Currents Glossary

Currents Glossary

A student’s guide to understanding important physical oceanography terms.

For more terms, see the glossary for ocean surface currents at the University of Miami Rosenstiel School of Marine and Atmospheric Science, a SECOORA partner.

Alphabetical Index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Global Conveyor Belt
Adiabatic
An absence of heat transfer. In thermodynamics, adiabatic heating and cooling are processes that commonly occur due to changes in hydrostatic pressure. When a substance undergoes a pressure change, compression or expansion takes place, resulting in a temperature change. In an adiabatic system, the temperature change occurs without the addition or loss of heat.
Alaskan Gyre
A small sub-polar current in the Northeast Pacific, near the Gulf of Alaska. It rotates counterclockwise, with the dominant flow being the poleward-flowing Alaska Current.
Antarctic Bottom Water
The densest ocean water, formed primarily in Antarctica's Weddell Sea during Southern Hemisphere winters. The dense water sinks and forms a deep-water mass that flows north, eventually reaching all ocean basins.
Antarctic Circumpolar Current
Also called the ‘West Wind Drift’ because it is driven by powerful westerly winds north of Antarctica. This current continues eastward encircling Antarctica without changing direction. It is the largest volume current in all the oceans.
Antilles Current
This current transports warm tropical waters from the Atlantic Ocean's North Equatorial Current near the Lesser Antilles towards the northwest. It is a significant source of warm water for the Florida Current and Gulf Stream system.
Boundary Current
The poleward- or equatorward-flowing currents that form the western and eastern boundaries, respectively, of the subtropical circulation gyres. The east-west flowing currents that complete the gyres are called transverse currents.
Brackish Water
A mixture of fresh and salty water that often appears brown in color. Characteristics of brackish water are heavily influenced by tides because salt levels typically increase with an incoming tide (flood tide) and decrease with an outgoing tide (ebb tide).
Brine
Water containing large amounts of salts. These waters are created in areas of strong evaporation, for example the Dead Sea, and in polar regions when salt is extruded from seawater that freezes.
Buoyancy
The ability or tendency to float or rise in a liquid.
Canary Current
An eastern boundary current that flows along the west coast of Africa, towards the equator. It is the southwestward flow component of the North Atlantic subtropical gyre. Water from coastal upwelling is entrained as the Canary Current flows, thus relatively cool water is transported.
Coastal Downwelling
The vertical movement of water downward that results from the wind pushing water towards the shore. When the water piles up near the shore, it is forced to sink. (Also see Downwelling)
Coastal Upwelling
The vertical movement of water upward that results from the wind pushing water away from the shore. When the water is pushed away, deeper, often colder and nutrient-rich water rises to the surface to replace it. (Also see Upwelling).
Cold-core Ring
An oceanic vortex (that rotates counterclockwise) formed when a current with a large meander wraps around on itself and detaches from the main current. Cold-core rings have a core of cold water and are found south of the Gulf Stream.
Convection
The act of conveying (transporting). In ocean circulation, it refers to the movement within a fluid resulting from differential heating (different amounts of heating in different areas) and cooling of the fluid. Convection produces mass transport or mixing of the fluid. The sinking of cold, dense water in Polar Regions is an example of convection driven by changes in density with depth (density gradients).
Convergence Zone
The line along which waters of different density come together. Convergence zones form the boundaries of tropical, subtropical, temperate, and polar areas in the oceans. Downwelling often occurs in these areas.
Coriolis Effect

Coriolis Effect

The apparent deflection of a moving object from its initial course when its speed and direction are measured in reference to the surface of the rotating Earth. The object is deflected to the right of its anticipated course in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection occurs for any horizontal movement of objects with mass. No deflection occurs at the equator, as Coriolis effect values are zero. In the figure depicting force vectors for fluid flow (right), the Coriolis effect is represented by the small red arrows.

Density Currents
Circulation in a fluid that is driven by the action of gravity on adjacent water masses of different densities. Hydrostatic pressure differences result from the density gradient and this is what drives the flow. Because in the ocean density is largely a function of temperature and salinity, density-driven circulation is called thermohaline circulation. (Also see Thermohaline Circulation and Global Conveyor Belt)
Divergence Zone
A region where surface waters are moving apart resulting in upwelling and the arrival of waters from below to replace the diverging surface flow. These waters are often colder and nutrient-rich.
Downwelling
A circulation pattern in which surface water moves vertically downward. It occurs when surface waters converge or when winds force water to pile up along the coast (Also see Coastal Downwelling).
Eastern Boundary Current
A slow-moving current at the eastern boundary of an ocean (off the west coast of a continent). Unlike Western Boundary Currents which carry warm water towards the poles, Eastern Boundary Currents carry cold water towards the equator.
Ebb Current
The movement of a tidal current away from the shore or down a tidal river or estuary. Water moves towards the sea during a decrease in the height of the tide.
Eddy

Eddy

A current of any fluid forming on the side of or within a main current. Eddies usually move in a circular path and develop where the main current encounters obstacles or where currents flow at different speeds past one another.

Ekman Spiral

Ekman Spiral

A theoretical model of the effect of a steady wind blowing over an ocean of unlimited depth and breadth and of uniform viscosity. Because of the Coriolis Effect, the surface layer is expected to flow at a 45° angle to the wind in the Northern Hemisphere (to the left in the Southern Hemisphere). Water at increasing depth will drift in directions increasingly to the right of the wind, until it is moving in a direction opposite to that of the wind. The depth at which this occurs depends on wind speed, but is typically between 10 m and 100 m. The net water transport is 90° to the wind, and current velocity decreases with depth.

Ekman Transport
The net transport of surface water set in motion by wind. Because transport is the sum of layer movement due to the theoretical Ekman spiral, it moves in a direction 90°to the right of the wind in the Northern Hemisphere and 90°to the left of the wind in the Southern Hemisphere.
Estuarine Circulation
Circulation characteristic of estuaries or other bodies of water that have restricted water exchange with the ocean. It is governed by the coming together of freshwater running into the estuary from the land, with saltwater entering the estuary from the sea. Because freshwater is less dense than seawater, it floats above it and surface flow is toward the ocean. The denser seawater moves out of the estuary in a subsurface counter flow. Mixing of the two water bodies does occur, the extent depending on the speed and volume of the two masses.
Florida Current
A warm current flowing north along the coast of Florida. It is part of the Gulf Stream System and becomes the southern ending point of the Gulf Stream.
Friction
Surface resistance to relative motion caused by the rubbing of the surface of one mass against that of another. In ocean circulation, the force of friction allows winds over water to move the underlying water, initially in the direction of the wind. In coastal regions, where the water is relatively shallow, ocean currents are slowed by friction from contact with the sea floor and interaction with the coastline.
Geostrophic Current
A current that develops out of Earth's rotation and is the result of a near balance between gravitational force and the Coriolis Effect.
 

Global Conveyor Belt

Differential heating at the poles and equator (the poles and the equator get very different amounts of heat from the sun) drives this system of thermohaline circulation (thermo = temperature, haline = salinity.) Water that travels to the poles gets cold and sinks. Water that is warmed on the surface at the equator evaporates and leaves salt behind, making surface water heavier than the water below it and causing it to sink. These two basic mechanisms along with major ocean currents drive the Global Conveyor Belt, which moves ocean water all over the globe over thousands of years.

Gulf Stream

Gulf Stream

The strong western boundary current of the North Atlantic Ocean subtropical gyre that flows poleward off the east coast of the United States. It was first identified by William Gerand De Brahm of Britain in 1769. Benjamin Franklin published the first map (right) shortly after it was identified.

Gyre

Gyre

A circular motion. Used mainly in reference to the circular motion of water in each of the major ocean basins centered in subtropical high-pressure regions. Circulation around a subtropical high is clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. (See figure, right). There are also sub-polar gyres that rotate in the opposite direction, for example the Alaskan Gyre rotates counter-clockwise. (Also see Alaskan Gyre)

Halocline
A layer in the ocean in which salinity changes rapidly with depth.
In Situ
In place, i.e., in situ density of a sample of water is its density at its original depth.
Intermediate Water
water mass situated between a surface water mass and a bottom water mass.
Irminger Current
A warm current that branches off from the Gulf Stream and moves up along the west coast of Iceland.
Isobar

Isobar

A curve connecting values of equal pressure.

Isohaline
A curve that connects points of equal salinity
Isopycnal
A curve connecting points of equal density.
Isotherm
A curve connecting lines of equal temperature.
Knot
A unit of speed often used in oceanography equal to one nautical mile per hour.
Labrador Current

Labrador Current

The Labrador Current is a cold current in the North Atlantic Ocean which flows from the Arctic Ocean south along the coast of Labrador and passes around Newfoundland, continuing south along the east coast of Nova Scotia. It meets the warm Gulf Stream at the Grand Banks southeast of Newfoundland and again north of the Outer Banks of North Carolina.

Longshore Current
A current running parallel to shore in the surf zone caused by the way waves break when they approach the beach at an angle.
Loop Current

Loop Current

A clockwise-moving flow that pushes northward into the Gulf of Mexico. Receiving its warm water from the Caribbean and Yucatan Currents, it flows out of the Gulf of Mexico through the Straits of Florida where it meets with the Antilles Current. These currents then feed warm water into the Florida Current. Like many ocean currents, the Loop Current is variable in position. Its extreme positions range from nearly in line with the Florida Current to northern expansion up near the Mississippi River Delta. While the cause(s) for the position fluctuations are unclear, evidence suggests wind forcing plays a major role.

Meander
A loop-like bend or curve in the flow path of a current.
Nautical Mile
A distance approximately equal to 1.15 statute miles.
North Atlantic Deep Water
A deep-water mass that forms primarily at the surface of the Norwegian Sea near Greenland and moves south along the floor of the North Atlantic Ocean.
Sargasso Sea

Sargasso Sea

A region of convergence in the North Atlantic lying south and east of Bermuda. Water in the Sargasso Sea is a clear, deep blue color and contains large quantities of Sargassum, a floating brown bushy algae.

Synoptic Flow
Large-scale ocean circulation features, such as a basin-wide gyre or the global conveyor belt.
Thermocline
A layer in the ocean in which temperature changes rapidly with depth. This often occurs in the area directly below the ocean surface layer that is most warmed by the sun and well-mixed by wind-derived energy.
Thermohaline Circulation
The vertical movement of ocean water driven by density differences resulting from the combined effects of variations in temperature and salinity. (Also see Global Conveyor Belt)
Turbidity
The amount of solids (such as sediment or plankton) suspended in a sample of water. This can be affected by wind speed and wind fetch, small-scale circulation features, and many other factors.
Upwelling
The process by which deep water is brought to the surface (often cold and nutrient-laden). Upwelling can be caused by the wind, divergence of equatorial currents, or coastal winds pushing water away from the coast. (Also see Coastal Upwelling)
Warm-core Ring
An oceanic vortex formed when a current with a large meander wraps around on itself and detaches from the main current. Warm-core rings have a core of warm water and are found north of the Gulf Stream.
Water Mass
A large volume of water with a distinctive set of properties, typically identified by its temperature and salinity.
Western Boundary Current
A strong, warm, concentrated, fast-moving current at the western boundary of an ocean (off the east coast of a continent). When a mound of water is formed in the ocean due to winds and Ekman transport, the shape and rotation of the Earth cause the peak of the mound to be closer to the western side of the ocean basin than to the eastern side. The tilt of the mound's surface results in much stronger currents on the western side of ocean basins, where the western margin or boundary of a subtropical gyre is found. The Gulf Stream off the east coast of the United States and the Kuroshio Current off the east coast of Japan are examples.
Wind-driven Circulation
Any movement of ocean water that is driven by winds. This includes most horizontal movements in the surface waters of the world's oceans.
Wind Fetch
The uninterrupted distance over which the wind blows without a significant change in direction. Fetch is a factor in the development of gravity waves (waves formed by the transfer of wind energy into water), and a significant factor in forecasting storm surge heights.
Wind Speed
The distance over which the wind blows in a given amount of time. Wind speed is usually measured in knots (kt, nautical miles per hour); meters per second (m/s) or miles per hour (mph).

Sources

  • Garrison, Tom (2004) Essentials of Oceanography. Brooks/Cole-Thomas Learning, Inc., Pacific Grove, CA USA. 352 p
  • Pinet, Paul R. (1992) Oceanography: an Introduction to the Planet Oceanus. West Publishing Company, St. Paul MN USA. 571 p
  • Thurman, Harold V. (1997) Introductory Oceanography. Prentice Hall College Div, Upper Saddle River, NJ USA. 513-533 pp.