Pn-Pz

PNA

Abbreviation for the Pacific/North American teleconnection pattern, a prominent mode of low frequency variability in the extratropics of the northern hemisphere. It appears in all months except June and July, and reflects a quadrapole pattern of height anomalies. Anomalies of similar sign are found south of the Aleutian Islands and over the southwestern U.S., while those with opposite sign are located near Hawaii and over the intermountain region of North American (central Canada) during the winter and fall (spring). The spatial scale of the PNA is largest in winter, when the Aleutian center covers most of the northern latitudes of the North Pacific. It contracts in the spring when the subtropical center near Hawaii reaches a maximum. It disappears in June and July and reappears in the late summer and fall. During this period, the midlatitude centers are dominant and appear as a wave pattern emanating from the eastern North Pacific. The PNA pattern shows substantial interseasonal, interannual and interdecadal variability. See Wallace and Gutzler (1981) and Horel and Wallace (1981).

P-N-J method

A wave spectrum method for wave forecasting developed by Pierson, Neumann and James (Pierson et al. (1955)) in the mid-1950s. Each wind velocity produces a certain range of wave periods with a well-defined maximum, with the total range of periods increasing with the wind velocity along with the energy within the total spectrum. The significant wave height can be found with method along with the spectrum information. See Komar (1976).

POC

Abbreviation for particulate organic carbon.

POCEX

Acronym for Pacific Ocean Color Experiment, See the POCEX Web site.

PODAAC

Acronym for Physical Oceanography Distributed Active Archive Center, an element of EOSDIS and located at JPL, is responsible for archiving and distributing data relevant to the physical state of the ocean. Most of the data archived at PODAAC is derived from sensors on satellites.

[http://podaac-www.jpl.nasa.gov/]

PODS

Acronym for Pilot Ocean Data System.

POEM

Acronym for Physical Oceanography of the Eastern Mediterranean. See Malanotte-Rizzoli and Robinson (1988) and Robinson et al. (1992).

[http://sit.iuav.unive.it/mednet/ocean/poem.html]

POGO

A free-fall vertical velocity profiler. POGO is an acoustically-tracked dropsonde intended to measure the depth-averaged horizontal velocity from the sea surface to some preselected depth. See Rossby et al. (1991).

POGO

Acronym for Partnership for Observation of the Global Oceans, an international network of major oceanographic institutions established to promote the integration and implementation of global oceanographic activities.

[http://www.oceanpartners.org/]

Poincare wave

A gravity wave in a rotating system. One of the fundamental wave solutions of the linearized barotropic equations. The properties of these waves depend on how the wavelength compares with the Rossby radius. If they are short compared with the Rossby radius, then they are ordinary nondispersive shallow-water waves (when the Rossby radius is additionally large compared to the fluid depth). If they are long compared with the the Rossby radius, the frequency is approximately constant and equal to f or twice the rotation rate. Gravity has no effect in this limit and thus fluid particles move under their own inertia at the inertial frequency f and are called inertial waves. The dispersion relation for Poincare waves is where is the square of the horizontal wavenumber. (The use of this term is occasionally restricted to those waves that satisfy the boundary conditions for a channel.) See Gill (1982), pp. 196-197, 249-256.

polar domain

The northernmost of three hydrographic domains into which the waters of the North Atlantic Ocean are sometimes divided for the purpose of describing water mass formation in the region, with the other two being (to the south) the arctic domain and the Atlantic domain. The polar domain provides an upper layer source water mass for the arctic domain that is colder ($<$ 0$^\circ$ C), less saline (30 to 34), and less dense ($\sigma_t$ ranging from 24 to 27.6) than those from the Atlantic domain. The low salinity of this surface water is derived from both river runoff and through the melting of ice and it is carried southward through the western Denmark Strait by the East Greenland Current. Small amounts of this water are carried eastward into the interior basins of the arctic zone by the Jan Mayen Current and the East Icelandic Current. See Swift (1986).

Polar Front (PF)

In physical oceanography, a region of rapid transition in the Southern Ocean (SO) between the Polar Frontal Zone (PFZ) and the Antarctic Zone (AZ). The position of the PF is usually indicated by the large temperature gradient along the temperature minimum of the Antarctic Surface Water (AASW). where it starts to rapidly descend northward. The PF is the northern boundary to cold (1.5$^\circ$ to 2$^\circ$C) near-surface water formed by winter cooling, i.e. the Antarctic Surface Water.

The property indicators within the front are $\theta$ $<$ 2$^\circ$ along the $\theta$-minimum at Z $<$ 200 m, a $\theta$-minimum at Z $>$ 200 m, and $\theta$ $>$ 2.2$^\circ$ along the $\theta$-maximum at Z $>$ 800 m. The PF is one of three distinct fronts in the Antarctic Circumpolar Current (ACC), the others being the Subantarctic Front (SAF) to the north and the Southern ACC Front (SACCF) to the south.

This has previously been referred to as the Meinardus Line (1926), the Oceanic Polar Front (1928), the Antarctic Convergence (1933) and the Antarctic Polar Front (1960). See Orsi et al. (1995), Belkin and Gordon (1996), Peterson and Stramma (1991) and Moore et al. (1999).

Polar Frontal Zone

In physical oceanography, the name given to a transition region in the Southern Ocean (SO) or Antarctic Circumpolar Current (ACC) between the Subantarctic Front (SAF) to the north and the Polar Front (PF) to the south. This was first called the Antarctic Polar Front Zone in the mid-1970s but later modified to the present name. It is identified as a region bound between the 3-9$^\circ$ C surface isotherms. The PFZ is one of four distinct surface water mass regimes in the Southern Ocean, the others being the Subantarctic Zone (SAZ) to the north and the Antarctic Zone (AZ) and Continental Zone (CZ) to the south. See Orsi et al. (1995).

Polar Front Survey

An investigation in the North Atlantic that took place during IGY under the auspices of ICES. See Dietrich (1969).

polar halocline catastrophe

A hypothesized and modeled situation where the presently dominant mode of thermohaline circulation is unstable and evolves to a much weaker overturning circulation pattern. See McWilliams (1996).

Polar Intermediate Water (PIW)

A water mass found in the polar domain in the Northern Atlantic Ocean. It is identified as a distinct temperature minimum layer underneath the East Greenland Current and has salinities in the range 34.4 to 34.7 and is colder than 0$^\circ$ C. Since there is no sharp interface between this and the upper Arctic Intermediate Water, it is distinguished chiefly by geographic location. See Swift (1986).

polar orbit

An orbit in which a satellite passes directly over or close to the poles. The characteristic orbital period is around 90 minutes at an altitude of between 500 and 1500 km. Such satellites are usually Sun synchronoussunsynchronous, and have a field of view such that it takes about 15 orbits to cover the globe, with a specific location being seen about twice a day.

Polar Water (PW)

Any water with salinity values less than 34.4 that enters the arctic domain from the polar domain in the North Atlantic. The temperatures of PW are typicall lowy ($<$ 0$^\circ$ C) although they can reach 3 to 5$^\circ$ C in the summer. The lowest salinities observed are summer salinities less than 30 in the East Greenland Current. The total transport of PW into the arctic domain is not well known but usually estimated at around 1 Sv. See Swift (1986).

POLARIS

A GPS-navigated ocean acoustic current profiler. See Leaman et al. (1995).

polarization relations

The relationships between the velocity components and pressure for a progressive wave. They are found by substituting the assumed wave form into the relevant equations. See Gill (1982), p. 262.

POLDER

Acronym for Polarization and Directionary of the Earth's Reflectances, a wide field of view imaging radiometer that will provide the first global, systematic measurements of spectral, directional and polarized characteristics of the solar radiation reflected by the earth/atmosphere system. POLDER will better allow the radiation scattered in the atmosphere from that reflected by the surface. The data will be processed to determine the physical and optical properties of aerosols so as to classify them and study their variability and cycle; improve the climatological description of certain physical, optical and radiative properties of clouds; precisely determine the influence of aerosols and clouds on the earth's radiation budget; and quantify the role of photosynthesis from the continental biosphere and oceans in the global carbon cycle. It will fly on the ADEOS mission and the results will contribute to the WCRP and the IGBP. See the POLDER Web site.

POLE Experiment

See Davis et al. (1978) and Simpson and Paulson (1979).

POLES

Acronym for Polar Exchange at the Sea Surface, a component of the NASA EOS program that investigates the exchange of mass and energy at the air-ice-ocean interface in the polar regions. See the POLES Web site. See also Barry et al. (1993).

poleward energy flux

The flux process on Earth made inevitable by the fact that more heat is incipient on and absorbed at low than high latitudes and that the Earth is surrounded by a fluid envelope. This excess heat then moves from the tropics to the poles in both hemispheres, i.e. down the gradient, via the atmosphere and the oceans. The partitioning of this flux between the atmosphere and the oceans is as yet not well estimated. If there were no fluid envelope on the Earth, then the tropics would be much warmer and the poles much colder.

Poleward Undercurrent (PUC)

A current that flows from Peru to northern and central Chile over the slope and outer shelf, and is identified by a subsurface maximum. According to Strub et al. (1998):

The Poleward Undercurrent is clearly identified by its water mass characteristics. At a given location it is saltier, richer in nutrients and lower in oxygen than the surrounding water. Maps of salinity at approximately 150 m depth and of the minimum value of oxygen found beneath 50 m depth depict its path within 100 km of the coast from 20$^\circ$S to approximately 42$^\circ$S. Alongshore sections of salinity next to the coast from 15 to 42$^\circ$S reveal a tongue of high salinity between 100 and 300 m depth, spreading from north to south, descending from 150 m depth north of 20$^\circ$S to 200-300 m depth south of 25$^\circ$S. It has been traced to 48$^\circ$S using geostrophic velocities and T-S characteristics.

The Poleward Undercurrent is also evident in onshore-offshore sections of geostrophic velocity from hydrographic cruises. A section of mean, north-south geostrophic velocities shows it to be maximum over the continental slope at 150-200 m depth. Off Chile at 30$^\circ$S, hydrographic sections made in each season in 1991-1992 show it within 40 km of the coast, often extending to the surface. Consistent with the values of these geostrophic velocities, direct measurements of velocities from shipboard profiling current meters and parachute drogues have found values ranging from 0.1 to 0.5 m s$^{-1}$ a depths of 100-300 m.

This has also been called the Gunther Current (after Gunther (1936)) and the Peru-Chile Undercurrent. See Wooster and Gilmartin (1961) and Strub et al. (1998).

POLEX

Acronym for Polar Experiment, a FGGE project.

POLYGON

An oceanographic program to measure the eddy currents in the North Atlantic Equatorial Current for several months using moored current meters and hydrographic surveys. This was a program carried out in 1970 by the Soviet Union. See Brekhovskikh et al. (1971).

POLYMODE

A joint US/USSR oceanographic program to study mesoscale processes in the North Atlantic in the late 1970s and early 1980s. It included a Synoptic Dynamical Experiment (SDE), a Local Dynamics Experiment (LDE), and a statistical geographical experiment. The field phase of POLYMODE ended in 1979.

The seven intensive hydrographic surveys of temperature, salinity and oxygen of the LDE took place between May 15 and July 15, 1978 and were carried out in a 200 km wide octagonal region centered at 31$^\circ$05'N, 69$^\circ$30'W, a location within the southern portion of the Gulf Stream recirculation region. The recirculation region is a northwestern intensification of the wind-driven subtropical gyre in the North Atlantic. The location was chosen as a compromise among a desire to work well within the gyre, with its associated large m ean and eddy currents, a conflicting desire to avoid the peculiar measurement problems associated with especially intense currents, such as those of Gulf Stream rings, and a desire to use the familiar MODE results (from 28$^\circ$N, 70$^\circ$W) as a basis for LDE experimental design. The LDE was designed to meet five scientific objectives:

• to design sampling schemes to yield estimates of the various components in the dynamical balance equations for mesoscale eddies, i.e. to demonstrate the degree of potential vorticity balance and to diagnose dynamical processes from the manner of the balance;
• to provide a high resolution sampling of the energetic eddy scales in all four dimensions, i.e. 50 km horizontally, 500 m vertically and 15 days temporally;
• to obtain a statistical description for the low-frequency, hence plausibly geostrophic, variability on finer scales, both in flow variables and passive tracers;
• to take measurements near the ocean surface to allow a description of mesocale variability in the surface layer during a time of general warming and formation of the seasonal thermocline; and
• to make a further contribution to mapping the means, variances and covariances of currents, density and other observables in the North Atlantic subtropical gyre.

See McWilliams et al. (1983) and Taft et al. (1986) (and several other related papers in the same issue as the latter).

[http://www.aip.org/history/ead/mit_jointsoviet/19990041_content.html]

polynya

An oceanic area which remains either partially or totally ice free at times and under climatological conditions where the surface waters would be expected to be ice covered. They appear in winter when air temperatures are well below the freezing point of sea water and are bordered by water that is covered with ice. They are typically rectangular or elliptical in shape and occur quasi-continuously in the same regions. The size of polynyas can range from a few hundred meters to hundreds of kilometers.

Polynyas are of interest for several reasons. They are sites for active brine formation which may affect the local water density structure and current field and may also influence large-scale water mass modification. They are also a locus for gas exchange between the ocean and atmosphere in polar regions. The large sensible heat fluxes (along with fluxes due to evaporation and longwave radiation) tend to dominate regional heat budgets. They are also of biological interest since their regular occurrence makes them important habitats, e.g. the open water can lead to localized plankton blooms and large mammals tend to use them as feeding grounds.

There are two mechanisms for polynya formation. In the first ice may form within a region and be continually removed by winds, currents, or both. Here the heat required to balance loss the atmosphere and hence to maintain the open water is provided by the latent heat of fusion of the ice being continually formed. The second mechanism involves oceanic heat entering a region in quantities sufficinet to prevent local ice formation. The first mechanism creates ``latent heat polynyas'' and the second ``sensible heat polynyas'', and both mechanisms may operate simultaneuously in the same region. See SMith et al. (1990).

POM

Abbreviation for Particulate Organic Matter. This is usually split into large (or sinking) POM and small (or suspended) POM. Large POM is typically greater than 50 $\mu$m in diameter, sinks at rates around 100 m/day, and is usually sampled with sediment traps. It consists mainly of marine snow, zooplankton fecal pellets and intact organisms. Small POM is typically between about 1 and 50 $\mu$m in diameter, sinks very slowly (if at all), and is sampled by filtering sea water. See Najjar (1991).

PONAM

Acronym for Polar North Atlantic Margins, an LESC program to investigate the Late Cenozoic evolution of the Polar North Atlantic Margin. This project features scientists from seven European nations studying the major climatic variations over the last 5 million years and their impact on the environment in European Arctic regions during a period when glacial cycles dramatically changed the landscape and depositional environment along the Polar North Atlantic margin.

The program studied the long-term climatic signal documented in the marine sediment fans deposited adjacent to glacially overdeepened fjords and shelf troughs, the latest interglacial-glacial cycle by absolute dating and high resolution stratigraphic work to obtain environmental parameters such as ice distribution and oceanic circulation patterns, and the present-day interglacial setting as an interpretational tool for studying the glacial-interglacial sedimentary record with emphasis on sediment transfer processes. See the PONAM Web page.

ponente

A westerly wind blowing in the Mediterranean area.

POP

Abbreviation for principal oscillation patterns. In linear cases, PIPs reduce to damped normal modes or POPs, that represent the eigenoscillations of the reduced linear dynamical system. See Hasselmann (1988) and Hasselmann (1993).

POPS

Abbreviation for Particle and Optics Profiling System.

PORES

Acronym for Physical Oceanography Research of the East Sea, a subproject of the MECBES program of KORDI.

[http://key.kordi.re.kr/home/pores.htm]

pororoca

The tidal boretidalbore of the Amazon River.

PORTS

Acronym for Physical Oceanographic Real-Time System.

positive feedback

A type of feedback in which a perturbation to a system causes an amplification of the process, and thus enhances itself. An example is the ice-albedo feedback mechanism.

potassium-argon dating

A radioisotopic dating method based on the decay of the radioisotope $^{40}$K (potassium) to a daughter isotope $^{40}$Ar (argon). This has been used to date sea-floor basalts as well as to provide the accurate dating needed to establish and correlate on a world-wide basis the geomagnetic polarity time scale. It has also seen limited use in dating lava flows juxtaposed with glacial deposits, thus enabling the glacial event to be dated. The $^{40}$K decays into both $^{40}$Ar and $^{40}$Ca with a half-life of 1.31 $\times\,{{10}^{9}}$ years, although the relative of abundance of the latter precludes its use for dating purposes.

Rocks of volcanic origin are dated using this method since argon is driven off by heating which leaves the samples argon free as they initially cool. The $^{40}$K builds up over time as the potassium decays until it is heated, released, and measured in the laboratory. The $^{40}$K content is derived from a measurement of the total potassium content or by measurement of another stable isotope, $^{39}$K, since the abundance ratios of the potassium isotopes are known. The potassium and argon measurements have to be made on different parts of the same sample, which led to the development of the argon-argon dating method. The extremely long half-life of the argon restricts the use of this procedure to samples greater than 100,000 years old, with volcanic rocks formed over the last 30 million years the most common specimens dated.

This dating method assumes that no argon was present in the material after formation and that the system remains closed from the time of formation. The first assumption can be violated in the case of the formation of deep-sea basalts which retain argon during formation under high hydrostatic pressure, and some material can retain argon from argon-rich source materials during formation. This can result in an overestimation of the sample age. The second assumption can be violated when argon is absorbed on to the surface and interior of a sample, although the degree of atmospheric concentration can be adjusted using known atmospheric isotopic argon ratios. See Bradley (1985).

potential evaporation

The amount of water that would be evaporated from a land or water surface if the water supply were unlimited, as opposed to actual evaporation. The latter will fall below the former when the water at the evaporating surface is somehow limited.

potential evapotranspiration

The theoretical maximum amount of water vapor that can be convyed to the atmosphere by the combined processes of evaporation and transpiration by a surface covered by green vegetation with no lack of available water in the soil.

potential density

A physical oceanographic term for the density of a sample calculated from its salinity, potential temperature, and at a selected pressure, i.e. ${\sigma_\theta}\,=\,\sigma (S,{T_\theta},p)$. This is the effective density of a parcel of water after removing the heat associated solely with the effects of compression. Up until about 1970 calculations of potential density values were routinely performed with atmospheric pressure at the sea surface as the selected pressure, but later investigators found it sometimes convenient to instead calculate potential densities at other pressure levels. The 4000 dbar pressure level (abbreviated $\sigma_4$) is probably the next most often used level. Other levels (usually at 1000 dbar increments) are also sometimes used and similarly abbreviated.

potential surface

See geopotential surface.

potential temperature

A physical oceanographic term for the temperature that a water sample gathered at depth would potentially have if brought adiabatically (i.e. without thermal contact with the surrounding water) to the surface, i.e. the effective temperature of a water parcel after removing the heat of the parcel associated solely with compression. A sample brought from depth to the surface will, due to the slight compressibility of sea water, expand and therefore tend to cool, and as such potential temperatures at great depths are always less than measured temperatures.

In meteorology this is defined as a measure of temperature that removes the effects of dry adiabatic temperature changes experienced by air parcels during vertical motion. This can be calculated as

$$\theta\,=\,T\,{{\left({{P_0}\over P}\right)}^{{R_d}/{C_p}}}$$

where $\theta$ is the potential temperature, $P_0$ a reference pressure, $R_d$ the gas constant for dry air and $C_p$ the specific heat.

potential thickness

In physical oceanography, a quantity equal to the local thickness of a water layer divided by the local sine of latitude. See Stommel (1987).

potential vorticity

In the simplest case, this is a quantity equal to

$${{\zeta\,+\,f}\over {D}}$$

in a barotropic fluid (or at least in a fluid layer of constant density within a larger body of fluid) where $\zeta$ is the relative vorticity, $f$ the planetary vorticity, and $D$ the depth. This relation permits predictions to be made about how vorticity will change in a column or parcel of water if it moves northward or southward or into shallower or deeper water, assuming that frictional processes are negligible. More general and complicated versions of this quantity can be defined, but this simplest case well illustrates the essential physical processes without confusing the issue.

To confuse the issue, a general form with wide applicability is Ertel's potential vorticity. It follows from his potential vorticity theorem and, according to Muller (1995), plays the most fundamental role of all potential vorticity theorems because:

• it is most general, i.e. other vorticity and circulation theorems can be derived from Ertel's theorem;
• potential vorticity becomes conserved in a variety of circumstances, thus making the theorem a conservation law; and
• Ertel's potential vorticity equation is the governing equation for an important class of motion where the equation becomes the sole prognostic equation determining the time evolution of the flow, with all other variables expressed in terms of the potential vorticity by diagnostic equations.

The most general form of Ertel's theorem for a materially conserved property $\psi$ is

$${D\over{Dt}}q\,=\,\upsilon J(p,\upsilon ,\psi)$$

where $D/Dt$ is the material derivative, $q$ is Ertel's potential vorticity, $\upsilon$ is the specific volume, $p$ is the pressure, and $J$ the Jacobian given by

$$J(p, \upsilon , \psi )\,=\,(\nabla p \times \nabla \upsilon )\cdot \nabla\psi$$

. The corresponding potential vorticity is

$$q\,=\,\upsilon (\omega\,+\,2\Omega )\cdot \nabla\psi$$

where $\omega$ is the relative vorticity, $\Omega$ is the constant angular velocity of the rotating frame, i.e. the Earth's rotation rate,

Ertel's theorem in the Boussinesq approximation is

$${D\over{Dt}}\left[{1\over{\rho_*}}(\mathbf{\omega}\,+\, 2\mathbf{\Omega})\cdot\nabla\psi\right]\,=\,{1\over{\rho_*^3}} J(\rho ',{p_r},\psi)$$

$\rho_*$ is a constant reference density, $\rho '$ is the deviation of the density from a reference state, and $p_r$ is a reference pressure. The corresponding potential vorticity is

$$q\,=\,{1\over{\rho_*}}(\omega\,+\,2\Omega )\cdot \nabla\psi$$

where the constant density $\rho_*$ can be dropped.

In the f plane approximation, where spherical effects are neglected, the expression for the potential vorticity is:

$$q\,=\,{f_0}\,+\,{\omega_z}\,-\,{f_0} { {\partial\zeta}\over{\partial z} }\,-\,\omega\cdot\nabla\zeta$$

where

See Rhines (1986), Bryan (1987) and Muller (1995).

potentiotropic

Of any fluid whose density is function only of pressure and potential temperature, i.e.

$$\rho\,=\,\rho (p, \theta )$$

where $p$ is the pressure and $\theta$ the potential temperature.

power spectrum

The presentation of the square of the amplitudes of the harmonics of a time series as a function of the frequency of the harmonics.

PPP

1. Abbreviation for principal prediction patterns, used synonymously for EOF. 2. Abbreviation for Pool Permutation Procedure, a method for testing the significance of difference in the means, temporal and spatial variances, and spatial patterns between two data sets. See Preisendorfer and Barnett (1983).

Practical Salinity Scale (PSS-78)

In oceanography, a scale on which the salinity of ocean water is evaluated. It is a unitless scale that was developed to unify two separate salinity determination methods that were previously used for laboratory and in-situ measurements. The results are reported in a unitless manner since it is based on chlorinity ratios rather than measurements of absolute quantities, although the results are mostly consonant with earlier ones reports in units of parts per thousand.

The practical salinity is defined in terms of the ratio of the electrical conductivity of a seawater sample at atmospheric pressure at 15$^\circ$C to that of KCl solution containing 32.4356 g of KCl in a mass of 1 kg of solution at the same pressure and temperature. This ratio $K_{15}$ defines practical salinity of a sample according to

$$S({{^\circ}/{_{\circ\circ}}})\,=\,{a_0}\,+\,{a_1}{K_{15}^{1/2}}\,... ...{K_{15}}\,+\,{a_3}{K_{15}^{3/2}}\,+\,{a_4}{K_{15}^{2}}\,+\, {a_5}{K_{15}^{5/2}}$$

where $a_o$ = 0.0080, $a_1$ = -0.1692, $a_2$ = 25.3851, $a_3$ = 14.0941, $a_4$ = -7.0261, and $a_5$ = 2.7081. This definition suffices for laboratory determination of salinity for samples at the aforementioned pressure and temperature, but corrections must be made for in-situ measurements in water of salinity $S$ and temperature $T$. These are available in the form of additional tables and equations. This equation is valid for a practical salinity $S$ from 2 to 42.

The history and development of PSS-78 is summarized in ():

In 1964, a panel of scientists were appointed jointly by UNESCO, ICES, SCOR and IAPSO to advise on the establishment of international tables and standards. The first task of this Joint Panel of Oceanographic Tables and Standards (JPOTS) was the preparation of tables for computing salinity of seawater from determinations of electrical conductivity. Before these tables could be prepared, it was necessary to redefine salinity in terms of conductivity. After discussions (UNESCO, 1965), the JPOTS recommended (UNESCO, 1966a) a definition of salinity based on determinations of chlorinity and conductivity on samples of natural seawater from all the oceans of the world. In October 1966, the ``International Oceanographic Tables'' (UNESCO, 1966b) for computing salinity from conductivity, based on the above definition, were published. They included a tabulation of this definition at 15$^\circ$C for salinities from 29 to 42 ppt, along with a correction table for measurements at temperatures other than 15$^\circ$C, from 10 to 31$^\circ$C.

However, the use of in situ measurements of conductivity for estimating salinity increased rapidly in the early seventies, rendering the ``International Oceanographic Tables'' unsuited for use in the majority of in situ measurements because the tables do not go below 10$^\circ$C. Furthermore, a comparison to the conductivities of seven batches of standard seawater, relative to a KCl solution revealed that the conductivity of some batches was higher than than calculated from the certified chlorinity (UNESCO, 1976). This raised the problem of the calibration of the conductivity salinometers and CTD probes, as well as the definition of salinity itself. After discussion, the principle of calibrating standard seawater in electrical conductivity with a potassium chloride solution, was adopted and the establishment of a practical salinity scale was recommended by the JPOTS (UNESCO, 1978). Intensive work was then carried out in differnet laboratories with radically different measuring equipment. This resulted in considerable data on which are based the Practical Salinity Scale 1978, as well as the recommended algorithms for the calculation of practical salinity from the conductivity ratio at all temperatures and pressures over the range of oceanographic interest (UNESCO, 1979). This was finally adopted by the JPOTS during its meeting in Sidney, B.C., Canada, 1-5 September 1980 (UNESCO 1981a).

Whereas the previous salinity scale (UNESCO, 1966b) was based on a conductivity-chlorinity relation using natural seawater, the Practical Salinity Scale 1978 is different in that the standard seawater used was diluted by weight with distilled water or evaporated to obtain other salinity values. This procedure was followed to ensure the constancy of composition of this seawater over the salinity range of interest. A precisely specified solution of potassium chloride was chosen as a reproducible electrical conductivity standard; an evaluation was then made of the concentration of this solution which yields a conductivity ratio of unity at 15$^\circ$C with respect to a standard seawater (from the North Atlantic Ocean) whose certified chlorinity was 19.3740 ppt, i.e. its salinity was exactly 35 ppt on the previous salinity scale. By convention, its practical salinity, on the new Practical Salinity Scale 1978, is 35, to ensure continuity at that salinity with the previous scale.

Poisson and Gadhoumi (1993) extended PSS-78, which was 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:

$$S\,=\,35{R_t}\, + \,{R_t}({R_t}\,-\,1)\,\times\,$$

$$({A_0}\, + \,{A_1}{R_t}\,+\,{A_2}t\,+\,{A_3}{R_t^2}\,+\,{A_4}{R_t}t\,+\, {A_5}{t^2}\,+\,{A_6}{R_t^3}\,+\,{A_7}{R_t^2}t\,+\,{A_8}{R_t}{t^2}\,+\, {A_9}{t^3}$$

where ${R_t}$ is the measured conductivity ratio, $t$ the measured temperature, and the coefficients are:

${A_0}$ = 77.37 $\times$ 10$^{-1}$	${A_5}$ = 39.89 $\times$ 10$^{-4}$
${A_1}$ = -98.190 $\times$ 10$^{-1}$	${A_6}$ = -26.25 $\times$ 10$^{-1}$
${A_2}$ = 34.73 $\times$ 10$^{-3}$	${A_7}$ = 48.205 $\times$ 10$^{-3}$
${A_3}$ = 86.635 $\times$ 10$^{-1}$	${A_8}$ = -66.82 $\times$ 10$^{-5}$
${A_4}$ = -10.018 $\times$ 10$^{-2}$	${A_9}$ = -46.56 $\times$ 10$^{-6}$

The coefficients were calculated with eight decimal digits and rounded off to obtain a salinity value that is different from the salinity calculated by the eight decimal digital coefficient by $<2\times{10^{-4}}$. The standard deviation of the difference between the experimentally measured salinities and those calculated using this equation is $3\times{10^{-3}}$. This equation is valid for the temperature range 10-30$^\circ$C and the salinity range 35-50. See Lewis (1980), Lewis and Perkin (1981) Poisson and Gadhoumi (1993) and ().

Practical Temperature Scale

A temperature scale created to provide an operational method for measuring temperatures that is precise and reproducible. See Comite International des Poids et Measures (1969).

Prandtl number

A dimensionless number expressing the ratio of the Peclet number to the Reynolds number. It is expressed by

$$Pr\,=\,{{Pe}\over{Re}}\,=\,{\nu\over\kappa}$$

where $Pe$ is the Peclet number, $Re$ the Reynolds number, $\nu$ the kinematic viscosity, and $\kappa$ the thermal diffusivity. When $Pr$ = 1, the viscous time scale is equal to the time scale of thermal diffusion, and similarity exists between viscous dissipation and thermal diffusion. The Prandtl number is equal to about 0.7 for air, and is about 13 at 0$^\circ$C and 7 at 20$^\circ$C for water. See Kraus and Businger (1994), p. 33.

precession

Also called precession of the equinoxes, this component (the other two being eccentricity and obliquity) of the orbital perturbations that comprise the Milankovitch theory is actually two components. The first is axial precession, where the earth's axis of rotation wobbles likes a spinning top due to the torque of the sun and the planets on the non-spherical earth. Therefore the North Pole describes a circle in space with a period of 26,000 years. The second is elliptical precession in which the ellipse that is the earth's orbit is rotating about one axis. Both effects combined are known as the "precession of the equinoxes" where the equinox (March 20 and September 22) and solstice (June 21 and December 21) shift slowly around the earth's orbit with a period of 22,000 years. The eccentricity modulates and splits the precession frequency into periods of 19,000 and 23,000 years. The precession causes warm winters and cool summers in one hemisphere and the opposite in the other, with the effect being largest at the equator and diminishing towards the poles.

precision

The repeatability of an instrument, measured by the mean deviation of a set of measurements from the average value. Contrast this to accuracy. As an example of the difference, an instrument can measure a quantity a hundred times and if all the measurements are within a percent of each other it is a precise instrument, but if it has measured the correct value as, say, twice the correct value every time then it is not an accurate instrument or, alternatively, it is precisely wrong.

predictability of the first kind

The prediction of sequential states of the climate system at fixed values of external parameters and assigned variations of initial conditions. See Lorenz (1975) and Kagan (1995).

predictability of the second kind

The prediction of an asymptotically equilibrium response (of the limiting state) of the climatic system to prescribed changes in external parameters. See Lorenz (1975) and Kagan (1995).

pressure coordinates

A vertical coordinate system often used in numerical circulation models in which the vertical coordinate is pressure. The equations are created by replacing the vertical velocity in the equations of motion with the total derivative of the pressure following the motion.

Prestwich, Joseph (1812-1896)

See Peterson et al. (1996), p. 96.

prewhitening

A method for dealing with nonstationarity in time series analysis where a new series is creating by forming the differential of the original series. In practice this is done by taking the difference between successive points in the original series, although to be strictly correct this should be done between successive midpoints. This procedure removes both the trend and low-frequency components of the original series while retaining information about the short-variance. Another method for dealing with this problem is detrending. See Burroughs (1992).

primary productivity/production

The amount of organic material produced by organisms from inorganic material or, more technically, the amount of carbon fixed by autotrophic organisms through the synthesis of organic matter from inorganic compounds such as carbon dioxide and water using energy derived from solar radiation or chemical reductions. This should be compared with:

• gross primary production, the total amount of organic matter produced by photosynthesis or chemosynthesis per unit time; and
• net primary production, the amount that remains after plants metabolize and respire.

Most of the primary production in the oceans is due to photosynthesis of phytoplanktonic algae. in the upper 100 m, i.e. the euphotic zone. Primary production is expressed in units of gC m$^{-2}$ yr$^{-1}$ where gC is grams of carbon. The total ocean productivity ranges from 75-150 gC m$^{-2}$ yr$^{-1}$, with photosynthesis accounting for around 95% of this and chemosynthesis the rest. See Fogg (1975) and Woodwell (1995).

PRIMER

An experiment taking place during the summer of 1996 and the winter/early spring of 1997 that focused on the shelfbreak front just south of Cape Code on Nantucket Shoals. See Pickart et al. (1999).

primitive equations

A set of filtered equations obtained from the fundamental equations of motion of a fluid by applying the hydrostatic approximation and neglecting the viscosity. They comprise three prognostic and three diagnostic equations, the former of which are the x and y (or horizontal) components of the momentum equation and the thermodynamic equation of energy, and the latter the continuity equation, the hydrostatic equation and the equation of state. These equations form a closed set in the dependent variables which are the three components of velocity, pressure, density and temperature. The PEs filter out vertically propagating sound waves.

primum mobile

A theory of literally a ``first mover'' expostulated by Aristotle that was used to explain a perceived general broad pattern of westward flow in the world oceans. The first mover, a theological being, was itself unmoved but acted on the circumference of the universe to cause it to move. The theory asserted that the shape of heaven is spherical and it encloses successively smaller spheres down to the center, i.e. earth, with the motion of the outermost sphere being uniform and that of the inner spheres increasingly irregular as the center was approached. Since the sun and stars appeared to move to the west and they were in an outer sphere, the first mover must be moving things in that direction and therefore the motion of the seas should be generally to the west, although more irregular. This theory and the consequent belief in general westward motions in the seas held sway for many centuries until the weight of observational evidence made it untenable. See Peterson et al. (1996).

Prince Henry the Navigator (1394-1460)

The third surviving son of Portugal's King John I who, to help attain his goals of conquest and the conversion of pagans to Christianity, founded what some have called the first modern school of oceanography in the town of Sagres. He summoned seamen, cartographers, astronomers, shipbuilders and instrument makers from all over Europe to engage in activities that would provide a large part of the foundation for the European exploration of the world. His efforts earned him the surname the Navigator.

Significant advances initiated by the school included the systematic keeping of logbooks and annotation of charts, replacing the astrolabe with the quadrant, and the development and construction of the Portugeuse caravel as a durable ship for long voyages of exploration. Although Henry (who never participated in any significant voyages himself) did manage to convince someone (Gil Eannes) to sale beyond the Cape of Bojador (on the western Sahara coast southeast of the Canary Islands) in 1433, his men did not cross the equator in his lifetime. See Peterson et al. (1996).

Princess Elizabeth Trough

A gap in the topography between the Kerguelan Plateau and the Antarctic continent, with a sill depth of 3750 m. It provides a route for the exchange of Antarctic Bottom Water (AABW) between the Australian-Antarctic Basin and the Weddell-Enderby Basin. See Heywood et al. (1999).

PRIRODA

A Russian remote sensing module (named for the Russian word for nature) planned to provide the experimental basis for a scientific research program for the development and verification of remote sensing methods and investigations of regional and global problems in climatology, oceanography, and ecology. The module carries optical and infrared scanners, an imaging spectrometer, a LIDAR, scanning and pointing microwave radiometers, SAR, and high resolution digital (stereo) cameras. The launch data for PRIRODA is March 1996 and the operation is expected to cover 1996-97. This mission is conducted by the Russian Space Agency (RKA). See the PRIRODA Web site.

PRISM

The Pliocene Research, Interpretation, and Synoptic Mapping Project, the goals of which include providing modelers with improved quantitative global paleoenvironmental information associated with the warm climates of the Pliocene and providing a forum for data and modeling experts to collaborate in establishing what boundary conditions are needed, planning model experiments, and interpreting and evaluating model results. See Dowsett et al. (1994) and the PRISM Web site.

PROBES

Acronym for Processes and Resources of the Bering Sea Shelf.

PROFILE

Acronym for Processes in Regions of Fresh Water Influence, a project whose overall aim is to develop process understanding and tested numerical models for regions of freshwater influence (i.e. ROFIs). This EC MAST project studies the role of the physical processes controlling water property distributions and the role of suspended sediments in controlling the availability of light, nutrients, and phytoplankton growth. See the PROFILE Web site.

prognostic

In numerical modeling, an equation is prognostic if the future value of a dependent variable is predicted from the present value(s) of one or more dependent variables.

Project FAMOUS

The French American Mid-Ocean Undersea Study began in 1971 as part of the IDOE. It was based on the general acceptance of the plate tectonic theory and investigated the possibility of underwater hot springs. The site selected was along the Mid-Atlantic Ridge near the Azores, and was selected after detection of particularly strong magnetic stripes in that region. During FAMOUS over 100,000 photographs were taken and nearly 1,300 kg of geological samples were collected by Alvin and two French submersibles. Water samples were also collected and a data logger was developed to automatically record depth, altitude, heading and time.

[http://www.nap.edu/books/0309063981/html/index.html]

PROMISE

Acronym for Pre-Operational Modeling in the Seas of Europe, a project whose primary objective is to optimize the application of existing dynamical models of the North Sea such that the rates and scales of sediment exchange between the coast and the nearshore zone can be quantified for management applications. See the PROMISE Web site.

PROTEUS

Acronym for Profile Telemetry of Upper Ocean Currents, a NOAA PMEL project to develop a real-time capability for satellite transmission of ADCP data from deep water surface moorings. The first PROTEUS mooring was successfully deployed in April 1990 at 0$^\circ$, 140$^\circ$ W as part of the EPOCS program. See McPhaden et al. (1990).

Joseph Proudman (1888-1975)

See Cartwright and Ursell (1976).

Proudman-Taylor theorem

See Taylor-Proudman theorem.

PROVESS

Acronym for PROcesses of Vertical Exchange in Shelf Seas, a joint European funded project for an interdisciplinary study of the vertical fluxes of through the water column and the surface and bottom boundaries based on the integrated application of new measuring techniques, new advances in turbulence theory, and new models.

[http://www.pol.ac.uk/provess/index.html]

proxy data

Paleoclimate data inferred indirectly via the use of transfer functions. The underlying idea is that organisms exhibit a high degree of differentiation according to their physical environment, and that physical variables can be estimated from biotic distributions once the degree of relationship has been objectively established. For example, some present plankton species live in cold waters and others prefer warmer waters. If we make the additional assumption that fossil assemblages of these species (or their related ancestors) exhibited similar temperature tendencies, then we can infer, within limits, the temperature of the water in which they existed. See Crowley and North (1991), Appendix B. Compare to instrumental data.

Prydz Bay

The third largest embayment in the Antarctic continent. Prydz Bay lies in the Indian Ocean sector, with typical depths of 500-600 m. A deep basin in the inner part of the bay descends to 800 m in places, and to 1085 m adjacent to the Amery Ice Shelf. Moving offshore from within the Bay, the bed rises from approximately 800 m to a sill at 400-500 m along 67-68$^\circ$S, and then falls to 699 m at the edge of the continental slope some 100 km seaward. The sill has a saddle point with 500 m depth at about 67$^\circ$S, 71$^\circ$30'E, where the bathymetry rises zonally and falls meridionally. Relatively shallow banks (Frame Bank to the west and Four Ladies Bank to the east) rise to 200 m depth and extend over much of the shelf width to either side of Prydz Bay, so deep connections between the ocean and the Bay are concentrated through a broad region at the central longitude of about 73$^\circ$E. A much narrow deep connection to the shelf waters of the West Ice Shelf region east of the Bay is found along a trough in the inner part of the shelf north of Davis station. The waters of the southwestern part of the Bay come into direct contact with a large floating ice shelf, the Amery Ice Shelf.

The large-scale circulation is dominated by a large cyclonic gyre centered on the mid- to western part and extending from within the Bay to the Antarctic Divergence northwest of the Bay (at about 65$^\circ$S). The gyre is associated with a relatively narrow coastal current running from the southern limits of the Bay past the Amery Ice Shelf, and continuing westward after leaving the Bay along the MacRobertson Land Shelf (with velocities reaching 1 m/s in the latter part). Part of the current flows offshore near 63$^\circ$E, while another unknown fraction continues westward. See Nunes Vaz (1996).

Prydz Bay Bottom Water (PBBW)

A type of regionally modified Circumpolar Deep Water (CDW) which, along with several other types of regionally modified and previously defined types of CDW, is now usually defined as one of several regional types of a more general water mass called Modified Circumpolar Deep Water. See Middleton and Humphries (1989) and Whitworth et al. (1998).

pseudoenergy

See Andrews and McIntyre (1978).

pseudomomentum

See Andrews and McIntyre (1978).

pseudospectral method

In numerical modeling, an approximation which uses interpolating functions to estimate derivatives of fields represented on a grid in physical space. It is so-called because the interpolating functions used are usually the same as are used in the spectral method. All operations other than differentiation are carried out in the physical space defined by the grid rather than in spectral space. This allows, for example, the calculation of the nonlinear terms, a dauntingly onerous task in spectral space, to be easily performed. The trade-off is that the calculations are aliased, although various remedies for the problem have been proposed. See Gottlieb et al. (1984).

P17N

A WOCE hydrographic program in the North Pacific.

[http://blackburn.ims.uaf.edu:8000/~musgrave/P17N/]

PSMSL

Abbreviation for Permanent Service for Mean Sea Level, an archive based at the Proudman Oceanographic Laboratory which contains monthly and annual mean sea level information from over 1600 tide gauge stations from around the world.

[http://www.pol.ac.uk/psmsl/]

PSS-78

See Practical Salinity Scale.

psychrometrics

The study of the physical and thermodynamic properties of the atmosphere. The properties mainly of concern are dry-bulb temperature, wet-bulb temperature, dew-point temperature, absolute humidity, relative (or percent) humidity, sensible heat, latent heat, enthalpy (or total heat), density and pressure.

psychrosphere

One of two regions into which the ocean depths are sometimes divided according to temperature, the other being the thermosphere. The psychrosphere is those ocean depths where the temperature is less than 10$^\circ$ C, which can range anywhere from 100 to 700 m beneath the surface depending on oceanic conditions. This coincides with the ocean stratosphere.

pteropod ooze

Ooze composed of the shells of small, planktonic swimming molluscs with a calcareous shell that live in tropical and subtropical waters. These are coarser than globigerina oozes, are found between 1500-3000 m depth and cover no more than 1% of the sea floor. See Tchernia (1980).

Purdy, John (1773-1843)

See Peterson et al. (1996), p. 59.

PW

Abbreviation for Polar Water.

pycnocline

In physical oceanography, a layer where density changes most rapidly with depth. It can be associated with either a thermocline or a halocline.

pycnostad

In physical oceanography, a layer where the vertical change of density is very small and displays a local minimum.