Effects of Currents
III
4.3 Effects of Currents
4.3.1 Surface Currents. Of all ocean currents, the Gulf Stream is perhaps the most
familiar (Fig. 4. 14a). It is the largest and most important current of the Nothern
Hemisphere, and is mighty indeed, transporting nearly one hundred million cubic
meters per second (100 x 10 6 m\ For comparison, extraordinary peak floods of the
Mississippi might carry as much as fifty thousand cubic meters per second (50 x 10 3
m 3 /s) - one half of a thousandth of the Gulf Stream transport.
The Gulf Stream, of course, is part of the great North Atlantic gyre, complete with
west wind drift in the north, trade wind currents in the south, and an eastern boundary
current (the Canary Current) connecting the two. The center of the gyre is near 30°
N, in the Sargasso Sea. There are five such gyres on the globe: North and South
Atlantic, Nqrth and South Pacific, and southern Indian Ocean. In each case, the west
winds and the trade winds are the driving force, and the western and eastern boundary
currents complete the circle (Fig. 4.14b, c). Normally, the currents of the open ocean
reach down to about 100 to 200 m, and their velocities are low: a fraction of a knot
(a knot, a nautical mile per hour, is close to 0.5 m/s). The Gulf Stream, however, and
other fast narrow boundary currents (e. g. Kuroshio off Japan) reach to a depth of
some 1000 m and have velocities of a couple of knots or so (- 100 cm/s). The Florida
Current, which issues from the Gulf through the narrow Florida Straits, reaches
velocities of 6 knots (300 cm/s).
The general outlines of the gyral circulation have been known for some time.
Present research including remote sensing of surface temperatures from satellites,
focuses on the meanders within currents like the Gulf Stream, on the eddies separating from such a current, and on the mechanisms of mixing with the surrounding
ocean.
The surface currents have profound geologic effects. They strongly influence
weather and climate, by transport of heat and moisture. They also write the record of
climatic changes on the sea floor, by controlling the production of biogenous
deposits. Planktonic organisms, which mostly live in the upper few hundred meters,
can actually be used as tracers of currents, much like drift bottles. Those planktonic
organisms which form hard parts, of course, can trace out the path of surface currents
on the sea floor, for the geologic record.
Currents, on the whole, run parallel to isotherms, and are strongest where temperature gradients are strongest. The reason is that temperature distributions are largely
congruent to density distributions, which in tum are generally in equilibrium with
currents. From these rules, ancient currents can be mapped from a reconstruction of
the temperature field, based on plankton remains (see Sect. 7.2.1).
Fig. 4.13 a-c. Defense against surf action. Westerland, Sylt Island, German North Sea. a Power of
storm surf (Feb. IS, 1962) demonstrated by damage to beach wall and transport of heavy tetrapods.
b Gently sloping wall breaks the power of storm tide breakers (fall 1961). c Interlocking tetrapods
serve to dissipate wave energy before it hits the wall. [Photos E. S. a and 1. Newig b, c]
III
4.3 Effects of Currents
4.3.1 Surface Currents. Of all ocean currents, the Gulf Stream is perhaps the most
familiar (Fig. 4. 14a). It is the largest and most important current of the Nothern
Hemisphere, and is mighty indeed, transporting nearly one hundred million cubic
meters per second (100 x 10 6 m\ For comparison, extraordinary peak floods of the
Mississippi might carry as much as fifty thousand cubic meters per second (50 x 10 3
m 3 /s) - one half of a thousandth of the Gulf Stream transport.
The Gulf Stream, of course, is part of the great North Atlantic gyre, complete with
west wind drift in the north, trade wind currents in the south, and an eastern boundary
current (the Canary Current) connecting the two. The center of the gyre is near 30°
N, in the Sargasso Sea. There are five such gyres on the globe: North and South
Atlantic, Nqrth and South Pacific, and southern Indian Ocean. In each case, the west
winds and the trade winds are the driving force, and the western and eastern boundary
currents complete the circle (Fig. 4.14b, c). Normally, the currents of the open ocean
reach down to about 100 to 200 m, and their velocities are low: a fraction of a knot
(a knot, a nautical mile per hour, is close to 0.5 m/s). The Gulf Stream, however, and
other fast narrow boundary currents (e. g. Kuroshio off Japan) reach to a depth of
some 1000 m and have velocities of a couple of knots or so (- 100 cm/s). The Florida
Current, which issues from the Gulf through the narrow Florida Straits, reaches
velocities of 6 knots (300 cm/s).
The general outlines of the gyral circulation have been known for some time.
Present research including remote sensing of surface temperatures from satellites,
focuses on the meanders within currents like the Gulf Stream, on the eddies separating from such a current, and on the mechanisms of mixing with the surrounding
ocean.
The surface currents have profound geologic effects. They strongly influence
weather and climate, by transport of heat and moisture. They also write the record of
climatic changes on the sea floor, by controlling the production of biogenous
deposits. Planktonic organisms, which mostly live in the upper few hundred meters,
can actually be used as tracers of currents, much like drift bottles. Those planktonic
organisms which form hard parts, of course, can trace out the path of surface currents
on the sea floor, for the geologic record.
Currents, on the whole, run parallel to isotherms, and are strongest where temperature gradients are strongest. The reason is that temperature distributions are largely
congruent to density distributions, which in tum are generally in equilibrium with
currents. From these rules, ancient currents can be mapped from a reconstruction of
the temperature field, based on plankton remains (see Sect. 7.2.1).
Fig. 4.13 a-c. Defense against surf action. Westerland, Sylt Island, German North Sea. a Power of
storm surf (Feb. IS, 1962) demonstrated by damage to beach wall and transport of heavy tetrapods.
b Gently sloping wall breaks the power of storm tide breakers (fall 1961). c Interlocking tetrapods
serve to dissipate wave energy before it hits the wall. [Photos E. S. a and 1. Newig b, c]