Page 225 - NS-2 Textbook

220 NAUTICAL SCIENCES

Philippines and into the South China Sea and evenhwlly directions. As a result, there is an uplvard h"a11Sfer of rich
into Vietnanl. As the ""varm Ktuoshio Current spreads nutrients, which is responsible for large numbers of fish
out north of Japan, it passes south of but close to the in these regions. These distinct layers of water also influ-
cold Oyashio Current coming out of the Bering Sea. ence the h'ansmission of underwater sOlmds, an impor-
The Kuroshio Current travels eashvard across the North tant consideration in undersea ,variare.
Pacific and splits into two branches. One of these
branches is the Alaskan Current, which travels counter- TIDES
clockwise around the Gulf of Alaska and weshvard south
of the Aleutian Islands. The other branch becomes the Earth's nearest neighbor in space, the Moon, is the main
California Current ,vhich travels southward along the cause of the rise and fall of ocean tides. Anyone who has
west coast of the United States. lived by or visited an ocean shore has seen the ebb and
Subslilface or COlilliercurreu/s. While the frictional flow of the tide twice daily. The ancient Greeks first rec-
force of Earth's winds sets the major surface currents of ognized the relationship between the tides and the
the ,varld in motion, a COtulterforce caused by gravity Moon's Inonthly moven1ent afolmd Earth. It was not
and the Coriol1s effect, particularly in higher latitudes, tlltil Sir Isaac Newton (1642-1727) worked out his theory
often creates an opposite Ination in the deeper 'vater of gravity in 1687, howevel~ that this relationship could
layers. be explained.
Near the equator, the deepest water may be moving Science has determined that everything in the uni-
exactly 180 degrees (opposite) from the surface flow. This verse exerts a gravitational force or pull on everything
amazing phenomenon was discovered in 1952 by else. The pull of gravity is very small for small objects,
Townsend Cromwell, a scientist working with the u.s. but for a planet, mOOll, or star, the force is enormous,
Fish and Wildlife Sen'ice. He was experimenting with tending to pull every other object into its own center of
deep-sea fishing teclmiques. gravity. The mass (amOlU1t of material) of the body and
Letting down long lines into the South Equatorial the distance it is from the other object or body determine
Cunent in the Pacific Ocean, a ·west-flowing current the gravitational effect. It is gravity that holds the planets
Townsend discovered that the lines drifted eastward. in their orbits arOllld the Stll and keeps the Moon and
TIlis indicated the existence of a strong undercurrent. Earth "tied" together as cOlnpanions in space.
Later T€search showed that this undercurrent, or COU1lter- The pull of the Moon's gravity causes the oceans on
currellt, proceeds 3,500 miles to the Galapagos Islands off the Moon's side of Earth to bulge out toward it. TI,e
EcuadOl~ carrying 30 million tons of water eashvard gravitational pull, however, is not the same everf"\,here.
every second. The points of Earth closer to the Moon are pulled more
In 1955 oceanographer Henry Stornrnel theorized strongly, and those farther away are pulled less. This ef-
that a countercurrent flowed beneath the Gulf Stream. 10 fect, in addition to an outward centrifugal force on the
1957 the combined United Kingdom-United States Inter- far side of Earth caused by the rotation of the Earth-
national Geophysical Year (IGY) investigation proved Moon systenl about their cornman center of gravity,
that Stammel's theory ,vas correct. causes the water on the far side of Earth to bulge out-
The oceanographers used in their investigations a 'ward as "well, though not as much as on the near side.
floating underwater device called the Swallow buoy. In- The Stm also causes tides, but this effect is only about
vented by Dr. Jolm Swallow, this equipment can be made two-fifths as strong as that caused by the Moon. Though
to free-float while remaining at any chosen depth. It car- it is of course much more nlassive than the Moon, the
ries a simple "beeper" or "pingel''' that sends out elec- Stll'S effect on tides is smaller because it is 390 times far-
tronic signals that can be picked up by a receiver aboard ther away.
ship. Using Swallow buoys at different depths, oceanog- TIle variations in position of the Slill and Moon in re-
raphers found that the Gulf Stream surface current lation to Earth produce the high and low ranges of tides.
moves about 100 miles a day northeashvard, ,vrue at At times of the new and full moons, the tides are highest
depths from 1,350 to 1,500 fathoms, cOlmtercurrents and lowest because the forces of the Moon and StU1 are
move in the opposite direction about I» to 15 miles per working together. The result is spring tides. (The term has
day. Just above the ocean floor at 1,750 fathoms, the nothing to do with the spring season.) Halfway between
countercurrent was found to move 2}2 miles a day in the the new and full moons, "when ,ve see the half moon dur-
opposite direction. ing the first and third quarters, the tidal forces of the
The different directions of motion and speed of the Moon and Sun are opposed. At this time the difference
surface and the countercurrents create a turbulence be- between high and low tides is much less. These are called
tween the two layers of ·water, resulting in considerable Ileap tides.
vertical mixing. TIUs Inixing is particularly strong at the The ebb of a tide is the fall of the tide, that is, the mov-
equatol~ ,'vhere the hvo currents travel in nearly opposite ing of the tide away from the shore. TI,e flood of the tide