistributing the net heat flux due to large amounts of incoming solar radition in (sub-)tropical regions to more (sub-)polar latitudes where there is a net loss of heat from the ocean into the atmosphere especially in western boundary currents and their extensions.
Latitude and Longitude are used to represent position on a spherical earth. Latitude is given by degrees south and north of the equator, where the latitude is zero, between the south pole, latitude = - 90° South, and the north pole, latitude = 90° North. Longitude is zero between the segment of the great circle arc through Greewich, England, the South pole, and the North pole. Longitude varies by 360 degrees, -180°W to 180°East.
Meanders are large-amplitude wave-like features evident in ocean currents. They can either propagate in the direction of the current (small meanders), remain stationary, or propagate against the direction of the current (meanders with long wavelengths). In strong currents, like the Gulf Stream, meanders can grow and pinch off and form either cold-core or warm-core rings. See this figure for an example of Gulf Stream meanders and the animations here (figures 9-14).
Measurements of ocean current are collected by a variety of methods. One popular way to measure ocean currents is to determine the water's velocity at one fixed place in the ocean. This type of measurement is called Eulerian, in honor of the Swiss mathematician Leonhard Euler. This is typically accomplished by the use of a electro-mechanical current meter (which measures the velocity at a single depth) or Acoustic Doppler Current Profiler (ADCP) (which can provide a profile of velocity with depth). Current meters are usually mounted on a wire that is part of a mooring. Moorings are deployed from a ship, while ADCPs can be mounted on a mooring, the bottom, or the underside of a vessel. These measurement techniques provide a time series of the velocity of the ocean's water at a single geographic location. Current measurements are also obtained using radar-based measurements.
Another direct way to measure ocean currents is by tagging a water material with either floats or dyes. This viewpoint of following a tagged water parcel is called Lagrangian, named in honor of Joseph Louis Lagrange, a French mathematician. (Near-)surface ocean currents are measured by so-called drifters, which is a buoy which rides at the ocean surface and is usually drogued at some depth to negate the direct effects of wind on the buoy itself. Tracking this drifter (by satellite, radar, radio, sound, etc.) will give a description of the ocean current. Other examples of this type of measurement are SOFAR, RAFOS, and (P)ALACE, floats. See Tom Rossby's ocean float history and the Global Drifter Program. Other measurements techniques including using ship-drift estimates, and indirect methods using hydrographic data based on the thermal wind relationship.
Meditterian Salt Water is described under intermediate waters in this glossary.
Nansen Bottles, named after Norwegian explorer and scientist Fridtjof Nansen, and are used to collect water samples at specific depths from ships. These bottleres were developed in the early 20th century. A heavy brass messenger is sent down a metal cable attached to a metal or plastic cylinder to trip the bottle and close it. A reversing thermometer is attached to record the temperature. These bottles have been replaced by Niskin bottles that are closed electronicallly and are equipped with modern sensors. Pictures and more details can be found HERE.
North Atlantic Deep Water (NADW) is the major source of deep water for the Atlantic Ocean and is characterized by temperatures and salinites of 2-3°C, 34.7-34.95°F, respectively. NADW is formed by winter-time convection in the Norwegian (1.8-3°C, 34.9°F), Greenland (2.5°C, 34.9°F), and Labrador (3-4°C, 34.92°F) seas with the given (temperature, salinity) values.
An ocean is a large volume of salty water that occupies each of the world's primary basins that are bounded by continental land masses. The world ocean is usually divided into the Pacific, Atlantic, Indian, Southern/Antarctic, and Arctic Oceans.
Oceanography is the study of the biology, chemistry, geology, and physics of the ocean.
Physical oceanography is the study of the mechanics and thermodyanicms of the ocean and its coastal seas. Physical oceanographers study and model many different types waves (sound waves, storm-generated waves and breaking waves on the beach, internal waves, inertial waves, tsunamis, nonlinear waves such as solitons, and planetary waves known as Rossby waves), average ocean currents, the variability of ocean currents, air-sea interactions, sea surface temperature and subsurface temperatures, the salinity of the ocean, the distribution of mass by measuring water density, mixing of biogeochemical material in the ocean, the spread of pollutants such as oil, the dynamics of the surf zone, transport of sand and suspended material, and the role of the ocean in global climate, to name a few research areas. Click here for more information about physical oceanography as a career.
Potential Vorticity is a fundamental quantity that is used to study the dynamics of both the atmosphere and ocean. The conservation of potential vorticity combines a statement both mass conservation and conservation of absolute vorticity. Absolute vorticity is the sum of relative vorticity and planetary vorticity due to the spin of the earth. For a constant density fluid column of height h, the potential vorticity is (\zeta + f)/H, while for a variable density fluid, potential vorticity is given by (\zeta + f) d\rho /dz.
Pressure is a funamental dynamical variable defined as a force/area. For a fluid at rest, the pressure is due to the weight of the water that equals to the mass multiplied by gravity. Pressure is then mass*gravity/area = density*volume*g/area = density*height*gravity. Thus the distribution of pressure in the ocean is determined by both the density of the sea water and the slope of the sea surface. A pressure gradient is the change of pressure with density and is responsible for fluid motion.
Q_net is the net heat flux at the ocean surface. It is due to the sum of incoming solar radiation, Q_s, turbulent convection with the atmosphere known as the sensible heat flux, Q_h, that primarily depends on wind speed and the temperature difference between air and sea; the latent heat of evaporation/condensation, Q_l or Q_e, that primarily depends on wind speed, temperature, and water vapor at the sea surface; and outgoing black back radition, Q_b, modeled by Stefan-Boltzman as proportional to temperature to the fourth power.
Retroflection of ocean currents occurs when the current turns back on itself. It is believed that this process occurs when the current experiences large changes in potential vorticity due to flow over topography that has large changes in depth or the Coriolis force changes as a current crosses the equator; well-known examples are the Agulhas Current and the North Brazil Current, respectively.
The Rossby number is a fundamental nondimensional number that measures the importance of nonlinear acceleatrion to the Coroilis acceleration for different flows. It is name in the honor of CARL-GUSTAF ROSSBY. The Rossby number can be defined as U/fL, where U is a characteristic velocity scale, O(10 cm/s) to O(100 cm/s), in the ocean, f is the Coriolis parameter given by twice the earth's angular speed multiplied by the sine of the latitude, and L is a characteristic horizontal length scale of O(1 km) to O(1000 km). The Rossby number can also be defined as the relative vorticity divided by the Coriolis parameter. Rossby numbers less than 0.1 have been estimated for slower ocean flows found in the middle of ocean gyres, O(1) in strong currents like the Gulf Stream and ocean rings, and O(10) in the Florida Current that is a very strong flow close to shore.
The Rossby radius of deformation is a fundamental length scale in ocean flows. It is name in the honor of CARL-GUSTAF ROSSBY. It is the distance over which the tendency for the Coriolis acceleration to balance pressure gradients versus gravity that flattens the sea surface and reduce the pressure gradient to zero. It is given by the square root(gH) divided by f, g is gravity (=9.81 m/s^2), f is the Coriolis parameter given by twice the earth's angular speed multiplied by the sine of the latitude, and H is a characteristic vertical length scale. The baroclinic Rossby radius of deformation is defined in a similar fashion but with reduced gravity that depends on the density difference between the upper and lower fluid. The Rossby radius of deformation is an important length scale for the approximate radius of ocean eddies. The baroclinic Rossby Radius of deformation is on the order of 10 km in polar regions, 40-50 km in mid-latitude, and hundreds of kilometers in the tropics.
Salinity is defined as the total amount of salts in grams in one kg of water. Typical ocean water is between 34 and 36 parts per thousand. Saline water is defined to be between 30 amd 50 parts per thousand. Though there are many different salts in sea water, under the assumption of a binary fluid, sea water is composed of freshwater and salt and just one number is used to represent all of the salts in density calculations.
Sargasso Sea is the area of the N. Atlantic ocean enclosed by the subtropical gyre circulation. The Sargasso Sea is east of the U.S. and south of the Gulf Stream. It is relatively warm, salty and is populated by many oceanic eddies.
Scales are the characteristic size of organized fluid flows. A spatial scale of an oceanic vortex (eddy, ring) is its radius. A temporal scale of a vortex is its rotational period. A spatial scale of a wave is its wavelength. A time scale of a wave is its period. Scales for vertical motion is usually determined by either the depth of the Ekman layer, depth of the main thermocline, or the bottom depth. Horizontal scales for currents are given by the width of the current or by its wavelength, i.e., the distance between successive meander crests (or troughs). The scale of turbulent fluid flow is usually given by the e-folding scale (distance or time in that the autocorrelation function of the pressure/velocity field decreases by 1/e (37%) of its initial value), where the autocorrelation function becomes zero (zero-crossing scale) or the integral time scale calculated by integrating the autocovariance function. Fluid scales can be set by the geomorphology of the basin. Small-scale ocean flows occur for horizontal spatial scales on the order of 100 m to 10 km, and time scales measured in day(s). Mesoscale (or synoptic) flow is the oceanographic equivalent of atmospheric weather. Time scales are on the order of a week to months. Space scales are on the order of tens of km to hundreds of km. Large-scale flow is the oceanographic equivalent of atmospheric climatology. Time scales are on the order of one year to many years. Space scales are on the order of the size of ocean basins and range from many hundreds to a few thousand of km.
Stratification is a fundamental property of the ocean and is due to water with large density being deeper than low density water. The density of a stable ocean increases with depth. Ocean stratification impedes vertical motion and large scale motion is approximately horizontal along density surfaces.
Subpolar Mode Water is formed during wintertime convection in the Pacific subpolar gyre. The temperature and salinity range of Subpolar Mode Water is 3 to 4 C and less than 34.92, respectively.
Subtropical Mode Water is formed during wintertime convection in the N. Atlantic subtropical gyre. The temperature and salinity range of Subtropical Mode Water is 17.9 to 19°C and 36.4-36.6 respectively.
Sverdrup (Sv) is the basic unit of volume transport used in physical oceanography and is equal to one million cubic meters of water flowing per second. This unit is named after author and Scripps Director Harald Sverdrup. One Sv is about the average amount of water that flows in all of the world's rivers. During spring thaw, runoff from melting snow and rain from strong seasonal storms increases the amount of river flow to 2 Sv. In comparison, the transport of the Fl current is about 30 Sv and the peak flow of the Gulf Stream has a transport of approximately 150 Sv.
Sverdrup (Sv) Transport is the depth-integrated transport of the ocean due to spatially winds. The transport consists of two components, the direct wind-driven Ekman transport and the resulting geosotrophic transport that is due to the convergence/divergence of the Ekman transport. The meridional component (south-north direction) of the Sverdrup transport is given by the curl of the wind stress divided by β (see above). The zonal component (east-west) of the Sverdrup component is than found assuming that the transport is horizontal nondivergent.
Temperature is the fundamental thermodynamical quantity that is a measure of thermal energy such as the average kinetic energy of molecular vibrations. There are three temperature scales-Celsuis/Centigrade, Farenheit, and Kelvin-all defined in this glossary that are used to measure the coldness/hotness of water.
Thermohaline circulation is the flow of water induced by differences in temperature (thermo-) and salinity (haline). These differences in water properties leads to density differences. Deep water forms when sea water entering polar regions cools and freezes. This process leaves colder, saltier and denser water that can sink to great depths and flow into the ocean basins. Surface water must replace the sinking water leading, for example, to a large-scale poleward surface flow in the North Atlantic. A global "conveyor belt" is a simple model of the large-scale thermohaline circulation. Deep-water forms in the North Atlantic, sinks, moves south, circulates around Antarctica, and finally enters the Indian, Pacific, and Atlantic basins. This water is upwelled and returned in a surface circulation (ADD figure). It can take a thousand years for water from the North Atlantic to find its way into the surface waters of the North Pacific.
A tracer as the name implies, is anything that allows us to follow or 'trace' a process. For example, tracers can be used to follow a water mass as it flows from the ocean surface into the interior. Almost any property we can measure in the ocean can be considered a tracer as its distribution is due to some oceanographic process. For example, dye injected into water at the surface can be seen spreading through the water column and can be considered a tracer. Tracers can be classified into different types: 1) stable, conservative tracers, for example, salinity or potential temperature. A conservative tracer is one that can only be altered by mixing in the ocean interior. Potential temperature and salinity can be used to identify water masses for this reason. These properties are set by processes occurring in the ocean boundary, so once a water mass is isolated from the atmosphere below the mixed layer, it can be identified by its temperature and salinity. Another type of tracer includes: 2) stable, non-conservative, which can be altered by physical, chemical or biological processes occurring within the ocean. An example of this type of tracer is O2 or CO2. Dissolved oxygen is produced by phytoplankton photosynthesis and is consumed throughout the water column by respiration and bacterial oxidation of decaying material Two other classifications of tracers include: 3) radioactive, conservative (e.g. 222Rn) and 4) radioactive, non-conservative (eg.14CO2).
Transport is the amount of water flowing per unit time. Transport values are either given as a volume transport, VA, where V is the average velocity perpendicular to the plane with surface area A. Its units are m3/s. Since most ocean currents involve volume transports of millions to hundred millions m3/s of water, physical oceanographers use units of Sverdrup, 1 Sv = 1,000,000 m3/s. Mass transport is calculated as mdUA, where md is the mean density and has units of kg3/s. The Florida Current is bounded by the Florida Coast to its west and the Bahama Islands to its east. If you were to measure all the water flowing between Miami, FL and the Bahamas, from top to bottom, the mean volume transport would probably be around 30 Sv. That is, on the average, 30,000,000 m3 of water is flowing, every second, between Miami and the Bahamas. This transport is about 8 billion galleons of water per second.
Upwelling brings cold, nutrient-rich water from the depths up to the surface. Earth's rotation and strong seasonal winds push surface water away from some western coasts, so water rises on the western edges of continents to replace it. Marine life thrives in these nutrient-rich waters. Coastal upwelling is usually induced by Ekman transport (see below). Large-scale equatorial upwelling results at the equator due to the divergence of the Ekman transports at the Equator. The trade winds predominantly blow from east-to-west in the tropics. In the N. Hemisphere, this leads to an Ekman transport poleward to the north and in the S. Hemisphere, poleward to the south. Deeper, colder and more nutrient rich waters replace the poleward moving surface waters (add figure).
Velocity is a fundamental physical quantity that is the rate of change of position with time. Velocity consists of both a magnitude known as the speed and a direction of the motion. Oceanographers are interested in the three-dimensional fluid velocity and use the notation (u,v,w) to denote (west-east velocity, south-north velocity, up-down velocity).
The Vema Channel is an important pathway for water exchange between the deep western and eastern South Atlantic. It is located at, 31.3°S, 39.4°W and is a deep topographic trough in the mid-Atlantic ridge that allows the deep Antarctic Bottom water to enter the eastern Atlantic.
The Vema Fracture Zone is an important pathway for water exchange between the deep western and eastern equatorial Atlantic. It is located at, 10.7°N, 42.3°W and is a deep topographic fracture in the mid-Atlantic ridge.
Vorticity is a measure of spin for a fluid. The relative vorticity is a vector quantity equalling the the curl of the velocity field. The value of the vorticity depends on the spatial gradients of the velocity field. It is important for analyzing rotational dynamics of fluid motion.
Warm surface currents invariably flow from the tropics to the higher latitudes, driven mainly by atmospheric winds, as well as the earth's rotation. Subtropical western boundary currents, such as the Gulf Stream and Kuroshio, are warm, fast surface currents that transport a lot of water and heat of tropical origin to subpolar regions. This process is extremely important for maintaining the earth's heat balance.
Water masses are fairly large volumes of water with similar, nearly homogeneous, physical and chemical properties (temperature, salinity, oxygen, silicon, etc.). These properties are either acquired at the ocean surface in regions of strong atmospheric forcing or are the result of homogenization by oceanic mixing. Major examples include North Atlantic Deep Water (NADW), Antarctica Bottom Water (AABW) and the central or common waters of the Pacific and Indian Ocean. Tracking water masses from their formation regions to where you observe them is one way oceanographers learn about ocean circulation and mixing.
Wind-driven circulation is the large-scale oceanic circulation that results from the actions of the wind. There are two-components, a directly-driven Ekman component and an indirect component, due to the divergences and convergences of the Ekman transport that either leads to water piling up, creating a high pressure system in the ocean or to a low pressure system where surface waters diverge. These pressure systems have pressure gradients that drive the flow. The indirect flow component is in geostrophic balance with the pressure gradients. The indirect geostrophic flow is larger by a factor of 3-4 than the directly wind-driven flow.
The variable x is ususally used to represent longitude or position in the west to east direction.
XBT is the acronym for eXpendable Bathy-Thermagraph. The probe can be launched from a moving ship and measures the temperature of the sea water as it falls through the water. Many of these probes measured vertical profiles of temperature for depths from the surface down to 200 - 1500 m depending on which model probe.
XCP is the acronym for an eXpendable Current Profiler. These probes can be launched out of planes. XCPs and XCTD have been launched by hurricane hunters. Click here for a series of experiments by Dr. Nick Shay studying the air-sea interaction of hurricanes and the ocean.
XCTD is the acronym for an eXpendable version of the CTD (see above).
The variable y is ususally used to represent latitude or position in the south to north direction.
The variable z is ususally used to represent the vertical coordinate for depth in the ocean. A depth of 100 m is z =-100 m.
An excellent glossary for oceanography can be found at Ocean World.