On-Oz

OOPC

Acronym for Ocean Observations Panel for Climate, a panel succeeding and formed from the OOSDP. It is sponsored jointly with WCRP, GOOS, and GCOS. The OOPC continues the design of the ocean observing system begun by OOSDP, in addition to addressing:

• areas not considered by the OOSDP;
• prioritizing recommended elements;
• considering alternate sampling strategies; and
• assisting in making inventories of available data and products needed for the GCOS/GOOS common module.

[http://www.bom.gov.au/bmrc/ocean/OOPC/]

OOSDP

Acronym for Ocean Observing System Development Panel, established in 1990 by the CCCO of SCOR/IOC and the Joint Scientific Committee (JSC) of the ICSU/WMO. The panel's task was to formulate a conceptual design of a long-term, systematic observing system to monitor, describe, and understand the physical and biogeochemical processes that determine ocean circulation and the effects of the ocean on seasonal to decadal climate changes and to provide the observations needed for climate predictions.

The panel published seven background reports and a final report in 1995 before reforming as the OOPC. The reports were:

• The Role of Models in an Ocean Observing System - Neville Smith
• Scientific Rationale for Recommending Long-Term, Systematic Ocean Observations to Monitor the Uptake of CO$_2$ by the Ocean - Now and in the Future - Liliane Merlivat and Alain Vezina
• Surface Conditions and Air-Sea Fluxes - Robert Weller and Peter K. Taylor
• The Ocean Freshwater Cycle - Raymond Schmitt
• Monitoring Global Ocean Carbon Inventories - Douglas Wallace
• The Role of the Indian Ocean in the Global Climate System: Recommendations Regarding the Global Ocean Observing System - J. S. Godfrey et al.
• Sea Ice in the Global Climate System: Requirements for an Ocean Observing System - Ian Allison and Richard E. Moritz
• Scientific Design for the Common Module of the Global Ocean Observing System and the Global Climate Observing System: An Ocean Observing System for Climate - OODSP Final Report

The final report is available online.

[http://www-ocean.tamu.edu/OOSDP/oosdp.html]

OPA

Acronym for Océan PArallélisé, an ocean general circulation model developed by the ECUME team at the Laboratoire d'Océanographie Dynamique et de Climatologie (LODYC). It is a primitive equation model applied to both regional and global ocean circulation. It can be interfaced with several sea ice models, a passive and biogeochemical tracer model, and several AGCMs. It also has adjoint and tangent linear models.

[http://www.lodyc.jussieu.fr/opa/]

OPC

Abbreviation for Optical Plankton Counter. See Herman (1992).

open boundary conditions

See Palma and Matano (1998).

OPERA

Acronym for Observatoire Permanent de l'Atlantique Tropical.

Operation Cabot

A multiple-ship cruise to survey the Gulf Stream in 1950. See Fuglister and Worthington (1951) and Stommel (1966).

operational oceanography

The activity of routinely making, disseminating, and interpreting measurements of the seas and oceans so as to provide continuous forecasts of the future condition of the sea for as far ahead as possible, provide the most usefully accurate description of the present state of the sea including living resources, and assemble climatic long term datasets to provide data for the description of past states as well as time series showing trends and changes. This usually proceeds via the rapid transmission of observational data to computerized data assembly centers. There they are processed through numerical models to generate products for various applications.

operator splitting

In numerical ocean circulation modeling this is a technique for splitting the fast and slow dynamics into separate subproblems. When the partial differential equations governing large-scale ocean dynamics are discretized to achieve numerical solutions, dynamical phenomena with many temporal and spatial scales are usually included in the discretized equations. Most prominent in discretizations of the primitive equations are external and internal gravity waves, where the characteristic wave speeds are, respectively, 200 m/s and 1-2 m/s.

The size of the discrete time step used to integrate the equations is limited by the fastest motion that has to be resolved, in this case the external gravity waves. One way to get around this limitation is to separate the fast and slow motions into separate subproblems. The fast external motions are essentially 2-D due to approximate independence from depth, which leads to the common option of obtaining the 2-D velocity field from a vertical average of the horizontal velocity field in the original 3-D equations.

This procedure can give rise to computational instabilities since the operator splitting method is inexact except for the case of a linearized flow with a horizontal bottom and a rigid lid. In this case one solution is exactly independent of depth and the horizontal velocity field obtained corresponds exactly to the vertically averaged velocity. However, if any of the restrictions are relaxed the fast and slow motions can be mixed by variable bottom topography or nonlinearities and can result in numerical instabilities if an explicit method with a long time step is used to advance the slow motion component in time. See Higdon and Bennett (1996).

optical depth

See optical thickness.

optical oceanography

See Mobley (1994) and Reynolds and Lutz (2001).

optical thickness

A measure of the attenuation of solar radiation by the atmosphere that allows the convenience of considering as a single unit the losses due to scattering and absorption processes. The greater the thickness, the greater the attenuation of incoming solar radiation. This is also referred to as the optical depth.

Optimum Multiparameter Analysis (OMP)

A tool to analyze the water mass mixture in a water sample by calculating the contributions from the original water masses (the source water masses) to the sample. OMP is based on a simple linear mixing model that assumes all water mass properties undergo the same mixing processes, i.e. their mixing coefficients are identical. This allows their distribution is space to be determined via a linear set of mixing equations.

Mathematically speaking, OMP is an inversion of an overdetermined system performed individually for each observation point. The source water type contributions for each data point are obtained by finding the best linear mixing combination in parameter space defined by temperature, salinity, oxygen, nutrients and other water mass properties that minimizes the resisuals in a non-negative least squares sense.

[http://www.ifm.uni-hamburg.de/~karstens/omp_std/]

OPTOMA

Acronym for Ocean Prediction Through Observation, Modeling, and Analysis, a program that consisted of a lengthy set of surveys of the eddy field in the California Current. See Rienecker et al. (1985).

OPUS

Acronym for Organization of Persistent Upwelling Structures, a program taking place in 1983 that studied the inner part of a filament near Point Conception, California. See Atkinson et al. (1986).

OPYC

Abbreviation for an isopycnic ocean circulation model developed and used at the DKRZ. See Oberhuber (1993).

ORCA

Acronym for Oceanographic Remotely Controlled Automaton, a diesel powered semi-submersible designed to survey water depths from 10 to 300 meters. This instrument was designed for the cost effective collection of hydrographic and oceanographic data. See the ORCA Web site.

organic matter pump

Then name given to the cycle of organic matter and nutrients in the ocean. Since photosynthesis exceeds respiration only in the euphotic zone, there is a net sink of CO2, phosphate and nitrate in the euphotic zone and a net source in the aphotic zone. Thus a downward flux of organic matter and an upward flux of nutrients connects the euphotic zone sink and the aphotic zone source of nutrients. This has also been called the soft tissue pump, but the present name was suggested in recognition of the possible role of dissolved organic species in the transport cycle. See Najjar (1991).

orthobaric density

A density variable empirically corrected for pressure. It is constructed by first fitting compressibility (or sound speed) computed from global ocean datasets to an empirical function of pressure and in situ density (or specific volume). An exact Pfaffian differential form is then obtained by replacing true compressibility by the best-fit virtual compressibility in the thermodynamic density equation. The orthobaric density has advantages as a vertical coordinate variable for both descriptive and modeling purposes. The advantages of this over the potential density include:

• a geostrophic streamfunction exists for the momentum equations transformed to orthobaric density coordinates such that the gradients of orthobaric density surfaces give precisely the geostrophic shear;
• a form of Ertel's potential vorticity can be defined whose evolution equation contains no contribution from the baroclinity vector; and
• orthobaric density surfaces are invariant to the choice of reference pressure.

See de Szoeke (2000).

O-SCOPE

Acronym for Oceanographic Systems for Chemical, Optical, and Physical Experiments, a program addressing the need for next generation autonomous near real-time long-term time series measurements in critical regions of the world oceans. The goal is to systematically obtain high resolution, long-term, interdisciplinary oceanic data by improving the variety, quantity, quality and cost-effectiveness of observations using a network of strategically placed moorings. O-SCOPE capitalizes on several recent technological advances (e.g. pCO$_2$, pH and TAlk sensors, nitrate analyzers, spectral optical sensors, and data telemetry) to meet this goal.

The scientific goals of O-SCOPE are:

• quantification of trends in biogeochemical and bio-optical variables which can be caused by major changes in wind-driven and thermohaline circulation and seasonal, interannual, and decadal changes in upper ocean biogeochemical and bio-optical variability and carbon fluxes which affect global climate; and
• monitoring trends in ``ocean health'' in the form of chemical, biological, and optical indicators.

[http://www.pmel.noaa.gov/oscope/]

OSCR

Abbreviation for Ocean Surface Current Radar, a measuring system that uses high frequency radio pulses to probe the ocean surface to deduce near-surface currents. The shore-based system consists of two units which are deployed several kilometers apart. Each unit makes independent measurements of current speed along radials emanating from its phased-array antennae system. The data are then combined via UHF or telephone communication to produce accurate vector currents, store them to disk, and display them in near real time. Measurements can be made simultaneously at up to 700 grid points at either 1 km or 250 m resolution. The OSCR samples for about 10 minutes and then processes radar returns for about 10 minutes to create a quasi-synoptic surface current map every 20 minutes. See the OSCR Web site.

OSLR

Abbreviation for Intergovernmental Committee for Ocean Science and Living Resources, an IOC committee.

OTEC

Acronym for Ocean Thermal Energy Conversion, the use of the temperature difference between surface and deep sea water to generate electric power. This is done by taking a working fluid with a low boiling point, turning it to vapor by heating it up or depressurizing it, and then using the pressure of the expanding vapor to turn a turbine. The liquid used may be either the sea water or ammonia. The OTEC is called open-cycle if the ocean water itself functions as the refrigerant, transferring heat energy by changing between the liquid and gaseous phases. It is called closed-cycle if ammonia is used.

The idea was first propounded by a French engineer named Jacques D'Arsonval in 1881, although it was a student of his named Georges Claude who first tested the idea. Claude used warm seawater to create a low pressure vacuum system, i.e. an open-cycle system. D'Arsonval's original idea was to use another fluid such as ammonia, i.e. a closed-cycle system. At present the only operating OTEC plant is at the Natural Energy Laboratory of Hawaii.

OTH

Abbreviation for over-the-horizon radar, a type of radar originally developed to detect military targets far beyond the optical horizon. Radio waves in the 5 to 28 MHz range are reflected from the ionosphere and reach up to 3500 km in one hop. Properties of the ocean surface are extracted from the energy backscattered from the ocean surface. Properties that can be measured include surface wind direction, radial surface currents, sea state, surface wind speed, and more. See the OTH Web site.

OTRANTO

A research project whose goal is to improve the knowledge about the hydrodynamics of the Straits of Otranto and the evaluate the water and particulate fluxes across this strait at synoptic, seasonal, and interannual time scales. This MTP Core Project took place from Dec. 1993 to May 1996. See the OTRANTO Web site.

otter trawl

A device used in biological oceanography to trawl for pelagic organisms. As opposed to the beam trawl, the opening to this is kept open not by a rigid rectangular frame but rather by otter boards attached to either side of the net opening. These boards are forced apart by the force of the water when the trawl is towed and close when it is not being towed, an eventuality convenient for retaining the organisms caught. The open may be 20 to 26 m wide and the net up to 40 m in length. See Sverdrup et al. (1942).

outer sublittoral zone

See circalittoral zone.

overall Richardson number

A dimensionless number expressing the ratio of the removal of energy by buoyancy forces to its production by the shear in a flow. It is expressed by

$${Ri_0}\,=\,g'L/{U^2}$$

where $g'$ is the reduced gravity and $L$ and $U$ are, respectively, length and velocity scales imposed by the boundary conditions of the problem. The name comes from the fact that this is an overall parameter describing a whole flow as opposed to the gradient and flux Richardson numbers. See Turner (1973).

OVERFLOW '60

An ICES investigation of cold, sub-arctic, deep water overspill into the North Atlantic Ocean from the Iceland-Faroe Ridge. It was carried out from May 30 to June 18 in 1960 under the leadership of J. B. Tait and was an optimal coordination of 9 research ships in a small region to reach a maximum of synoptic work. This experiment is considered to be the starting point of current measurements in deep water by self-recording anchored instruments.

[http://www.ices.dk/ocean/project/data/ov60.htm]

OVERFLOW '73

An ICES expedition whose core period of observation was between August 15 and September 15 in 1973. The principal objective of this experiment was to describe in detail the kinematic and dynamical processes which lead to the renewal of the sub-Arctic bottom water of the northern North Atlantic across the Iceland-Faeroe Ridge and the Denmark Strait. This experiment, a follow-up to OVERFLOW '60, consisted of over 1700 hydrographic stations and 52 current meter moorings as well as numerous XBTs, tide measurements and drogue tracking. The chief scientist was J. Meincke of the University of Hamburg.

[http://www.ices.dk/ocean/project/data/ov73.htm]

overmixing

A condition that can exist in strongly stratified estuaries with net circulation out in the upper layer and net circulation in in the lower layer. This limits the amount of salt water available for mixing inside the estuary. This condition begins as mixing proceeds within the estuary by whatever processes are dominant. The mixing causes more salt water to be added to the net circulation and volume flow out of the estuary up to a critical condition past which any more increased mixing has no further effect on the discharge flow or the exiting salinity. See Officer (1976).

overturning potential energy

In a stratified ocean, the locally averaged change in potential energy produced by vertically rearranging the water column to achieve static stability. See McDougall et al. (1987).

OVIDE

A tomography experiment planned by IFREMER to study the variability in the subpolar gyre from seasonal to decadal time scales. The goals are to document the transformation of the subpolar mode water and the amplitude of the thermohaline circulation. OVIDE is planned to start in 2002 and includes hydrography, profiling floats, and tomography. It will be based on four tomography moorings that will be installed in the Western European Basin for monitoring the heat content varability of the waters entering and leaving the basin.

oxycline

A layer of maximum downward decrease in oxyty.

oxygen

See Richards (1957).

oxygen isotope analysis

The use of stable oxygen isotopes to extract paleoclimatic information from ice cores. The theoretical basis Bradley (1985) of the method is that two paleoclimatically important heavy isotopes (one containing deuterium and the other 18O) have vapor pressures lower than that of pure H20. Thus, evaporation leads to water vapor depleted in deuterium and 18O as well as a water body enriched in the same. Further, condensation of the vapor preferentially removes even more of these heavy isotopes, leaving the vapor even more depleted in deuterium and 18O. Therefore, to a first approximation isotopic concentration in the condensate can be considered as a function of the temperature at which the condensation occurred, although other considerations come into play.

A major use of this method is to gauge the waxing and waning of glacial periods since the deposition of large amounts of water on land in the form of glaciers leaves the water enriched in and the water depleted of the heavy isotopes.

oxygen isotope ratio

The ratio of oxygen-18 to oxygen-16, used as an indicator of paleotemperatures since it is related to ocean temperature.

oxystad

A layer of relatively small vertical change in oxyty as proposed in Seitz (1967).

oxyty

The concentration of dissolved oxygen. This was proposed as an analogy to salinity in Montgomery (1969). For a line of uniform oxyty, both isoxygen and isooxygen have been proposed, with isooxygenic as the adjectival form.

Oyashio Current

The western boundary current of the subpolar gyre in the North Pacific Ocean. Divergence in the center of this gyre causes the Oyashio to carry cold water rich in upwelled nutrients and full of marine life - hence the meaning of the name as ``parent current''. The Oyashio is formed by the confluence of the Alaskan Stream and the Kamchatka Current west of the Kamchatka Peninsula at about 55$^\circ$ N. It flows southward and splits into two paths called the First and Second Oyashio Intrusion just south of Hokkaido, after which the First Intrusion proceeds southward along mainland Japan (Honshu) where it turns west at about 38$^\circ$ N to rejoin the First Intrusion, which has proceeded more or less directly south from the splitting point. They merge at about 39$^\circ$ N and 145$^\circ$ E where the southern boundary of the Oyashio defines the Polar Front. This boundary and the northern edge of the Kuroshio maintain their identities at least through the Kuroshio Extension, although it is not well known how much further east they continue to be distinguishable and distinct from the broader eastward flow of the North Pacific Current. Thus the Oyashio forms the western and part of the southern limb of the North Pacific subpolar gyre. See Tomczak and Godfrey (1994).

Oyashio Front

A front delimiting the southern limit of subpolar waters in the North Pacific. According to Talley et al. (1995):

The Oyashio Front is defined for our purposes as the southern limit of waters that we characterize as ``subpolar'' based on their temperature-salinity relation, and which are often referred to as ``subpolar water.'' The Oyashio commonly meanders twice (the ``first and second intrusions'') after leaving the coast of Hokkaido. The meanders are separated by a warm core feature shown to originate from northward movement of warm core rings produced by the Kuroshio, possibly with interaction from westward propagating offshore warm core rings, and with considerable modification due to winter cooling and mixing with surrounding water. The warm core separating the Oyashio ``intrusions'' is not necessarily always a closed ring. The Oyashio is fairly barotropic, with little vertical shear, and has apparently more transport than can be inferred from a shallow reference level calculation. The Oyashio Front, if defined as a water mass boundary, continues eastward across the Pacific along 40-42$^\circ$N as the Subarctic Front, forming the boundary between the subarctic and modified subtropical water masses. Much farther east, past the date line, the surface Subarctic Front and the deeper front that forms the northern boundary of the subtropical water become separated, with the surface front found farther south.

See Talley et al. (1995).

Ozmidov scale

An important length scale in stratified flow representing the vertical length scale at which the buoyancy force is of the same order of magnitude as the inertial forces. This is expressed as:

$${L_O}\,=\,\sqrt{\epsilon/{N^3}}$$

where $\epsilon$ is the kinetic energy dissipation rate and $N$ the buoyancy frequency. The Ozmidov scale is the largest that can overturn, i.e. buoyancy has only a minor effect at smaller scales but dominates at larger ones. Overturning can occur at scales greater than $L_O$ if internal waves are present, however, with the buoyancy scale used instead of the Ozmidov scale if vertical velocity fluctuations due to internal waves are small compared to those due to turbulence. This scale varies from a few cm in the thermocline to several hundred meters in weakly stratified and/or highly energetic flows. See McDougall et al. (1987).