Bn-Bz

BNL

Abbreviation for Benthic Nepheloid Layer, the thickest and upper of three layers into which the bottom 1000 m of the ocean are sometimes divided, with the other two being the BML and BEL. The BNL is characterized by an increasing concentration of suspended material towards the bottom, and it extends from the clear water minimum (CWM) (at around 1000 meters above the bottom) down to the deep-sea bottom. See Klein and Mittelstaedt (1992).

BOFS

Abbreviation for Biogeochemical Ocean Flux Study, a U.K. contribution to JGOFS funded by the NERC. The goal of GOFS was to study differences in glacial-interglacial paleoenvironments of the eastern Atlantic Ocean, especially between the last glacial and the Holocene. The results are presented in a special issue of Paleoceanography (Vol. 10, No. 3, 1995). See especially the short review by Elderfield and Thomas (1995).

[http://www.bodc.ac.uk/projects/bofs.html]

Bohai Gulf

See Bohai Sea

Bohai Sea

See Guan (1994).

Bohnecke mechanical recording current meter

A mechanical current meter, first designed and used in the 1930s, in which the propellor and the compass both drive a set of horizontal dials with raised numbers on their vertical rims. A clockwork mechanism moves a strip of tin foil past the vertical rims of the dials and a hammer presses the the foil against the raised numbers on the rims every 5 or 10 minutes. The speed and direction can be obtained from the information on the foil. Wide use of this mechanism was forestalled by the difficulty in finding a material for the spring in the clockwork that could withstand the corrosive exposure to sea water. See Sverdrup et al. (1942).

Bohol Sea

A small sea centered in the Philippines at about 124$^\circ$ E and 9$^\circ$ S. It is surrounded by the islands of Mindanao to the southeast and Negros, Bohol and Leyte to the northwest. It is connected to the Sulu Sea to the west via a passage between Negros and Mindanao, the Visayan Sea to the north via the Tanon Strait, the Camotes Sea to the north via the Bohol Strait and a passage between Bohol and Leyte, and to the Leyte Gulf to the northeast via the Surigao Strait. Prominent geographic features include the islands of Siquijor and Camiguin and Sogod (in Leyte), Gingoog, Macajalar and Iligan (in Mindanao) Bays. This has also been called the Mindanao Sea.

Boltzman equation

The Navier-Stokes equations can be derived from the Boltzmann equation by considering appropriate limits, i.e. Knudsen and Mach numbers tending to zero, and appropriate averaging procedures to define new coarse-grained variables (velocity and pressure) and associated transport coefficients (viscosity and density). See Farge et al. (1996).

bolus velocity

See eddy-induced transport velocity.

BOMEX

Acronym for Barbados Oceanography Meteorology EXperiment, a joint experiment between NOAA and the Government of Barbados conducted over the tropical Atlantic east of Barbados in the summer of 1969. See Holland (1970), Pond et al. (1971) and Geernaert (1990).

[http://rainbow.ldeo.columbia.edu/data/NASAentries/nasa611.html]

BOOS

Acronym for Baltic Operational Oceanographic System.

BOP

Abbreviation for Bayesian oscillation patterns, patterns found using BSA. See Ruiz de Elvira and Bevia (1994).

BOPS

Acronym for Bio-Optical Profiling System, an instrument for measuring optical and physical parameters in the water column.

Boreal period

A post-LGM European climate regime. This refers to the period from about 7000-6000 BC when temperatures continued to rise, e.g. the colder seasons of the year gradually became milder (although probably with some dry and frosty winters) and the summers became generally warmer than today. It was preceded by the Pre-Boreal period and followed by the Atlantic period. See Lamb (1985), p. 372.

Bothnian Sea

See Gulf of Bothnia.

bottom boundary layer models

Killworth and Edwards (1999) review the use of these in numerical ocean models and present another model.

Bottom Cold Water (BCW)

See Isobe (1995).

Bourne, William (?1535-1582)

A British innkeeper who wrote what some consider to be the first popularization of the extent ideas constituting that which we now consider to be the field of oceanography. In this book, entitled A booke called the Treasure for Traveilers, deuided into fiue Bookes and published in 1578, he invoked the primum mobilus concept as the driving force beyond the movement of the moon, but also ascribed to the moon itself some unknown power responsible for the observed tides and steady currents (whose complexity beyond the steady westward flow of the primum mobilus he also deemed partially due to the distrupting presence of land masses).

Bourne's picture of the flow in the Atlantic started with the general westward flow around the southern tip of Africa merging with that in the Atlantic, with the combined volume being too great to squeeze through the Straits of Magellan. Thus part of the flow was diverted northwards along the South American coast, into the Gulf of Mexico, and then out between Florida and Cuba and eastwards towards Europe. He also proposed a second type of steady, non-tidal current that flows against the wind during periods of strong winds, with the driving force being a hypothesized upward tilt of the sea surface downwind caused by waves piling up water there. See Peterson et al. (1996).

Boussinesq approximation

A set of filtering approximations originally developed by Boussinesq. According to ():

In his attempts to explain the motion of the light in the aether Boussinesq (in 1903) opened a wide perspective of mechanics and thermodynamics. With a theory of heat convection in fluids and of propagation of heat in deforming or vibrating solids he showed that density fluctuations are of minor importance in the conservation of mass. The motion of a fluid initiated by heat results mostly in an excess of buoyancy and is not due to internal waves excited by density variations. In other words, the continuity equation may be reduced to the vanishing of the divergence of the velocity field, and variations of the density can be neglected in the inertial accelerations but not in the buoyancy term. Although used before him, Boussinesq's theoretical approach established a cardinal simplification for a special class of fluids which fundamentally differ from gases and may eliminate acoustic effects.

They result in an equation set applied to almost all oceanic motions except sound waves. The four approximation steps are:

• subtracting a motionless hydrostatically balanced reference state from the equations of motion;
• making the anelastic approximation;
• assuming that the vertical scale of motion is small compared to the scale depth (or height); and
• ignoring the inertial but not the buoyancy effects of variations in the mean density.

The term ``Boussinesq approximation'' is not always used identically with the above series of approximation steps, e.g. it may or may not include the assumption of incompressibility.

Mahrt (1986) addresses the issue of which assumptions properly constitute the Boussinesq approximations:

The derivation of conditions for the validity of the Boussinesq approximations is not as straightforward as many would assume. In the literature, a variety of sets of conditions have been assumed which, if satisfied, allow application of the Boussinesq approximations. The Boussinesq approximation can be divided into two parts. The first group of assumptions allows use of incompressible mass continuity and linearization of the ideal gas law, which are referred to as the shallow motion approximations. Additional restrictions allow neglect of the pressure influence on buoyancy. This more restrictive subclass of shallow motions is equivalent to the full Boussinesq approximations, also referred to as the shallow convection approximations.

The different derivations of the shallow motion approximations share the following conditions:

• the perturbations of variables of state must be small compared to basic state averaged values;
• the motion must be shallow compared to the scale depth of the basic flow; and
• restrictions on the time scale are required.

See Spiegel and Veronis (1960), Mihaljan (1962), Greenspan (1969), Phillips (1977) (pp. 15-20), Mahrt (1986), Zeytounian (1990) (pp. 142-176), Muller (1995), Thunis and Bornstein (1996) and Sander (1998).

Boussole Strait

See Okhotsk Sea.

Bowen ratio

The ratio of the amount of sensible to that of latent heat lost by a surface to the atmosphere by the processes of conduction and turbulence. See Hicks and Hess (1977) and Lewis (1995).

BPR

Abbreviation for bottom pressure recorder.

Bransfield Strait

A strait located between the northern tip of the Antarctica Peninsula and the South Shetland Islands. It is about 120 km wide and extends 460 km from Clarence Island in the northeast to Low Island in the southwest. It consists of three separate basins isolated from the surrounding ocean by relatively shallow sills, with local deep water formation processes resulting in different water characteristics in each of the basins. The basins deepen to the northeast, having a maximum axial depth in the west basin of 1100 m near Low Island and a maximum depth of 2700 m in the east basin south of Elephant Island. Sills shallower than 500 m almost entirely circle the strait, with the east basin having the deepest access to outside water with sills deeper than 500 m. There are no passages deeper than 500 m into either the central or west basins, and the central basin is isolated from the adjacent basins by sills of 1000 and 1100 m at its western and eastern boundaries. See Clowes (1934), Wilson et al. (1999), López et al. (1999) and Gordon et al. (2000).

brave west winds

See roaring forties.

Brazil Basin

An ocean basin located off the eastern coast of Brazil in the west-central Atlantic Ocean. It is bounded to the north by the Belem (formerly Para) Rise, at which end there is also a broad depression called the Recife (formerly Pernambuco) Abyssal Plain. This has also been called the Tizard Deep. See Fairbridge (1966).

Brazil Current

A western boundary current that forms the western limb of the subtropical gyre in the South Atlantic Ocean. This current is conspicuously weak as compared with other western boundary currents since only about 4 Sv of the water from the northern limb of the gyre, i.e. the South Equatorial Current (SEC), turns south, with the rest turning north to feed the North Brazil Current (NBC). The BC is not only comparatively weak but also much weaker than might be expected from observed wind fields, more about which later.

The portion of the SEC that feeds the BC turns south at about 10-15$^\circ$ S. The incipient BC is shallow and flows closely confined to the continental shelf, with direct current measurements at 23$^\circ$ S showing that nearly half of its transport of 11 Sv was inshore of the the 200 m isobath. There also seems to be a semi-permanent offshore meander near 22-23$^\circ$ S that may be related to local upwelling. South of 24$^\circ$ S the BC flow intensifies at a rate of about 5% per 100 km, with the intensification apparently linked to a recirculation cell south of about 30$^\circ$ S (although there is some evidence for an more extensive recirculation cell extending from 20 to 40$^\circ$ S).

Geostrophic transport estimates for the southern BC based on shallow or intermediate zero flow levels (1300-1600 m) have ranged from 18-22 Sv at 33-38$^\circ$ S. Evidence for much deeper flow (from the examination of water mass characteristics) has led to estimates ranging from 70-76 Sv at 37-38$^\circ$ S with a zero flow level at 3000 m. The latter estimates are at latitudes very close to where the BC separates from the coast and thus may be considered as estimates of the maximum BC flow.

The BC separates from the continental shelf between 33 and 38$^\circ$ S with the average being near 36$^\circ$ S. There is some evidence for a seasonal variation in the latitude of this point, with it being generally farther north in the (local) winter than in summer. After it separates from the boundary, it continues to flow in a general southward direction together with the return flow from the Falkland Current, with the southern limit to the warm water it bounds fluctuating between 38-46$^\circ$ S on time scales of about two months. After the flow reaches it maximal southern extent it turns back towards the north (as what is sometimes called the Brazil Current Front) and appears to close back on its source flow near 42$^\circ$ S. The north-south excursions of its southern limit result in eddies averaging about 150 km in diameter being shed at a rate of about one per week.

It was first proposed by Stommel that the reason the BC is weaker than expected from observed wind fields is because of an opposing effect of the thermohaline circulation. The formation of North Atlantic Deep Water requires a net transfer of thermocline water from the South Atlantic to the North as well as net northward fluxes of intermediate and bottom waters. This leads to the situation where the surface circulation of the South Atlantic subtropical gyre is not a closed system because the majority of the SEC flow turns north and crosses the equator due to the demands of the thermohaline circulation. See Peterson and Stramma (1991).

Brazil Current Front

See Peterson and Stramma (1991).

Brazil-Malvinas Confluence

A region where the Brazil Current meets the Malvinas Current at around 38$^\circ$S. This collision of subtropical and subantarctic waters produces one of the most spectacular of the oceanic fronts and complex SST fields seen in the world ocean. At the BMC, subantarctic surface waters meet subtropical thermocline water in a front that can have a gradient as strong as 8$^\circ$C per km. It is frequently marked by a ribbon of warm, low salinity water of Rio de la Plata origin that has folded over the northern tip of the cyclonic trough formed by the Malvinas Current and its return to the south. A warm, low salinity cap tens of meters thick often covers the western segments of the warm subtropical water. It is derived from the continental shelf north of the Rio de la Plata.

The specific configuration of the BMC at any given time is thought to depend on the relative strengths of the baroclinic and barotropic fields of the Malvinas and Brazil Currents. Its variability occurs over time scales ranging from the intra-annual to the inter-annual, with the spatial characteristics including changes in the latitudes of separation of the western boundary currents from the continental margin, and changes in the geometry of their extensions in the offshore region. On intra-annual time scales, the variability is dominated by the periodic production of transient, cold-core eddies from the Malvinas Current and warm-core eddies from the Brazil Current. These have associated SST anomalies that can be as large as 10$^\circ$C on space and time scales of 1000 km and two months, respectively.

On annual time scales the variability of the western south Atlantic is dominated by the seasonal displacements of the BMC. It is found farther north during austral winter (July-September) than during the summer. Besides the annual cycle, thought to be driven by variations in the strengths of the Malvinas and Brazil Currents, there is a semi-annual component of variability with near zero amplitude at 30$^\circ$S increasing to nearly half the magnitude of the annual signal at 50$^\circ$S. This is probably a response to the semi-annual cycle in zonal winds over the Southern Ocean. See Garzoli and Garraffo (1989) and Goni et al. (1996).

breaker zone

The portion of the nearshore zone where waves arriving from offshore become unstable and break. See Komar (1976).

Brewer-Dobson circulation

The meridional atmospheric circulation that transports air poleward and downward from the tropical middle atmosphere. Air is transferred between the equator and poles by this circulation on a time scale of months, indicative of the strong control by the Coriolis force that deflects the air stream zonally and inhibits meridional motions. See Salby (1992).

BROKE

Acronym for Baseline Research on Oceanography, Krill and Environment, a study conducted off east Antarctica in the Austral summer of 1995-96. The primary focus was to describe the distribution and abundance of Antarctic krill and to determine possible sources of Antarctic bottom water in the region. See Nicol et al. (2000).

Brunt frequency

See buoyancy frequency.

Brunt-Vaisala frequency

See buoyancy frequency.

BSA

Abbreviation for Bayesian signal analysis, a method designed to be optimal for analyzing short time series which can work with an SNR as low as 0.6. No hypotheses are made about the actual series belonging to any hypothetical ensemble or infinite series; only the given data are used to find the probability of some a priori signal being contained in the data. A measure of the accuracy of the estimate can also be obtained. See Ruiz de Elvira and Bevia (1994).

BSFOCI

Abbreviation for Bering Sea Fisheries Oceanography Coordinated Investigations, a NOAA COP program whose overall goal is to reduce uncertainty in resource management decisions through ecological research on recruitment and stock structure of walleye pollock, presently the largest single-species fishery in the world. A combination of basin circulation studies, analysis of recent and historical data, and development of genetic testing methods has advanced the definition of the stock structure of Bering Sea pollock in this program. See the BSFOCI Web site.

BSPFTE

Abbreviation for Barents Sea Polar Front Experiment. See the BSPFTE Web site.

BTM

Abbreviation for Bermuda Testbed Mooring Program.

Buchanan Deep

See Angola Basin.

Buchanan, John Young

More later.

bucket temperature

The surface temperature of the ocean as measured by a bucket thermometer. This can also be the temperature measured by immersing a surface thermometer into a freshly drawn bucket of water.

bucket thermometer

A thermometer with an insulated container around the bulb. It is used to measure ocean temperatures by lowering it on a line, allowing it to equilibrate with the temperature of the surface water, withdrawing it along with the water surrounding it, and reading the temperature. The water serves both as insulation for the thermometer (after withdrawal) and as a sample for a salinity determination.

buffer factor

Defined as the fractional change in atmospheric CO$_2$ divided by the fractional change in oceanic DIC after equilibrium has been reached. This factor characterizes the fraction of the CO$_2$ flux from the atmosphere to the mixed layer that will react to form carbonate and bicarbonate ions. This is also known as the Revelle factor. See Najjar (1991).

buffer sublayer

That part of a boundary layer where the viscous stress and the Reynolds stress have the same order of magnitude and the linear velocity profile turns smoothly into the logarithmic profile. See Kagan (1995).

bulk parameterizations

In studying air-sea interactions, it is difficult to obtain direct measurements of the surface fluxes, and those that are available are extremely limited in geographic scope. Therefore, extensive, global-scale estimates must be obtained via parameterizations of the surface fluxes that permit the use of more easily obtained quantities. The basic premise of the concept of bulk parameterizations is to relate the surface layer fluxes to logarithmic profiles of the mean quantities. The fluxes can then be determined from the mean wind, temperature and humidity at a single height by introducing bulk transfer coefficients of heat, moisture and momentum. This method is also used to quantify gas exchange processes. See Geernaert (1990) and Rogers (1995).

buoyancy

buoyancy frequency

The frequency with which a parcel or particle of fluid displaced a small vertical distance from its equilibrium position in a stable environment will oscillate. It will oscillate in simple harmonic motion with an angular frequency defined by:

$${N^2}\,=\,g\left(\alpha{{\partial\theta}\over{\partial z}}\,-\, \beta{{\partial S}\over{\partial z}}\right)$$

where $g$ is the gravitational acceleration, $\theta$ is the potential temperature, $\alpha$ is the thermal expansion coefficient, and $\beta$ is the saline contraction coefficient. In practice, the equivalent formula:

$${N^2}\,=\,{\left(-{g\over \rho}{{\partial{\rho}}\over{\partial z}} \right)}^{1\over 2}$$

is often used, where $c$ is the velocity of sound, although care must be taken to consistently evaluate $\rho$ and $c$. See Turner (1973) and McDougall et al. (1987).

buoyancy scale

An important length scale in stratified flow with internal waves. This is defined as:

$${L_B}\,=\,\sqrt{\bar{w}^2} / N$$

where $\bar{w}$ is the ratio of the vertical turbulent and internal wave fluctuations and $N$ the buoyancy frequency. This is used instead of the Ozmidov scale if the vertical velocity fluctuations due to internal waves are small compared to those due to turbulence.

Burger number

A dimensionless number indicative of the importance of baroclinicity in a flow field. It is the square of the ratio of the Rossby radius of deformation to the horizontal scale of the flow, and is given by:

$${B_u}\,=\,{{NH}\over{fL}}$$

where $N$ is the buoyancy frequency, $H$ the vertical scale of the flow (i.e. the depth), $f$ is the inertial frequency, and $L$ the horizontal scale of the flow. A Burger number of zero indicates a rotation dominated flow, while large values indicate stratification dominated flows. This was named for the mathematician/meteorologist Alewyn Burger.

Burma Sea

See Andaman Sea.

Buys Ballot's law

A synoptic meteorology rule stating that if, in the northern hemisphere, an observer stands with his back to the wind, pressure is lower on his left hand than on his right, while in the southern hemisphere the converse is true. This was enunciated by Buys Ballot of Utrecht in 1857 and is basically a restatement of the fact that winds blow clockwise around a depression in the northern hemisphere and anticlockwise in the southern hemisphere.