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ENACW
Abbreviation for Eastern North Atlantic Central Water.

ENM
Acronym for empirical normal mode.

ENPCW
See Eastern North Pacific Central Water.

ENSIP
A coordinated study to compare the simulations of ENSO in coupled ocean-atmosphere models.

[http://www.met.rdg.ac.uk/cag/NAO/Models.html]
[http://www1.imgw.gdynia.pl/lustro_dkrz/clivar/ensip.html]

ENSO
See El Niñno/Southern Oscillation.

enstrophy
This is defined as half of the area-mean vorticity squared in a fluid, mathematically expressed by

$\displaystyle E\,=\,{1\over A}{\int_A}{1\over 2}{\zeta^2}\,dA$

where $ A$ is the area over which the calculation is being made and $ \zeta$ the vorticity. The relation between vorticity and enstrophy is similar to that between velocity and kinetic energy, and the enstrophy budget is used in the study of large-scale motions in the ocean and atmosphere as an alternative to the more cumbersome vorticity budget. See Wiin-Nielsen and Chen (1993).

EOF
Acronym for empirical orthogonal function.

EOS-90
Abbreviation for International Equation of State for 1990, the officially recognized equation used by oceanographers to calculate the density of seawater. Why and how EOS-90 was developed EOS-90 are given in ():
Virtually all the computations of density of seawater made since the beginning of the [20th] century have been based on the direct measurements of density, chlorinity and salinity, made by Forch, Knudsen and Sörensen, published in 1902, and of compression of seawater, made by Ekman (1908). A new equation of state was considered urgently desirable because newly acquired data indicated slight discrepancies with the Knudsen-Ekman equation of state of seawater (Grasshoff, 1976). This old equation was obtained from measurements of density of natural seawater in which the proportions of the various ions are not exactly constant. To be consistent with the new definition of the Practical Salinity, 1978, the new equation of state is based on measurements of density of standard seawater solutions obtained by weight dilution with distilled water and by evaporation. As the absolute density of pure water is not known with enough accuracy, the density of distilled water used for the measurements was determined from the equation of the SMOW (Standard Mean Ocean Water) whose isotopic composition is well defined (IUPAC, 1976). Intensive work was then carried out in different laboratories with different measuring equipment. This resulted in considerable data on which the new International Equation of State of Seawater is based. The full equation is composed on a one atmosphere equation based on 467 data points, combined with a high pressure expression based on 2,023 data points. The density computed with these equations is relative to the IUPAC (1976) recommended equation for density of SMOW.

The density of seawater at one standard atmosphere is computed from the practical salinity (S) and the temperature (t) with the following equation:

$\displaystyle \rho (S,t,0)\,$ $\displaystyle =$ $\displaystyle \,{\rho_w}\,+\,(8.24493 \times {10^{-1}}\,-\, 4.0899 \times {10^{-3}} t \,+\, 7.6438 \times {10^{-5}}{t^2}$  
$\displaystyle \,$ $\displaystyle -$ $\displaystyle \,8.2467 \times {10^{-7}} {t^3} \,+\,5.3875 \times {10^{-9}}{t^4})S$  
$\displaystyle \,$ $\displaystyle +$ $\displaystyle \,(-5.72466\times{10^{-3}}\,+\,1.0227\times{10^{-4}}t\,-\, 1.6546\times{10^{-6}}{t^2}){S^{3/2}}$  
$\displaystyle \,$ $\displaystyle +$ $\displaystyle \,4.8314\times{10^{-4}}{S^2}$ (2)

where $ \rho_w$, the density of Standard Mean Ocean Water (SMOW) taken as pure water reference, is given by:
$\displaystyle {\rho_w}\,$ $\displaystyle =$ $\displaystyle \,999.842594\,+\,6.793952\times{10^{-2}}t\,-\, 9.095290\times{10^{-3}}{t^2}$  
$\displaystyle \,$ $\displaystyle +$ $\displaystyle \,1.001685\times{10^{-4}}{t^3}\,-\,1.120083\times{10^{-6}}{t^4}\,+\, 6.536332\times{10^{-9}}{t^5}$ (3)

This equation of state is valid for practical salinity values from 0 to 42 and temperature values from -2 to 40$ ^\circ$C.

The density of seawater at higher pressures is computed from the practical salinity (S), the temperature, and the applied pressure (p, bars) with the following equation:

$\displaystyle \rho (S,t,p)\,=\,{ {\rho (S,t,0)} \over {1\,-\,p/K(S,t,p)} }$ (4)

where $ \rho (S,t,0)$ is the one atmosphere value and $ K(S,t,p)$ is the secant bulk modulus given by:

$\displaystyle K(S,t,p)\,=\,K(S,t,0)\,+\,Ap\,+\,B{p^2}$ (5)

where:
$\displaystyle K(S,t,0)\,$ $\displaystyle =$ $\displaystyle \,{K_w}\,+\,(54.6746\,-\,0.603459 t\,+\,i 1.09987\times{10^{-2}}{t^2} \,-\,6.1670\times{10^{-5}}{t^3})S$  
$\displaystyle \,$ $\displaystyle +$ $\displaystyle \,(7.944\times{10^{-2}}\,+\,1.6483\times{10^{-2}}t\,-\, 5.3009\times{10^{-4}}{t^2}){S^{3/2}}$  
$\displaystyle A\,$ $\displaystyle =$ $\displaystyle \,{A_w}\,+\,(2.2838\times{10^{-3}}\,-\,1.0981\times{10^{-5}}t\,-\, 1.6078\times{10^{-6}}{t^2})S\,+\,1.91075\times{10^{-4}}{S^{3/2}}$  
$\displaystyle B\,$ $\displaystyle =$ $\displaystyle \,{B_w}\,+\,(-9.9348\times{10^{-7}}\,+\,2.0816\times{10^{-8}}t\,+\, 9.1697\times{10^{-10}}{t^2})S$ (6)

The pure water terms $ K_w$, $ A_w$ and $ B_w$ of the secant bulk modulus are given by:
$\displaystyle {K_w}\,$ $\displaystyle =$ $\displaystyle \,19652.21\,+\,148.4206t\,-\,2.327105{t^2}\,+\, 1.360477\times{10^{-2}}{t^3}\,-\,5.155288\times{10^{-5}}{t^4}$  
$\displaystyle {A_w}\,$ $\displaystyle =$ $\displaystyle \,3.239908\,+\,1.43713\times{10^{-3}}t\,+\, 1.16092\times{10^{-4}}{t^2}\,-\,5.77905\times{10^{-7}}{t^3}$  
$\displaystyle {B_w}\,$ $\displaystyle =$ $\displaystyle \,8.50935\times{10^{-5}}\,-\,6.12293\times{10^{-6}}t\,+\, 5.2787\times{10^{-8}}{t^2}$ (7)

The high pressure equation of state is valid for practical salinity from 0 to 42, temperature from -2 to 40$ ^\circ$C, and applied pressure from 0 to 1000 bars.

Poisson and Gadhoumi (1993) extended EOS-80 at one standard atmosphere, which was limited to salinities between 2-42, up to 50. A polynomial was developed from laboratory measurements via least-square regression fitting. The equation is:

$\displaystyle \rho (S,t,0)\,-\,{\rho_w}\,$ $\displaystyle =$ $\displaystyle \,S({A_0}\,+\,{A_1}t\,+\,{A_2}S\,+\, {A_3}{t^2}\,+\,{A_4}tS$  
  $\displaystyle +$ $\displaystyle \,{A_5}{S^2}\,+\,{A_6}{t^3}\,+\,{A_7}{t^2}S\,+\,{A_8}t{S^2} \,+\,{A_9}{S^3})$  

where $ S$ is the salinity, $ t$ the temperature, and the coefficents are:
$ {A_0}$ = 82.4427 $ \times$ 10$ ^{-2}$ $ {A_5}$ = -14.791 $ \times$ 10$ ^{-6}$
$ {A_1}$ = -52.753 $ \times$ 10$ ^{-4}$ $ {A_6}$ = 67.90 $ \times$ 10$ ^{-8}$
$ {A_2}$ = -51.17 $ \times$ 10$ ^{-5}$ $ {A_7}$ = -15.886 $ \times$ 10$ ^{-7}$
$ {A_3}$ = 40.261 $ \times$ 10$ ^{-6}$ $ {A_8}$ = -52.228 $ \times$ 10$ ^{-8}$
$ {A_4}$ = 11.5114 $ \times$ 10$ ^{-5}$ $ {A_9}$ = 20.750 $ \times$ 10$ ^{-8}$
The coefficients were calculated with eight decimal places and rounded off to obtain a value of density that differs from the one calculated with the eight decimal digit coefficient polynomial by $ <2\times{10^{-4}}$ kg m$ ^{-3}$. The quantity $ \rho_w$ is the density of pure water and is calculated the same as above. The standard deviation of the differences between the measured and calculated densities is $ 4\times{10^{-3}}$ kg m$ ^{-3}$. This equation is valid within the salinity range 35-50 and the temperature range 15-30$ ^\circ$C. See Millero et al. (1980), Millero and Poisson (1981) and Poisson and Gadhoumi (1993).

EOSS
Acronym for the European Sea-level Observing System, a project under which various European sea level activities are coordinated. The objectives of EOSS include:

[http://www.nbi.ac.uk/psmsl/eoss/eoss.html]

epeiric sea
A shallow inland sea with limited connection to the open ocean and having depths less than 250 meters. Compare to epicontinental sea and inland sea.

EPIC (CLIVAR)
Acronym for Eastern Pacific Investigation of Climate processes in the coupled ocean-atmosphere system, a process-oriented study of the VAMOS element of CLIVAR. EPIC focuses on the eastern Pacific Ocean, specifically the cold tongue ITCZ region and the stratus dreck region. The goal is to understand coupled ocean-atmosphere processes in these regions with the intent of building toward better models and prediction.

[http://www.cdc.noaa.gov/~ajr/epicwksh.html]
[http://www.physics.nmt.edu/raymond/epic2001/overview/]

EPIC
Acronym for Equatorial Pacific Information Collection, a system for management, display, and analysis of oceanographic in-situ data. This was developed at the NOAA PMEL to manage the large numbers of hydrographic and time series oceanographic in-situ data sets collected as part of NOAA climate study programs such as EPOCS, TOGA, WOCE and CLIVAR. There are over 100,000 individual data sets within the database, some of which can be accessed via a Web interface. See the EPIC Web site.

epicontinental sea
A shallow sea on a wide portion of a continental shelf or in the interior of a continent. The former type is also known as a shelf sea. Compare to epeiric sea and inland sea.

epilimnion
The layer of water above the thermocline in a fresh water lake, as opposed to the hypolimnion. This is equivalent to the mixed layer in the ocean.

EPILOG
Acronym for Environmental Processes of the Ice age: Land, Oceans, Glaciers, an IMAGES program. See Mix et al. (2001).

[http://www.images.cnrs-gif.fr/epilog.html]

epineutral
Along a neutral surface.

epipelagic zone
One of five vertical ecological zones into which the deep sea is sometimes divided. The epipelagic zone extends from the surface downward as far as sunlight penetrates during the day. It is a very thin layer, less than 100 meters thick in the eastern parts of the oceans in regions of upwelling and high productivity and up to 200 meters thick in clear subtropical areas. The endemic species of this zone either do not migrate or perform only limited vertical migrations, although there are many animals that do invade the epipelagic zone from deeper layers during the night or pass their early development stages in the photic zone. The epipelagic zone overlies the mesopelagic zone. See Bruun (1957).

EPOC
Acronym for Eastern Pacific Oceanic Conference.

EPOCS
Acronym for the Equatorial Pacific Ocean Climate Studies program, a project of the NOAA ERL initiated in 1979 to investigate the role of the tropical Pacific Ocean in influencing large-scale interannual climate fluctuations. The principal working hypothesis was that interannual variability of SST in the equatorial Pacific is intimately related to atmospheric fluctuations assocated with the Southern Oscillation, with the coupled signal known as ENSO. The goal of EPOCS was an improved understanding of the ENSO phenomena leading to the development of the capability to simulation the tropical Pacific and atmosphere conditions in near real time and to predict various aspects of the evolution of these conditions. EPOCS is a contribution to the large U.S. TOGA effort. See Hayes et al. (1986).

EqPac
Acronym for Equatorial Pacific Project, a U.S. JGOFS process study conducted in the central and eastern equatorial Pacific from 95-170$ ^\circ$W in 1992. The purpose was to determine the fluxes of carbon and related elements, and the processes controlling these fluxes, between the euphotic zone and the atmosphere and deep ocean. The pelagic studies principally addressed the mechanisms that make the equatorial Pacific a high nutrient-low chlorophyll (HNLC) zone, and the factors that control CO$ _2$-gas exchange and new and export production. Benthic studies investigated the fate of carbon in the deep sea and the preservation of the primary productivity signal in buried sediments.

Thirteen separate cruises were conducted consisting of 433 days of ship time on the R. V. Thompson, R. V. Wecoma, R. V. Baldridge and R. V. Discoverer. Remote sensing data were collected on NASA-sponsored P-3B aircraft overflights, an ONR-sponsored iron study (FeLine II) was conducted in March-April 1992, and data from the TOGA-TAO buoy network was obtained.

The scientific lessons learned during EqPac included:

See Murray et al. (1992) and Murray et al. (1995).

[http://usjgofs.whoi.edu/research/eqpac.html]
[http://www.aoml.noaa.gov/ocd/oaces/eqpac92.html]
[http://usjgofs.whoi.edu/jg/dir/jgofs/]

EQUALANT
A component of the ECLAT (Etudes Climatiques dans l'Atlantique Tropical) program, the French component of CLIVAR.

[http://www.aoml.noaa.gov/phod/COSTA/abstracts/equalant.html]

EQUAPAC
Acronym for Cooperative Survey of the Pacific Equatorial Zone, a joint France/Japan/USA project.

equation of mass continuity
An equation stating that because the mass of a fluid parcel is constant, the density must decrease/increase if the flow diverges/converges. This is mathematically expressed by

$\displaystyle {{d\rho}\over{dt}}\,=\,\rho\nabla\cdot v\,=\,0$

where $ \rho$ is the fluid density and $ v$ the vector velocity. See Dutton (1986).

equation of state
See Fine et al. (1974), Millero et al. (1980) and Brydon et al. (1999).

equatorial beta plane
An approximation for oceanic and atmospheric motions near the equator where the substitutions sin $ \phi\,\approx\,\phi$ and cos $ \phi\,\approx\,1$ are made into the governing equations of motion. In this approximation, beta is a constant given by

$\displaystyle \beta\,=\,2\Omega / r$

where $ \Omega$ is the rotation rate of the earth and $ r$ its radius, and $ f$ is given by

$\displaystyle f\,=\,\beta y$

where $ y\,=\,r\phi$ is distance northward from the equator. See Hendershott (1981), p. 304 and Gill (1982), p. 434.

Equatorial Countercurrent
In physical oceanography, a subsurface eastward flow that is about 100-200 m thick and 200-300 km wide. It is centered approximately on the equator, and its core lies just beneath the base of the mixed layer in the top of the equatorial thermocline. Such a current is found in all three oceans, although it appears to be a seasonal phenomenon in the Indian Ocean. See Leetmaa et al. (1981).

EquatorialIC
A westward flowing equatorial current in the Pacific Ocean that underlies the eastward-flowing Equatorial Undercurrent (EUC). See Delcroix and Henin (1988).

equatorial radius of deformation
A form of the Rossby radius of deformation applicable to wave motions near and at the equator. It is defined as

$\displaystyle {a_e}\,=\,{{c/\beta}^{1/2}}$

where $ c$ is the gravity wave speed, i.e. $ {gH}^{1/2}$ where $ H$ is the depth (or equivalent depth). This radius is about 2000 km ($ c$ = 200 m/s) for barotropic waves in the ocean, making it marginally applicable for use with the equatorial beta plane concept. The approximation is much more valid for the case of baroclinic waves where, for typical atmosphere (20-80 m/s) and ocean (0.5-3.0 m/s) values of $ c$, the equatorial deformation radius is, respectively, 650-1300 km and 100-250 km.

equatorial trough
A region of lower pressure located between the subtropical highs on each side of the equator. Within this zone the trade wind airstreams from either hemisphere meet causing ascending motion and large amounts of precipitation. It constitutes the equatorward, ascending portions of the Hadley mean meridional circulation cells of both hemispheres. Energetically this results in an import of water vapor concentrated in the trade wind layer and an export of geopotential energy and sensible heat in th eupper troposphere. This results in a net atmospheric heat export from the trough zone to the higher latitudes. This region, commonly called the doldrums, is centered near $ 5^\circ$ S in January and $ 12-15^\circ$ N in July. Its migration between these extremes influences the seasonal distribution of cloudiness and rainfall and the formation of tropical storms, and its annual mean position is known as the meteorological equator.

Equatorial Undercurrent (EUC)
In physical oceanography, a subsurface eastward flow centered approximately on the equator whose core lies just beneath the base of the mixed layer in the top of the equatorial thermocline. The flow generally ranges from 100-200 m thick and 200-300 km wide. Such a current is found in all three oceans, although it appears to be a seasonal phenomenon in the Indian Ocean. In the Atlantic its core is around 100 m deep with speeds exceeding 1.2 m/s and transports up to 15 Sv. It alternates between extreme positions 90 km on either side of the Equator on a 2-3 week time scale with speed and transport fluctuating between the previous figures and 0.6 m/s and 4 Sv.

The Pacific EUC was originally discovered and identified as a swift, subsurface current flowing eastward on the equator in opposition to the winds by Cromwell et al. (1954) and, as a result, is sometimes known as the Cromwell Current. Further studies by (,) showed it to be continuous along, symmetric about, and tightly confined to the equator with transports comparable to other major ocean currents. The Pacific EUC flows eastward as a narrow (about 500 km wide) tongue within the equatorial thermocline from north of New Guinea to the Galapagos Islands in the eastern Pacific. It has a thickness of only about 200 m, and typical velocities of 1.5 m s$ ^{-1}$, with the core depth ranging from 200m in the west to 40 m in the east. It is characterized by a high salinity core and a high concentration of dissolved oxygen. Results from hydrographic and modeling studies estimate a mean transport of 30-40 Sv, although instantaneous peaks of over 80 Sv have been calculated from various measurements.

The details are much more complicated and less well known for the Indian Ocean, although it appears to be present primarily during the northeast monsoon. This is also known as the Cromwell Current in the Pacific Ocean after Cromwell et al. (1954).

The dynamical explanation for a an undercurrent has an appealing qualitative explanation, i.e. fluid converging towards the equator conserves absolute vorticity. As a result, relative vorticity has to increase to make up for the vanishing of planetary vorticity there, with this providing a source of eastward momentum to drive the undercurrent. The balance of forces at the equator reduces to

$\displaystyle {{\partial p}\over{\partial x}}\,=\, {\partial\over{\partial z}}\left(\nu {{\partial u}\over{\partial z}}\right)$

where $ p$ is the pressure (baroclinic), $ x$ the coordinate along the equator, $ \nu$ the momentum diffusion coefficient, $ z$ the vertical coordinate, and $ u$ the along equator velocity component. This is a linear equation, and although the addition of nonlinearities has brought model results and observations into closer concordance, it is thought that they are not essential for maintaining the undercurrent and serve only to modify the linear dynamics. The unsteady flow represented by the dynamics of equatorial waves has also been postulated as an explanation for the observe time-varying characteristics of the undercurrent. See Cromwell et al. (1954), Knauss (1960), Knauss (1966), Philander (1973), Philander (1980), Leetmaa et al. (1981), Peterson and Stramma (1991), Tomczak and Godfrey (1994), Blanke and Raynaud (1997) and Lu et al. (1998).

equatorially trapped gravity wave

equatorially trapped Kelvin wave
An equatorially trapped wave similar in character to coastally trapped Kelvin waves. The motion is unidirectional and parallel to the equator everywhere, and in each vertical plane parallel to the equator the motion is the same as for a nonrotating fluid. A required geostrophic balance between the east-west velocity and the north-south pressure gradient leads to solutions that decay away from either side of the equator on a length scale called the equatorial radius of deformation. These dispersionless waves propagate eastward at the same speed as they would in a nonrotating fluid, with the dispersion relation being $ \omega\,=\,kc$. The magnitude of $ c$ for the first baroclinic mode for typical ocean values is around 2.8 m/s, which would take a Kelvin wave across the Pacific in about 2 months. See Gill (1982).

equatorially trapped Poincare wave
See equatorially trapped gravity wave.

equatorially trapped Rossby wave

equatorially trapped wave
A wave that is confined to propagate on and near the equator due to the local waveguide properties. The waveguide is caused by the vanishing of $ f_0$ at the equator. This means that the conditions for geostrophic balance theoretically fail there, although practically any wave motion having a finite expanse across the equator will feel the Coriolis force on either side. This will serve to turn that motion back towards the equator on either side, thus serving as a trap or a waveguide for motions there. See Gill (1982).

Equilant cruises
A series of research cruises in 1965-1966 that performed tightly organized surveys of the tropical Atlantic. Simultaneous data on temperature, salinity and currents were obtained for the then little-known area off the west coast of Africa. These cruises were done with ships from the U.S., the Soviet Union, France, Brazil and other nations. See Idyll (1969).

equilibration time
The time it takes for a system to re-equilibrate after being subject to a perturbation. This is usually expressed in terms of an e-folding time. Some typical equilibration times are: the atmosphere, 11 days; the ocean mixed layer, 7-8 years; the deep ocean, 300-1000 years; mountain glaciers, 300 years; ice sheets, 3000 years; the Earth's mantle, 30 million years.

equilibrium tide
The hypothetical tide which would exist if the ocean responded instantly to the tide producing forces and formed an equilibrium surface. The effects of friction, inertia, and the irregular distribution of land masses are ignored.

equivalent barotropic
An atmospheric state in which the temperature gradients are such that the isotherms are parallel to the isobars.

equivalent depth
When the solution of a differential equation set (e.g. the equations of motion for a baroclinic atmosphere or ocean) is approximated using the normal mode technique, each of the independent normal or baroclinic mode solutions obtained behaves equivalently to a homogeneous system with a depth that is called the equivalent depth. See Gill (1982).

equivalent potential temperature
In meteorology, the equivalent temperature of an air sample when it is brought adiabatically to a pressure of 1000 mb. It is a conservative property for both dry and saturated adiabatic processes.

ergodic hypothesis
The assumption that a process is statistically stationary, and therefore ensemble averaging is equivalent to averaging over time. See Kagan (1995).

error of representativeness
The spatial spectrum of the atmosphere or ocean shows variance at all scales, with generally less variance at smaller scales. The observation network, however, has a finite spacing between observation stations. If a network has an average spacing of, say, L between stations, then samples with scales much greater or smaller will be sample, respectively, very well or very poorly by the network. For instance, a network with 1000 km spacing will not see a tornado or thunderstorm with a 10 km characteristic length scale if it is between stations, but will see it if it overlies a station and, in addition, will misrepresent it as a larger scale motion. This is occasionally known as aliasing. See Daley (1991).

Ertel potential vorticity
A rigorous formulation of potential vorticity for any compressible, thermodynamically active, inviscid fluid in adiabatic flow. The Ertel potential vorticity $ \pi$ is defined by

$\displaystyle \pi\,=\,\nabla S \cdot \left( {{2\Omega\,+\,curl V}\over \rho}\right)$

where $ S$ is some conservative thermodynamic property of the fluid (the potential temperature, e.g.), $ \Omega$ is the angular velocity of the coordinate system, $ \rho$ is the density, and $ V$ the velocity of the fluid relative to the coordinate system. See Muller (1995).

Ertel's theorem
A theorem stating that in an incompressible Boussinesq fluid that is homogeneous and inviscid a quantity called the potential vorticity is conserved. See Hide (1978).

ESACW
Abbreviation for Eastern South Atlantic Central Water.

ESPCW
See Eastern South Pacific Central Water.

ESTAR
Acronym for Electronically Scanned Thinned Array Radiometer, a proposed remote sensing technique for monitoring the large scale distribution of surface salinity. It depends on the influence of salinity on microwave emissions, strongest at 1.4 GHz. Since temperature has a larger effect, high accuracy temperature measurements must also be made using another band at either 2.65 or 5.0 GHz. This yields a salinity accuracy of 0.05 parts per thousand, although this can be achieved only via long time (30 days) and space (100 km) averaging. A resolution of 10 km would degrade salinity measurement accuracy to 2 parts per thousand. See Swift (1993) and Schmitt (1995).

ESTOC
Acronym for European Station for Time-Series in the Ocean Canary Islands, established to complement existing open ocean stations in the eastern boundary regime of the North Atlantic. Regular observations started in 1994 at a nominal station position of 29$ ^\circ$ 10' N, 15$ ^\circ$ 30' W, a site about 100 km north of the islands of Gran Canaria and Tenerife at a depth of 3600 m.

[http://www.ifm.uni-kiel.de/ph/general/estoc.html]

estuarine Richardson number
A form of the Richardson number that gauges the relative effects of stratification and mixing in estuaries. It is given by

$\displaystyle R\,=\,{ {(\delta\rho/\rho )g{Q_f}} \over {W{U_t^3}} }$

where $ U_t$ is the RMS tidal velocity, $ W$ the channel width, $ \delta \rho$ the difference in density between river and ocean water, $ \rho$ the average density, $ g$ gravitational acceleration and $ Q_f$ the fresh water discharge rate. If $ R$ is large the estuary will be strongly stratified the flow dominated by density currents, and if it is small the estuary will be well mixed and density effects can probably be neglected. There is also a modified version of this in which $ U_t$ is replaced by the shear velocity $ u^*$ to include the effect of varying bottom friction. See Fischer et al. (1979).

estuary
A semi-enclosed body of water having a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. The term has traditionally been applied to the lower reaches of rivers into which sea water intrudes and mixes with fresh water as well as to bays, inlets, gulfs and sounds into which several rivers might empty and in which the mixing of fresh and salt water occurs.

Distinctions between estuaries are usually made based on the prevailing physical oceanographic conditions (principally the salinity distribution) which are governed by the geometry of the estuary, the magnitude of fresh water flow into the estuary, and the magnitude and extent of the tidal motion. The four principal categories into which estuaries are divided using these criteria are well mixed, stratified, arrested salt wedge and fjord entrainment estuaries, although a single estuary can vary seasonally from one type to another. See Emery and Stevenson (1957), Officer (1976), Hansen and Rattray Jr. (1966) and Scott (1993).

ETAMBOT
A French research program that took place from 1993 until April-May 1996. It was a program in the western Equatorial Atlantic Ocean wherein hydrographic cruises with tracers were conducted along three meridional and one zonal section off Northeast South America in the region west of 35$ ^\circ$ W and south of 7.5$ ^\circ$ N. This was followed up by the ARCANE program.

ETDP
Abbreviation for Expert Tsunami Database for the Pacific.

etesian
A Greek term for winds that blow at times in summer (May to September) from a direction ranging from northeast to northwest in the eastern Mediterranean. In Turkey these winds are known as ``meltemi''.

ETOPO5
A digital database of land and sea floor elevations on a 5 minute lat/lon grid. The resolution of the gridded data varies from true 5-minute for the ocean floors, the USA., Europe, Japan,and Australia to 1 degree in data-deficient parts of Asia, South America, northern Canada, and Africa.

[http://www.ngdc.noaa.gov/mgg/global/seltopo.html]
[http://fish.cims.nyu.edu/project_aomip/forcing_data/merged_topography.html]

ETP
Abbreviaton for Eastern Tropical Pacific.

EU
Abbreviation for Eurasian Oscillation.

EUBEX
Acronym for the Eurasian Basin Experiment.

EUC
Abbreviation for Equatorial Undercurrent.

Euler equations
More later.

Eulerian mean circulation
In oceanography, the time-averaged flow field in a fixed coordinate system. This can be remarkably different from the synoptic mean circulation. See Schmitz and McCartney (1993).

Eulerian velocity
That velocity which would be measured by a current meter at a fixed point. Compare and contrast to Lagrangian velocity and #.#>

euphotic zone
In the ocean, the sunlit layer from the surface to the depth of 1% light level wherein most of the primary productivity takes place. The depth varies geographically and seasonally and can range from a few meters in turbid, highly productive waters near the shore to around 200 m in tropical waters. The ocean average is around 100 m. It is a zone with sharp gradients in illumination, temperature and salinity, and overlies the aphotic zone. It is also known as the photic zone.

Eurafrican Mediterranean Water (EMW)
In physical oceanography, a water mass that leaves the Strait of Gibraltar with a temperature of about 13.5$ ^\circ$ C and a salinity of 37.8 but is transformed by mixing to a temperature and salinity of 11-12$ ^\circ$ C and 36.0-36.2 within 250 km. From there it spreads isopycnally across the ocean, mixing gradually with the water above and below.

EUROFLOAT
A MAST program for the observation and modeling of the large-scale movement of the Mediterranean Water (MW) and Labrador Sea Water (LSW) in the eastern North Atlantic Ocean. The principal objectives are: A large part of EUROFLOAT will be a lagrangian circulation experiment wherein subsurface neutrally buoyant floats will be used to observe the movement of deep water masses over a period of 3 years. The ARCANE project is a companion study to this.

[http://www.ifremer.fr/lpo/eurofloat/]

EuroGOOS
A program to support the European component of the GOOS. It exists to maximize the benefits to Europe from operational oceanography and the aims include indentifying European priorities for operational oceanography, promoting the development of various systems (i.e. scientific, technological, and computer) for operational oceanography, and establishing methods of routine collaboration between European national and multi-national agencies for the conduct of operational oceanography. See the EuroGOOS Web site.

eustatic
Descriptive of global sea level variations due to absolute changes in the quantity of seawater, the most recent significant examples of which have been caused by the waxing and waning of continental ice sheets during glaciation cycles.

eutrophic
A situation in which the increased availability of nutrients such as nitrate and phosphate (e.g. from the use of agricultural fertilizers and the combustion of fossil fuels) stimulates the growth of plants such that the oxygen content is depleted and carbon sequestered. It is hypothesized that this might serve as a negative feedback to an increase in atmospheric CO2.

evaporative cooling
A phenomenon wherein the evaporation of water from saturated air (when, for example, it mixes with drier air) cools the air due to the absorption of latent heat.

evolution of the ocean
See Holland (1984) and Walker (1977).

exergy
A concept and word invented in Rant (1956) for a quantity which can be defined as the available work of a system in connection with its environment in thermodynamic equilibrium, with the equilibrium characterized constant values of temperature and pressure. This is related to the concept of available potential energy (APE) in that it can be used to find the portion of potential energy in a system that can be transformed into kinetic energy. In the classical exergy concept, a state of thermodynamic equilibrium with constant temperature is used as a reference state. In meteorology, it has been shown that, in the case of stable stratification, a state of hydrostatic equilibrium will suffice as a reference state from which no portion of energy is available for conversion into kinetic energy. The APE theories can be derived from the exergy concept in certain situations. See Kucharski (1997).

explicit scheme
In numerical modeling, an integration algorithm that temporally advances an approximate solution via discrete steps using only information from previous time steps. These are computationally simpler than implicit schemes but require shorter time stepping intervals. See Kowalik and Murty (1993).

export flux
That organic matter (particulate and dissolved) exported from the upper productive layer of the ocean into the deep sea to balance primary production over large time and space scales. This is primarily studied with sediment traps moored in the deep ocean and with freely drifting traps in the upper 1000 m. See biological pump.

export production
In biological oceanography, the loss rate of organic carbon (and nitrogen) from the surface ocean layer to the ocean interior.

extensive parameter
A determining parameter of a system that is proportional to the size and mass of the system, e.g. volume, internal energy, enthalpy and entropy, as opposed to an intensive parameter.

extinction coefficient
A coefficient measuring the rate of extinction, or diminution, with distance of transmitted light in sea water. It is the attenuation coefficient for visible radiation.

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Manbreaker Crag 2001-08-17