Va-Vm

VACM

See Vector Averaging Current Meter.

Vaisala frequency

See buoyancy frequency.

vapor concentration

See absolute humidity.

vapor pressure

The vapor pressure of water vapor in moist air is given by

$$e'\,=\,{ {pr} \over {(0.62197\,+\,r)} }$$

where $p$ is the pressure and $r$ the mixing ratio.

Varen, Bernhard (1622-1650?)

A German physician who is commonly accredited with being the founder of modern general geography. He summarized, amongst many other things, the current state of knowledge about the sea in his book Geographia Generalis (later published in English as A Compleat System of General Geography), first published in 1650. The book was important for being the first comprehensive and objective collection of geographical knowledge since the Greek Classical Age and for its reflection of a growing appreciation and use of empirical knowledge to guide explanations rather than the reliance on fantasy and speculation that had prevailed for more than a millenia (although Varen did indeed lapse into the latter on more than a few occasions).

Varen's most important contribution to oceanography was probably the discussions in his book about steady currents being driven by the wind, the first time this notion had seen print. This led to motions in the sea finally being considered in terms other than Aristotle's primum mobile. He also attempted to categorize the motions of the sea, separating them into a continuous western flow, an observed periodic rise and fall of the sea surface that is the tide (although their connection with the moon was suspected though not as yet well-known), and various special flows including what are now known as the Florida, Kuroshio and Mozambique Currents.

Varen also discussed varying theories as to the causes of the perceived general westward flow and the tides. The explanations for the former included a magnetic pull from the moon, thermal expansion as a result of moonlight, downward pressure from the moon as trasmitted through an endless atmosphere, the sun pulling the water after it, the inability of the sea to keep up with the earth's rotation, and more. He concluded that the cause was uncertain although he favored the wind as a cause at least of the general westward currents in the tropics as well as of other non-tidal currents flowing counter to the supposed general western flow. He considered the moon as responsible for the tides although via a mechanism as yet unknown (and to be identified by Newton in 1687). See Peterson et al. (1996).

variance ellipse

Vector Averaging Current Meter

A current meter used in oceanography that senses speed with a Savonius rotor and direction relative to its case with a vane assembly. An internal compass senses the orientation of the case relative to magnetic north. Temperature is also recorded. The data recorded by the internal electronics is a true vector average with the sampling rate for speed and direction partly determined by the rotor rotation rate. See Heinmuller (1983).

Vector Measuring Current Meter

A current meter used in oceanography that is effective at measuring near-surface currents since it is not as susceptible as some other meters to contamination from vertical currents and high frequency horizontal currents. It uses two orthogonal propellors to achieve insensitivity to current flow at right angles to the propellor axis. See Weller and Davis (1980).

veering

Said of the clockwise change of the direction of a wind, as opposed to backing.

VEINS

Acronym for Variability of Exchanges in Northern Seas, a project whose overall objective is to measure and model the variability of fluxes between the Arctic Ocean and the Atlantic Ocean with a longer term view of implementing a system for taking critical measurements needed to understand the role of the high latitude oceans in decadal climate variability. This objective is based on the fact that interannual to decadal climate variability plays an important role in the water mass conversion processes in the northern seas. The VEINS scientific objectives are:

• to obtain time series measurements of heat, salt and water fluxes for the exchange routes through the Northern Seas, i.e. between the Atlantic Ocean and the Nordic Seas and between the Nordic Seas and the Arctic Ocean;
• to quantify the magnitude and variability of these fluxes;
• to improve the understanding of the processes responsible for the variability; and
• to develop a conceptual model of exchanges and water mass alterations between the Atlantic and Arctic Oceans which will be used to estimate the integrated effect of exchanges from the four measurement areas as well as to design an optimal measurement program for the long term.

VEINS started in Feb. 1997 and was designed to run for 3.5 years and possibly longer with a funding extension.

[http://www.ices.dk/ocean/project/veins/]

velocity potential

A scalar function that exists in irrotational fluid motion. If we denote the velocity potential by $\phi$, then it is defined by the equation

$${\bf V} = -\nabla\phi$$

where $\bf V$ is the velocity vector. This equation implies that $\bf V$ is normal to the equipotential lines and is directed from high to low potential.

Vema Channel

See Vema Gap.

Vema Gap

A deep-sea channel that connects the Hatteras Abyssal Plain to the Nares Abyssal Plain in the North American Basin. This is an important passasge for northward traveling deep water formed in the Antarctic. This is also known as the Vema Channel. See Fairbridge (1966) and Hogg and Zenk (1997).

vendavales

A name given to strong, squally, southwest winds in the Straits of Gibraltar and off the east coast of Spain. It is associated with depressions and occurs mainly between September and March. They usually bring stormy weather and heavy rain.

VENTEX

Acronym for Vent Experiment.

ventilated thermocline

See Luyten et al. (1983) for the original concept and Huang (1991) and Pedlosky (1990) for reviews.

VENTS

A NOAA PMEL program established in 1984 to focus research on determining the oceanic impacts and consequences of submarine hydrothermal venting, with most of the effort directed towards achieving an understanding of the chemical and thermal effects of venting along northeast Pacific Ocean seafloor spreading centers. See the VENTS Web site.

Veronis effect

See Gough and Lin (1994).

VERTEX

A multidisciplinary study of VERtical Transport and EXchange of material in the upper ocean performed in the California Current. It was organized to investigate the vertical exchange of materials between the photic zone and deeper waters in the Pacific Ocean. VERTEX was one of the first larger scale programs to focus on the couplings between new production and export. Components involved the use of particle traps and subsequent analyses of trapped particles for major elements, trace elements, radionuclides, fecal pellets and microbial populations to estimate vertical particulate fluxes. See Martin et al. (1987) and Broenkow et al. (1992).

vertical coordinates

Currently there are three main types of vertical coordinate systems in use in ocean models, each representing specific generalized vertical coordinate systems. These types are z, $\sigma$ or $\rho$ vertical coordinates.

The simplest type is z coordinates, where z represents the vertical distance from a resting ocean surface (i.e. a static ocean under hydrostatic balance) at z = 0, with z positive upwards and z = -H(x,y) the topography. () list the advantages of z coordinates as:

• allowing the simplest of numerical discretization approaches;
• easy representation of the horizontal pressure gradient for a Boussinesq fluid;
• clean and accurate representation of the equation of state for seawater; and
• natural parameterization of the surface mixed layer using a z-coordinate.

The disadvantages are:

• cumbersome representation of tracer advection and diffusion along inclined density surfaces in the ocean interior;
• unnatural representation and parameterization of the bottom boundary layer; and
• difficult representation of bottom topography.

Another choice for vertical coordinate is the potential density $\rho$ referenced to a given pressure. This is a close analogy to the use of the entropy or potential temperature in atmospheric models. In a stably stratified adiabatic ocean, potential density is materially conserved and defines a monotonic layering of the ocean fluid. The advantages are:

• well-suitedness for representing the dynamic of tracer transport in the ocean interior since it has a strong tendency to occur along directions defined by locally referenced potential density;
• representation of the bottom topography in a piecewise linear manner;
• better representation of the physics of overflows;
• easy representation of the horizontal pressure gradient in an adiabatic fluid; and
• conservation of the volume (Boussinesq fluid) or mass (non-Boussinesq fluid) between isopycnals.

The disadvantages are:

• cumbersome representation of the effects of a realistic, nonlinear equation of state; and
• an inappropriate framework for representing the surface mixed layer or bottom boundary layer.

The third popular vertical coordinate choice is the terrain following or $\sigma$ coordinate, originally introduced in atmospheric modeling in 1957 and usually defined as:

$$\sigma\,=\,{{z\,-\,\eta}\over{H\,+\,\eta}}$$

where $\eta (x,y,t)$ is the displacement of the ocean surface from its resting position $z\,=\,0$, and $z\,=\,-H(x,y)$ is the ocean bottom. The usual convention is that $\sigma\,=\,0$ is the ocean surface and $\sigma\,=\,-1$ the ocean bottom. Sine $\sigma$ is monotonic, the relation defines a unique mapping between the depth $z$ and $\sigma$. The advantages of this coordinate are:

• a smooth representation of the ocean bottom topography with isolines concentrated where bottom boundary layer processes are most important; and
• good representation of the thermodynamic effects associated with the equation of state.

The disadvantages include:

• variable representation of the surface mixed layer, i.e. more surface layers in shallow than in deeper water;
• cumbersome representation of advection and diffusion along inclined density surfaces in the ocean interior;
• difficulty in accurately representing the horizontal pressure gradient, which is the difference of two relatively large numbers.

vertical diffusivity

An elusive quantity to obtain for the ocean. Indirect and direct (e.g. via the release and measurement of inert tracers) measurements show that mixing across density surfaces within the thermocline is on the order of $\sim 10^{-5}$ m$^2$ s$^{-1}$. This is about a tenth of the value predicted by most ocean circulation models. The discrepancy is thought to be due to intense localized mixing at the boundaries, especially around sea mounts, i.e. topographically induced mixing. See Munk (1968) and Munk and Wunsch (1998).

virtual potential temperature

A temperature defined to include the buoyant effects of liquid water in the air. It is calculated identically to the virtual temperature.

virtual temperature

The temperature a sample of dry air at pressure $P$ would have in order that its density equal that of the sample of moist air at temperature $T$, pressure $P$, and water vapor mixing ratio $r$. It is given by

$${T_v}\,=\,T\,(1\,+\,0.6078\,r ).$$

a If there is also liquid water in the air this is modified as

$${T_v}\,=\,T\,(1\,+\,0.6078\,r\,-\,{r_L})$$

where $r_L$ is the liquid water mixing ratio (in grams of liquid water per gram of dry air). The virtual temperature is thus defined because it allows the ideal gas law to be used for situations in which the air is not dry, i.e. moist air of temperature $T$ behaves identically to dry air of temperature $T_v$.

Visayan Sea

A small sea located in the midst of the Visayan Islands that comprise the central portion of the Philippines. It is centered at about 124$^\circ$ E and 12$^\circ$ N and connected to the Sibuyan Sea to the northwest, the Samar Sea to the northeast, the the Camotes Sea to the southeast, the Bohol Sea to the southwest via the Tanon Strait, and to the Panay Gulf to the southwest via the Guimaras Strait. Prominent geographic features include the Asid Gulf (in Masbate Island) and Bantayan Island.

viscous sublayer

That part of a boundary layer where the viscous stress is much larger than the Reynolds stress. See Kagan (1995).

VMCM

See Vector Measuring Current Meter.