5.2 Water Circulation
Wind-Driven Circulation
Wind blowing over the surface of a standing water
body causes both waves and motion of the surface
water. In the open ocean, waves transport energy but
not water masses (see Sect. 3.1). Hence in this case,
we must deal only with the second consequence of
atmospheric circulation: wind-generated currents.
On a gross scale, wind-driven currents reflect the
global wind pattern and only move surface waters.
However, surface currents are modified by the ocean
basin boundaries and by Coriolis forces. The combined effect ofthese factors is shown in Fig. 5.le for
a hypothetical rectangular ocean resembling the
present-day Atlantic Ocean.
Trade wind belts to the north and south of the equator give
rise to a north equatorial and south equatorial current flowing westward. Near the coast they turn either to the north or
to the south until they reach the west wind belts in the midlatitude regions. Then the surface waters flow eastward,
until they are divided into two branches, a northeast current
and a southeast current. In the northem hemisphere, the
southeast current turns south along the eastem continental
margin and finally feeds into the north equatorial current,
thus forming a large gyre with clockwise water circulation.
In the southem hemisphere, similarly a counter-clockwise
gyre is completed. Those parts of the eastward-directed
mid-latitude currents which turn poleward can again form
gyres, a counter-clockwise gyre in the northem hemisphere
and a clockwise gyre in the southem hemisphere.
Another conspicuous phenomenon, related to the rotation of the Earth, is that the currents become narrower and
faster on the west side of an ocean, where the water is piled
up somewhat before it flows back in other directions, both
in the northem and southem hemisphere. Between the
westward-flowing equatorial currents, an eastward-directed
counter-current may develop.
This idealized current pattern in a rectangular ocean is,
of course, substantially modified by deviating atmospheric
circulation and, in particular, by the irregular topography
of a single or several connected ocean basins. The present
South Atlantic, for example, is openly connected with the
other large oceans. Therefore, water rnasses flowing southward feed into an Antarctic circumpolar current system,
where large volumes ofwater sink due to cooling and initiate thermohaline bottom circulation flowing northward
(Antarctic bottom water).
Due to the Coriolis force, surface current directions
deviate from those of the wind (Fig. 5.2b). In the
northern hemisphere, the Coriolis force deflects
wind-driven currents to the right, in the southern
hemisphere to the left. The clockwise gyre north of
the equator and the counter-clockwise gyre south of
the equator reflect this mechanism (Fig. 5.le).
At the surface of a deep water body, the angle between the
directions of wind and current may be up to 45° (Fig.
5.2b), in shallow water less. Below the surface, each layer
ofwater is dragged along by the layer above it. Hence, due
to the Coriolis force, each sublayer flows to the right ofthe
overlying layer. Since the Coriolis deflection increases with
187
slower current speeds, the slower moving deeper waters
turn even more to the right than the surface currents. The
resulting distribution of current directions at different
depths is called the Eckman spiral. The direction of net
water transport over all depths is 90° to the right of the
wind direction. In shallow waters the deflection of net water transport is less than 90°.
Combined Effects of Thermohaline and
Wind-Driven Circulation
The phenomenon of the Eckman spiral clearly demonstrates that wind-driven surface currents and
thermohaline circulation, prevailing in deeper waters,
cannot act independently. Ocean currents are the result of the combined effects of both factors. Wind
primarily causes horizontal motion, whereas the
thermohaline effects primarily generate motion with
a vertical component. Combined horizontal surface
currents can locally accelerate vertical motion
(downwelling and upwelling) and thus intensify deep
bottom currents. However, regions with substantial
exchange between the upper water layer (300 to 1000
m deep including the thermocline) and deep water
are limited in extent. Much larger areas in the ocean
display little exchange between these two water
masses.
In the open ocean, currents generated by easterly
winds near the equator tend, due to the Coriolis effect, to diverge to the north or to the south of the
equator (Fig. 5.2c). The resulting equatorial divergence of surface waters leads to upwelling of deeper
water along this zone (Fig. 5.1e). The same phenomenon is created at the boundary of oppositely directed
currents if, for example, in the northern hemisphere
the more northerly current flows westward (Fig.
5.2c).
In contrast, converging surface currents tend to
pile up water and thus generate a downward motion
(downwelling). The most prominent example in the
present-day oceans is the subtropical convergence
and the Antarctic polar front in the southern hemisphere, where eastward-directed currents are forced
to converge. Here, as weil as in northern polar regions, downwelling is intensified by the cooling of
surface waters. Downward moving water reaches,
according to its density, intermediate depths or the
sea bottom and feeds into relatively slow subsurface
or bottom currents. These currents are also subjected
to the Coriolis effect and thus tend to turn to the right
in the northern hemisphere and to the left in the
southern hemisphere.
Of particular interest to sedimentologists are bottom currents, because they may be able, at least 10cally, to transport sedimentary particles and even to
rework silty to fine-sandy sediments. Equator-directed bottom currents, derived from downwelling
high-Iatitude waters, travel preferentially on the west-
Wind-Driven Circulation
Wind blowing over the surface of a standing water
body causes both waves and motion of the surface
water. In the open ocean, waves transport energy but
not water masses (see Sect. 3.1). Hence in this case,
we must deal only with the second consequence of
atmospheric circulation: wind-generated currents.
On a gross scale, wind-driven currents reflect the
global wind pattern and only move surface waters.
However, surface currents are modified by the ocean
basin boundaries and by Coriolis forces. The combined effect ofthese factors is shown in Fig. 5.le for
a hypothetical rectangular ocean resembling the
present-day Atlantic Ocean.
Trade wind belts to the north and south of the equator give
rise to a north equatorial and south equatorial current flowing westward. Near the coast they turn either to the north or
to the south until they reach the west wind belts in the midlatitude regions. Then the surface waters flow eastward,
until they are divided into two branches, a northeast current
and a southeast current. In the northem hemisphere, the
southeast current turns south along the eastem continental
margin and finally feeds into the north equatorial current,
thus forming a large gyre with clockwise water circulation.
In the southem hemisphere, similarly a counter-clockwise
gyre is completed. Those parts of the eastward-directed
mid-latitude currents which turn poleward can again form
gyres, a counter-clockwise gyre in the northem hemisphere
and a clockwise gyre in the southem hemisphere.
Another conspicuous phenomenon, related to the rotation of the Earth, is that the currents become narrower and
faster on the west side of an ocean, where the water is piled
up somewhat before it flows back in other directions, both
in the northem and southem hemisphere. Between the
westward-flowing equatorial currents, an eastward-directed
counter-current may develop.
This idealized current pattern in a rectangular ocean is,
of course, substantially modified by deviating atmospheric
circulation and, in particular, by the irregular topography
of a single or several connected ocean basins. The present
South Atlantic, for example, is openly connected with the
other large oceans. Therefore, water rnasses flowing southward feed into an Antarctic circumpolar current system,
where large volumes ofwater sink due to cooling and initiate thermohaline bottom circulation flowing northward
(Antarctic bottom water).
Due to the Coriolis force, surface current directions
deviate from those of the wind (Fig. 5.2b). In the
northern hemisphere, the Coriolis force deflects
wind-driven currents to the right, in the southern
hemisphere to the left. The clockwise gyre north of
the equator and the counter-clockwise gyre south of
the equator reflect this mechanism (Fig. 5.le).
At the surface of a deep water body, the angle between the
directions of wind and current may be up to 45° (Fig.
5.2b), in shallow water less. Below the surface, each layer
ofwater is dragged along by the layer above it. Hence, due
to the Coriolis force, each sublayer flows to the right ofthe
overlying layer. Since the Coriolis deflection increases with
187
slower current speeds, the slower moving deeper waters
turn even more to the right than the surface currents. The
resulting distribution of current directions at different
depths is called the Eckman spiral. The direction of net
water transport over all depths is 90° to the right of the
wind direction. In shallow waters the deflection of net water transport is less than 90°.
Combined Effects of Thermohaline and
Wind-Driven Circulation
The phenomenon of the Eckman spiral clearly demonstrates that wind-driven surface currents and
thermohaline circulation, prevailing in deeper waters,
cannot act independently. Ocean currents are the result of the combined effects of both factors. Wind
primarily causes horizontal motion, whereas the
thermohaline effects primarily generate motion with
a vertical component. Combined horizontal surface
currents can locally accelerate vertical motion
(downwelling and upwelling) and thus intensify deep
bottom currents. However, regions with substantial
exchange between the upper water layer (300 to 1000
m deep including the thermocline) and deep water
are limited in extent. Much larger areas in the ocean
display little exchange between these two water
masses.
In the open ocean, currents generated by easterly
winds near the equator tend, due to the Coriolis effect, to diverge to the north or to the south of the
equator (Fig. 5.2c). The resulting equatorial divergence of surface waters leads to upwelling of deeper
water along this zone (Fig. 5.1e). The same phenomenon is created at the boundary of oppositely directed
currents if, for example, in the northern hemisphere
the more northerly current flows westward (Fig.
5.2c).
In contrast, converging surface currents tend to
pile up water and thus generate a downward motion
(downwelling). The most prominent example in the
present-day oceans is the subtropical convergence
and the Antarctic polar front in the southern hemisphere, where eastward-directed currents are forced
to converge. Here, as weil as in northern polar regions, downwelling is intensified by the cooling of
surface waters. Downward moving water reaches,
according to its density, intermediate depths or the
sea bottom and feeds into relatively slow subsurface
or bottom currents. These currents are also subjected
to the Coriolis effect and thus tend to turn to the right
in the northern hemisphere and to the left in the
southern hemisphere.
Of particular interest to sedimentologists are bottom currents, because they may be able, at least 10cally, to transport sedimentary particles and even to
rework silty to fine-sandy sediments. Equator-directed bottom currents, derived from downwelling
high-Iatitude waters, travel preferentially on the west-