Da-Dm

daily retardation

In tidal studies, the amount of time by which a tidal phase lags the previous day's corresponding phase. These lags average about 50 minutes.

Dampier, William (1652-1715)

See Peterson et al. (1996), p. 33.

Darwin, George H.

More later.

data assimilation

See Anderson et al. (1996).

Davidson Current

See Inshore Countercurrent.

Davidson Inshore Current

See Inshore Countercurrent.

D-BAD MOCNESS

Acronym for Dual-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmental Sensing System, an instrument designed to collect acoustic data and net samples simultaneously from the same portion of the water column. See Greene et al. (1998).

DBE

Abbreviation for Deep Basin Experiment.

DCP

Abbreviation for Data Collection Platform.

dead-water phenomenon

A phenomenon caused by a thin layer of fresh meltwater overlaying an otherwise salty sea. It is exceptionally hard to propel a boat (by rowing or any other method) in such a situation since energy has to be expended to generate not only surface waves but also internal waves at the saltwater-freshwater interface. Ekman (1904) first performed systematic analytical and experimental studies of this phenomenon. See also Kraus and Businger (1994), p. 246.

Deacon Cell

A name often given to the meridional circulation of the Southern Ocean wherein deep water upwells, is blown north by wind stress in an Ekman layer, and sinks at the Antarctic Convergence. According to Speer et al. (2000):

The conception of the Deacon cell was based on the idea of a transformation of water from cold, dense layers to warmer, lower density layers. A second, deeper, cell is associated with bottom-water formation, sinking, and entrainment next to the Antarctic continent, equatorward flow near the bottom, and the southward inflow of deep water. Thus, deep inflow must compensate both near-bottom and near-surface northward outflow; this deep inflow is thought to be accomplished geostrophically since ridges provide lateral boundaries that support east-west pressure gradients below their crests.

Similar, although much weaker, cells are seen in the northern subpolar gyres as well as cells flowing in the opposite sense in the tropics. See McWilliams (1996) and Speer et al. (2000).

The conception of the Deacon cell was based on the idea of a transformation of water from cold, dense layers to warmer, lower density layers. A second, deeper, cell is associated with bottom-water formation, sinking, and entrainment next to the Antarctic continent, equatorward flow near the bottom, and the southward inflow of deep water. Thus, deep inflow must compensate both near-bottom and near-surface northward outflow; this deep inflow is thought to be accomplished geostrophically since ridges provide lateral boundaries that support east-west pressure gradients below their crests.

de Brahm, William (1718-1799)

See Peterson et al. (1996).

decibar

More later.

Deep Basin Experiment (DBE)

A component of WOCE focused on increasing the knowledge and understanding of deep circulation processes. The primary objectives were:

• to observe and quantify the deep circulation within an abyssal basin,
• to distinguish between boundary and interior mixing processes,
• to understand how passages affect the water flowing through them, and
• to study the means by which deep water crosses the equator.

Specifically, the DBE examined the deep interior flow of the Brazil Basin with these objectives in mind. See Hogg et al. (1996).

[http://www.whoi.edu/science/PO/people/mvanicek/DBE/]

deep convection

In physical oceanography, the sinking of surface waters to form deep water masses, a process of fundamental importance for ocean climate and the maintenance of a stably stratified world ocean. There are two main types of deep convection, the physics of which are very different. The first is convection near an open boundary, which involves the formation of a dense water mass which reaches the bottom of the ocean by descending a continental slope. The second type is open-ocean deep convection, where the sinking occurs far from land and is predominantly vertical.

There are five separate ingredients involved in the formation of deep water near ocean boundaries:

• a reservoir in which to form dense water, which takes the form of either a very wide shelf or one that slopes upward away from the coast to form a sill;
• a source of dense water within the reservoir which, in polar regions, is the brine release that occurs during wintertime ice formation;
• a reason for the dense water to leave the reservoir and descend the slope, which takes the form of an already existing circulation that drives at least part of the dense water off the shelf, e.g. the Ross Sea features a cyclonic circulation pattern, an onshore Ekman flux driven by prevailing easterlies, and katabatic wind forcing;
• a requirement that more than one water mass be involved in dense water formation, with this requirement not playing a dynamical role but apparently necessary from the lack of observations to the contrary;
• a requirement that the densities, geography, and dynamics involved actually permit the water to sink.

There is another list of ingredients involved in open-ocean deep convection:

• a background cyclonic circulation, needed to form an upward doming of isopycnals in the center of the gyre to reduce the stability of the water column;
• preconditioning which, operating over a period of week, creates a region of very weak static stability within the cyclonic dome which can then begin convection if the surface forcing is sufficiently intense;
• similar to the ocean boundary convection case, more than one water mass be involved, i.e. the existence of several water masses provides subsurface sources of heat and salt which can be exposed to the surface during convection, both of which can be destabilized by cooling;
• sufficiently strong surface forcing must be involved, which usually takes the form of heat loss by sensible and latent flux to cold winds. This forcing causes a violent mixing phase consisting of rapid vertical convection and mixing, which takes place in cellular structures with horizontal and vertical scales of similar size. The convection mode also seems to be nonpentrative, i.e. mixing occurs such that the density structure remains a continuous function of depth;
• a breakup or sinking and spreading phase, accomplished by some combination of the processses of baroclinic instability, vertical shear, topography and mixing by internal waves that is not yet well understood.

Jones and Marshall (1993) discuss the phases of open-ocean deep convection:

[There are] three successive phases that characterize open-ocean deep convection: preconditioning, on the large scale (of order 100 km); violent mixing occurring in localized, intense plumes (on scales of order 1 km); and sinking and spreading of the convectively tainted water, on a scale of 5-10 km.

During preconditioning, the gyre-scale circulation and buoyancy forcing combine to predispose a particular site to overturn. For example, in the Gulf of Lions the background cyclonic circulation is subject to persistent surface heat loss priming the center of the gyre, where isopycnals dome up toward the surface. With the onset of strong surface forcing the near-surface stratification, over an area up to 100 km across, can be readily erased exposing the very weakly stratified water mass beneath the surface. Subsequent cooling events can then initiate violent mixing in which the whole of the fluid column overturns, drawing buoyancy from depth, in numerous cells of horizontal scale of order 1 km; downward velocities of order 10 cm s$^{-1}$ can devleop in only a few hours in this violent-mixing phase. The largest ascending and descending currents penetrate the whole depth of the mixed-water column. In concert the plumes are thought to rapidly mix properties over the preconditioned site, forming a ``chimney'' of homogeneous fluid. Chimneys ranging in scale from several to many tens of kilometers have been observed. At the density front between the homogeneous and stratified water, geostrophic eddies develop on a scale comparable with the local Rossby radius of deformation. With the cessation of strong forcing there is a sharp decline in convective overturning; the predominantly vertical heat transfer of the mixing phase gives way to horizontal advection associated with eddying on geostrohpic scales. The mixed fluid ``slumps'' under gravity and rotation, spreading out at middle depths and leading, on a time scale of days, to the disintegration of the chimney. As the dense fluid sinks, water from outside the chimney is drawn in, restratifying near-surface layers.

See Killworth (1983), Jones and Marshall (1993) and Marshall and Schott (1999).

deep scattering layer

A layer of organisms found in most oceanic waters that scatters sound. These layers are usually found during the day at depths ranging from 600 to 2400 feet, are rarely less than 150 feet thick, and can be as thick as 600 feet. Several layers are often recorded simultaneously and can range horizontally for many kilometers. Most of these layers undergo diurnal vertical movements. There are also shallow (over continental shelves) and surface scattering layers.

deep water wave

More later.

Defant, Albert

More later.

deformation radius

See Rossby radius of deformation.

degenerate amphidromic point

An amphidromic point whose center or nodal point appears to be located over land rather than water.

Delaware Coastal Current

See Sanders and Garvine (1996).

Demerara Eddy

The name given to what was once thought to be a semipermanent anticyclone north of the North Brazil Current (NBC) retroflection. Better temporal and spatial sampling has shown this to be a series of eddies pinched off by the retroflection rather than a persistent feature.

denitrification

In the ocean this is the process by which bacteria use nitrate instead of oxygen as an oxidant of organic matter. It may be considered as the biological reduction of nitrate or nitrite to nitrogen or nitrous oxide. This takes place under low oxygen conditions. See Riley and Chester (1971).

Denmark Strait Overflow

The flow through and over the 600 m sill depth in the Denmark Strait between Iceland and Greenland. This is thought to be around 3.0 Sv of which 2.5 Sv is Arctic Intermediate Water and 0.5 Sv is Upper Polar Deep Water. The mixture of these water masses after they pass the strait is called Northwest Atlantic Bottom Water (NWABW). This is the coldest and densest of the source waters for North Atlantic Deep Water and is characterizied by a salinity minimum. See Dickson and Brown (1994).

densimetric Froude number

See Froude number.

density (of sea water)

Much more later.

density current

See turbidity current.

density ratio

The ratio of the effect of a temperature change on density divided by the effect of a salinity change. This was found in the Spice Experiment to be roughly equal to one for horizontal scales ranging from 10 m to 1000 km. See Figueroa (1996).

dependent variable

In numerical modeling and general mathematics, a variable whose value changes as a function of another variable, i.e. the latter is first specified and the latter then calculated. The specified variables are called independent variables. Examples of variables that are usually dependent in numerical modeling include velocities, temperatures, and densities, with the independent variables usually being the spatial positions and time, although some variables can be either depending on the situation. For example, when pressure coordinates are used the pressure is an independent variable and the height or depth a dependent one, but when level coordinates are used the positions are reversed.

depth of frictional resistance

The depth at which the wind-induced current direction is 180 degrees from that of the wind in an Ekman spiral.

depth of no motion

See level of no motion.

design wave

More later.

detrital

The most voluminous of three major components of deep sea sediments, the other two being authigenic and biogenic. Detrital material is derived from the mechanical and chemical fragmentation of continental materials, most of which is in the form of alumino-silicate minerals. It is transported chiefly by rivers into coastal waters and by the wind onto the sea surface. See Broecker and Peng (1982).

detritus

A general collective term for loose mineral and rock that is broken or worn off by mechanical means, as by disintegration or abrasion.

diagenesis

The chemical, physical, and biological changes sediment undergoes after it is initially deposited. This includes such processes as compaction, cementation, reworking, authigenesis, replacement, crystallization, leaching, hydration, bacterial action, and formation of concretions that normally occur at temperatures and pressures characteristic of surface conditions. Weathering and metamorphic processes are usually excluded from this category.

diagnostic

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

dianeutral

Across a neutral surface.

diapycnal

Motion or transport directed across surfaces of constant density or isopycnals.

diapycnal mixing

See Gregg (1987).

diatom ooze

A soft, siliceous, deep-sea deposit of which more than 30% is composed of silica-rich diatom cell walls. This type of siliceous ooze (another of which is radiolarian ooze) predominates in high latitudes around the coast of Antarctica and in the North Pacific, but is overwhelmed by sediment of continental origin in the North Atlantic. This type of ooze covers about 9% of the sea floor. Compare to calcareous ooze. See Tchernia (1980).

DIC

Abbreviation for Dissolved Inorganic Carbon, which includes the sum of dissolved CO$_2$ gas and the ions HCO$_3$ and CO$_3$. This is dominated by the bicarbonate (HCO$_3$) ion is sea water and occasionally referred to as total CO$_2$.

dicothermal layer

A vertical ocean layer sometimes found in high northern latitudes. It is a cold (as low as -1.6$^\circ$ C) layer from 50 to 100 m sandwiched between warmer surface and deeper layers. The water column remains stable due to a salinity gradient that counters the unstabilizing effects of the temperature gradient.

differential heating

The difference in how land and water surfaces absorb heat, with water having a higher heat capacity than land. The same amount of solar radiation will heat the same area of ground more than it will the ocean. The heat absorbed by the ocean will be distributed over a greater vertical extent than on land due to mixing in the water column. These factors lead to the difference between land and ocean temperatures being greatest in the summer when the amount of solar radiation is the highest, with the land being warmer than the ocean. In the winter the ocean surface is warmer than the land, although the differential isn't as great as in winter. Diurnal variations in differential heating lead to the phenomenon known as a sea breeze, while long term (i.e. over weeks to months) variations lead to prevailing winds often called monsoons.

dilution basin

See mediterranean sea.

DIM

Acronym for Dissolved Inorganic Matter.

dimensionless number

See Biot number, Ekman number, Froude number, gradient Richardson number, Grashof number, overall Richardson number, Peclet number, Prandtl number, Rayleigh number, Rossby number, Strouhal number and Weber number.

direct tide

A tide which is in phase with the apparent motion of the attracting body, whether it be the sun or the moon. It has its local maximums directly under the tide-producing body and on the opposite side of the earth. See also reversed tide. From Baker, Jr. (1966).

discretization

In numerical modeling, the process of converting differential equations governing processes occurring in a continuum into equivalent algebraic equations governing processes occurring in a computational grid. This process is guided by considerations of consistency, convergence, and stability.

dispersion

The dependence of wave velocity on the frequency of wave motion. The name comes from the fact that waves starting at the same place will, if they have different frequencies, move away at different speeds and thus disperse or spread out.

dispersion curve

A graph showing the dependence of the frequency on the wavenumber for dispersive waves. This is usually created by first using a dispersion relation to obtain frequency/wavenumbers pairs, and then plotting them.

dispersion relation

An equation with which one can determine the frequency (and thus phase speed) of waves of a given wavenumber, or occasionally vice versa.

diurnal

1. Generally, occurring once a day. 2. Descriptive of a tide that has only one high and one low water per day, as opposed to semidiurnal.

divergence

The divergence of the flux of a quantity expresses the time rate of depletion of the quantity per unit volume. Negative divergence is called convergence and relates to the rate of accumulation. When applied to the velocity vectors of air or water motion, the divergence is positive when the parcels are expanding. In mathematical terms, the divergence of a vector function is defined by

$$\nabla\circ A\,=\,{{\partial{A_x}}\over{\partial x}}\,+\, {{\partial{A_y}}\over{\partial y}}\,+\, {{\partial{A_y}}\over{\partial y}}$$

where $\nabla$ is the gradient operator that operates with a scalar product on the vector field $A$, and ${A_n}/{\partial n}$ are the scalar components of $A$ in a Cartesian coordinate system. See Dutton (1986).

divergence theorem

A theorem stating that no matter how the divergence of a vector field varies over a volume, its integral depends only on the integral of the components of that field normal to the surface of the boundary. It is mathematically stated in vector notation by

$$\int\int{\int_V} \nabla\cdot A\,dV\,=\,\int{\int_S}A\cdot\eta\,d\sigma$$

where $A$ is the vector field, $V$ the volume, $S$ the surface, $dV$ an differential volume element, and $d\sigma$ a differential surface element. See Dutton (1986).

DMS

Abbreviation for dimethylsulphide, a gas emitted by phytoplankton in seawater where it escapes to the air and and reacts to form aerosols and presumably has a non-negligible climate effect. DMS often has a maximum at the surface or within the euphotic zone and decreases rapidly below this. It also has a strong seasonal cycle with a maximum in the summer and a minimum in the winter. See DMS-cloud-climate hypothesis for the exposition of one mechanism. See Charlson et al. (1987).

DMS-cloud-climate hypothesis

The main source of sulfate particles and CCN over the oceans is the oxidation of DMS. As such the DMS may determine the concentrations and size spectra of cloud droplets and therefore the cloud albedo over large regions of the oceans. Marine stratiform clouds are of particular importance in this regard as they cover about one-quarter of the world's oceans and therefore play a major role in the Earth's radiative balance. This scenario is the basis for what is called the DMS-cloud-climate hypothesis. See Charlson et al. (1987) and Jaenicke (1993).

DMSP

Abbreviation for DiMethyl Sulfonium Propionate, thought to be the dominant precursor to DMS in the oceans. DMS can be formed by the enzymatic cleavage of DMSP as well as by the oxidation of DMSP with OH-, oxygen or hydrogen peroxide. DMSP is present in both particulate and dissolved forms, with the latter thought to be the larger source of DMS. DMSP most likely originates in phytoplankton where it is believed to serve in maintaining osmotic pressure (i.e. an osmolyte). See Najjar (1991).