Present ocean circulation
33
Additional Material
D: To become more familiar with the forcing fields and their variability it is recommended to explore several data sets available at
http://iridl.ldeo.columbia.edu/SOURCES/. Elementary statistical analysis can
be performed on many of these data sets at Geert Jan van Oldenborgh’s Climate Explorer, available at http://climexp.knmi.nl/.
2.2. Ocean circulation
Apart from hydrographic measurements from which current information can be
extracted (we will see later how this is done), there are two methods to measure
currents directly. One of these methods is measurement of the Eulerian velocity,
for example with a current meter along a rope in the water column. The other is the
measurement of the velocity of an object that moves with the currents using, for
example, a satellite tracking system. Examples of these Lagrangian instruments
are the ARGO floats which were briefly mentioned at the end of section 1.2. These
floats can also be positioned at a certain density level.
2.2.1. Surface circulation
A textbook picture (Peixoto and Oort, 1992) of the surface ocean circulation is plotted in Fig. 2.4a together with a snapshot of a surface
speed plot (Fig. 2.4b) computed with the high-resolution NLOM model (see
http://www7320.nrlssc.navy.mil/global nlom32/skill.html). As could be expected, the surface currents follow the patterns of the annual mean winds
(Fig. 2.1a), with westward currents near the equator and eastward currents at midlatitudes. This gives the characteristic ‘gyre’ flows in the Atlantic and the Pacific,
consisting of a subpolar gyre and a subtropical gyre. Strong poleward currents appear at the western side of each ocean basin with the Gulf Stream, the Kuroshio,
the Brazil Current and the Agulhas Current being pronounced examples.
A short overview of volume transports of some major currents in the ocean is
provided in Table 2.1. These transports are measured in Sverdrup (SV) where 1 Sv
=10 6 m 3 s −1 , or about one million ‘bathtubs’ per second. The zonal width of these
currents is relative small compared to the zonal basin scale (Fig. 2.4b). At the
eastern side of each basin are less strong equatorward currents with the California
Current, the Canary Current and the Benguela Current as clear examples.
In
Ex. 2.4
the equatorial region there are strong currents both in the Pacific and Atlantic: the
South Equatorial Currents and North Equatorial Currents (Fig. 2.4). North of the
equator (at about 4 ◦ N), there is the Equatorial Countercurrent that is directed in
the opposite direction to the wind.
Finally, the Antarctic Circumpolar Current (ACC) is the dominant current in
the Southern Hemisphere and a comparable current does not exist in the Northern
33
Additional Material
D: To become more familiar with the forcing fields and their variability it is recommended to explore several data sets available at
http://iridl.ldeo.columbia.edu/SOURCES/. Elementary statistical analysis can
be performed on many of these data sets at Geert Jan van Oldenborgh’s Climate Explorer, available at http://climexp.knmi.nl/.
2.2. Ocean circulation
Apart from hydrographic measurements from which current information can be
extracted (we will see later how this is done), there are two methods to measure
currents directly. One of these methods is measurement of the Eulerian velocity,
for example with a current meter along a rope in the water column. The other is the
measurement of the velocity of an object that moves with the currents using, for
example, a satellite tracking system. Examples of these Lagrangian instruments
are the ARGO floats which were briefly mentioned at the end of section 1.2. These
floats can also be positioned at a certain density level.
2.2.1. Surface circulation
A textbook picture (Peixoto and Oort, 1992) of the surface ocean circulation is plotted in Fig. 2.4a together with a snapshot of a surface
speed plot (Fig. 2.4b) computed with the high-resolution NLOM model (see
http://www7320.nrlssc.navy.mil/global nlom32/skill.html). As could be expected, the surface currents follow the patterns of the annual mean winds
(Fig. 2.1a), with westward currents near the equator and eastward currents at midlatitudes. This gives the characteristic ‘gyre’ flows in the Atlantic and the Pacific,
consisting of a subpolar gyre and a subtropical gyre. Strong poleward currents appear at the western side of each ocean basin with the Gulf Stream, the Kuroshio,
the Brazil Current and the Agulhas Current being pronounced examples.
A short overview of volume transports of some major currents in the ocean is
provided in Table 2.1. These transports are measured in Sverdrup (SV) where 1 Sv
=10 6 m 3 s −1 , or about one million ‘bathtubs’ per second. The zonal width of these
currents is relative small compared to the zonal basin scale (Fig. 2.4b). At the
eastern side of each basin are less strong equatorward currents with the California
Current, the Canary Current and the Benguela Current as clear examples.
In
Ex. 2.4
the equatorial region there are strong currents both in the Pacific and Atlantic: the
South Equatorial Currents and North Equatorial Currents (Fig. 2.4). North of the
equator (at about 4 ◦ N), there is the Equatorial Countercurrent that is directed in
the opposite direction to the wind.
Finally, the Antarctic Circumpolar Current (ACC) is the dominant current in
the Southern Hemisphere and a comparable current does not exist in the Northern