Blakely, R. J., 1995. Potential Theory in Gravity and Magnetic
Applications. Cambridge, UK: Cambridge University Press.
Bouligand, C., Glen, J. M. G., and Blakely, R. J., 2009. Mapping
Curie temperature depth in the western United States with a fractal model for crustal magnetization. Journal of Geophysical
Research, 114, B11104, doi:10.1029/2009JB006494.
Buschow, K. H. J., 2001. Encyclopedia of Materials: Science and
Technology. Amsterdam: Elsevier. ISBN 0-08-043152-6.
Li, C.-F., Shi, X., Zhou, Z., Li, J., Geng, J., and Chen, B., 2010.
Depths to the magnetic layer bottom in the South China Sea area
and their tectonic implications. Geophysical Journal International, 182, 1229–1247, doi:10.1111/j.1365-246X.2010.04702.x.
Zhu, J., Lin, J., Chen, Y. J., Tao, C., German, C. R., Yoerger, D. R.,
and Tivey, M. A., 2010. A reduced crustal magnetization zone
near the first observed active hydrothermal vent field on the
Southwest Indian Ridge. Geophysical Research Letters, 37,
L18303, doi:10.1029/2010GL043542.
Cross-references
Hydrothermalism
Hot Spots and Mantle Plumes
Regional Marine Geology
CURRENTS
Jüri Elken
Marine Systems Institute, Tallinn University of
Technology, Tallinn, Estonia
Synonyms
Not to be confused with electrical current; Ocean currents;
Water flow
Definition
Currents are large-scale (mainly horizontal) translational
motions of seawater, as opposed to the wave motions
where water particles perform periodic back-and-forth
excursions relative to the state of rest. Ocean currents
can flow at great distances; they carry seawater and its
inherent properties (heat and salt content, dissolved and
particulate matter, etc.) from one region to another.
Ocean currents are expressed in terms of horizontal
velocity (m/s) of fluid motion as a function of space and
time, given either by the velocity vector components or
by the direction and speed of the current. This is the
Eulerian description of currents. Current velocity components are often presented as (eastward) zonal current and
(northward) meridional current. When using the current
presentation by flow direction, zero direction has been
agreed for the current flowing to the north; the direction
angle is further counted positive in clockwise rotation. In
Lagrangian description of currents, trajectories of fluid
particles are followed over time.
Development of observation techniques
The first ancient tracks of ocean currents were obtained by
investigating the ship logs. As an example, historical
cross-Atlantic voyages discovered regions of very strong
currents like Gulf Stream, where the sailing of ships was
slowed down or speeded up (depending on the sailing
direction) because “the currents are more powerful than
the wind”; the first Gulf Stream map was drawn in the
1770s by Benjamin Franklin. Drift data of objects floating
on the sea surface (boats, message bottles, etc.) carry
information about an object-dependent combination of
currents, waves, and wind in creating the drag forces.
Modern current-tracking drifters as Lagrangian observing
platforms have specially designed drogues to minimize the
wind and wave effects on the surface; the drifters are satellite
tracked (e.g., WOCE-SVP), and the recorded data are transferred over satellite communication links to the shore stations.
Currents and stratification of deeper ocean layers are effectively observed using the profiling ARGO drifters, which
move most of the time, neutrally buoyant in deep layers at
about 1,000-m depth, but rise periodically to the sea surface
for the profile measurement and data communication.
Direct water flow measurements at fixed locations are
technically complicated. The flowmeter needs to be fixed
in the dynamic ocean environment; or alternatively, the
sensor motions due to waves and currents need to be determined with high precision. Since the flow field is usually
fluctuating, averaging of current data over time is important. The earliest mechanical rotor-based current meter
capable of recording direction and speed from an anchored
ship on different depths has been introduced in the 1890s
by John Elliott Pillsbury. Vagn Walfrid Ekman introduced
in the 1930s reliable current meter design which included
mechanical averaging of speed and direction data. This
instrument was in service until the 1960s, when Ivar
Aanderaa introduced modern electronically recording current meters. Although concepts for acoustic current measurement methods were elaborated already before the
1950s, reliable technologies for replacement of mechanical rotor-dependent speed counters with acoustic point
sensors were developed by Neil Brown at the end of the
1970s. A breakthrough of current measurements was the
introduction of acoustic Doppler current profilers
(ADCP) in the 1980s that allow self-recording measurements of full current profiles from the instruments
mounted either on the bottom or on the moving ship.
Complementary to the direct current measurements,
indirect dynamic method based on charting of the density
fields has been widely used since the beginning of the
1900s to identify the main large-scale current patterns.
The method is based on geostrophic relations. Relative
currents of one vertical level in reference to another level
can be easily calculated from temperature and salinity profiles. Historically absolute currents were calculated with
the assumption that deep levels around 1,500-m depth
were motionless. In the recent decades, satellite-based
altimeters measure sea-level topography (determines the
pressure gradients at the sea surface) with high precision;
therefore pressure gradients forming the geostrophic currents throughout the water column can be calculated also
with high accuracy.
CURRENTS
139
Applications. Cambridge, UK: Cambridge University Press.
Bouligand, C., Glen, J. M. G., and Blakely, R. J., 2009. Mapping
Curie temperature depth in the western United States with a fractal model for crustal magnetization. Journal of Geophysical
Research, 114, B11104, doi:10.1029/2009JB006494.
Buschow, K. H. J., 2001. Encyclopedia of Materials: Science and
Technology. Amsterdam: Elsevier. ISBN 0-08-043152-6.
Li, C.-F., Shi, X., Zhou, Z., Li, J., Geng, J., and Chen, B., 2010.
Depths to the magnetic layer bottom in the South China Sea area
and their tectonic implications. Geophysical Journal International, 182, 1229–1247, doi:10.1111/j.1365-246X.2010.04702.x.
Zhu, J., Lin, J., Chen, Y. J., Tao, C., German, C. R., Yoerger, D. R.,
and Tivey, M. A., 2010. A reduced crustal magnetization zone
near the first observed active hydrothermal vent field on the
Southwest Indian Ridge. Geophysical Research Letters, 37,
L18303, doi:10.1029/2010GL043542.
Cross-references
Hydrothermalism
Hot Spots and Mantle Plumes
Regional Marine Geology
CURRENTS
Jüri Elken
Marine Systems Institute, Tallinn University of
Technology, Tallinn, Estonia
Synonyms
Not to be confused with electrical current; Ocean currents;
Water flow
Definition
Currents are large-scale (mainly horizontal) translational
motions of seawater, as opposed to the wave motions
where water particles perform periodic back-and-forth
excursions relative to the state of rest. Ocean currents
can flow at great distances; they carry seawater and its
inherent properties (heat and salt content, dissolved and
particulate matter, etc.) from one region to another.
Ocean currents are expressed in terms of horizontal
velocity (m/s) of fluid motion as a function of space and
time, given either by the velocity vector components or
by the direction and speed of the current. This is the
Eulerian description of currents. Current velocity components are often presented as (eastward) zonal current and
(northward) meridional current. When using the current
presentation by flow direction, zero direction has been
agreed for the current flowing to the north; the direction
angle is further counted positive in clockwise rotation. In
Lagrangian description of currents, trajectories of fluid
particles are followed over time.
Development of observation techniques
The first ancient tracks of ocean currents were obtained by
investigating the ship logs. As an example, historical
cross-Atlantic voyages discovered regions of very strong
currents like Gulf Stream, where the sailing of ships was
slowed down or speeded up (depending on the sailing
direction) because “the currents are more powerful than
the wind”; the first Gulf Stream map was drawn in the
1770s by Benjamin Franklin. Drift data of objects floating
on the sea surface (boats, message bottles, etc.) carry
information about an object-dependent combination of
currents, waves, and wind in creating the drag forces.
Modern current-tracking drifters as Lagrangian observing
platforms have specially designed drogues to minimize the
wind and wave effects on the surface; the drifters are satellite
tracked (e.g., WOCE-SVP), and the recorded data are transferred over satellite communication links to the shore stations.
Currents and stratification of deeper ocean layers are effectively observed using the profiling ARGO drifters, which
move most of the time, neutrally buoyant in deep layers at
about 1,000-m depth, but rise periodically to the sea surface
for the profile measurement and data communication.
Direct water flow measurements at fixed locations are
technically complicated. The flowmeter needs to be fixed
in the dynamic ocean environment; or alternatively, the
sensor motions due to waves and currents need to be determined with high precision. Since the flow field is usually
fluctuating, averaging of current data over time is important. The earliest mechanical rotor-based current meter
capable of recording direction and speed from an anchored
ship on different depths has been introduced in the 1890s
by John Elliott Pillsbury. Vagn Walfrid Ekman introduced
in the 1930s reliable current meter design which included
mechanical averaging of speed and direction data. This
instrument was in service until the 1960s, when Ivar
Aanderaa introduced modern electronically recording current meters. Although concepts for acoustic current measurement methods were elaborated already before the
1950s, reliable technologies for replacement of mechanical rotor-dependent speed counters with acoustic point
sensors were developed by Neil Brown at the end of the
1970s. A breakthrough of current measurements was the
introduction of acoustic Doppler current profilers
(ADCP) in the 1980s that allow self-recording measurements of full current profiles from the instruments
mounted either on the bottom or on the moving ship.
Complementary to the direct current measurements,
indirect dynamic method based on charting of the density
fields has been widely used since the beginning of the
1900s to identify the main large-scale current patterns.
The method is based on geostrophic relations. Relative
currents of one vertical level in reference to another level
can be easily calculated from temperature and salinity profiles. Historically absolute currents were calculated with
the assumption that deep levels around 1,500-m depth
were motionless. In the recent decades, satellite-based
altimeters measure sea-level topography (determines the
pressure gradients at the sea surface) with high precision;
therefore pressure gradients forming the geostrophic currents throughout the water column can be calculated also
with high accuracy.
CURRENTS
139