On-Oz

 

ontogeny

In biology, the origin and development of the individual organism, as opposed to phylogeny. See also Biogenetic Law.

 

Oort Cloud

A spherical zone surrounding and bound to the solar system between 20,000 and 100,000 AU from the Sun. It is hypothesized to contain 10**12 comets whose orbits, perturbed by passing stars or molecular clouds, can occasionally reach to within Jupiter's orbit. The existence of the Oort Cloud is a key element of the Nemesis theory.

 

Operation Cabot

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

 

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).

 

opposition

In tidal mechanics, that instant when the Earth-centered longitude of the Moon differs from that of the Sun by 180 degrees. This is also the time of the full moon and of one of two monthly spring tides. See also conjunction.

 

optical depth

See optical thickness.

 

optical oceanography

More later.

 

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.

 

OPYC

Abbreviation for an isopycnic ocean circulation model developed and used at the DKRZ.

 

orbital elements

The six pieces of mathematical data needed to completely determine a planet's orbit and its position in that orbit. These data are: (1) longitude of the ascending node, (2) inclination of the orbit, (3) longitude of perihelion, (4) semi-axis major, (5) eccentricity, (6) epoch, or date of planet's passing perihelion. Analogous elements are used for satellites.

 

Ordovician

The second period of the Paleozoic era, lasting from 505 to 438 Ma. It precedes the Silurian period and follows the Cambrian period, and is comprised of the Early (505-478 Ma), Middle (478-458 Ma), and Late (458-438 Ma) epochs. Named after the Ordovices, an ancient Celtic tribe in Central Wales. Widespread volcanic activity and the vast mountain-building movement known as the Caledonian Orogeny took place during this era.

 

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).

 

ORNL

Abbreviation for Oak Ridge National Laboratory.

 

orogeny

Mountain building when a belt of the Earth's crust is compressed by lateral forces to form a chain of mountains.

 

OTTER

Acronym for the Oregon Transect Ecosystem Research Project, a study of a climatic and elevational gradient extending from the Pacific coast 200 kilometers to the east. See the OTTER 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).

 

OTU

Abbreviation for operational taxonomic units. See numerical taxonomy.

 

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

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).

 

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).

 

Oxfordian

The first of three ages in the Late Jurassic epoch, lasting from 163 to 156 Ma. It is preceded by the Callovian age of the Middle Jurassic epoch and followed by the Kimmeridgian age.

 

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.

 

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 deg. 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 deg. N to rejoin the First Intrusion, which has proceeded more or less directly south from the splitting point. They merge at about 39 deg. N and 145 deg. 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).

 

ozone

An allotropic form of oxygen, a bluish toxic gas, O , with a characteristic sharp odor, produced from molecular oxygen by electrical discharge and in the upper atmosphere by ultraviolet light. Ozone is climatically significant for this ability to absorb ultraviolet light in the ozonosphere, which is being reduced by its being broken down by anthropogenic chlorofluorocarbon in the atmosphere.

 

ozonosphere

The region of the upper atmosphere, at an altitude of 10 to 50 km (6 to 30 miles), containing significant amounts of ozone, which aborbs short ultraviolet light. The region of maximum ozone concentration is between 20 and 25 km (12 and 15 miles).