be evident to those living in coastal areas. Because of its timing, the new moon in December nearly coincides with
perigee--when the moon makes its closest approach to the earth--and some of the highest and lowest tides of the
year are expected.
During new and full phase, the moon and sun align with the earth, so that the gravitational pull of both bodies are
combined. Although the tidal effects of the sun are only 44 percent as strong as those of the moon, when the two
are added together, they produce significantly higher tides, called spring fides (a term that has nothing to do with
the spring season but rather with the water's appearing to "spring" from the earth).
Tides and Forces of Gravity
All bodies of water, large or small, are subject to the tide-producing forces of the sun and moon. (Tides also occur
in the solid earth and in the atmosphere, but the displacements are small and can only be detected by sensitive
instruments.) In inland bodies of water, the regular rise and fall of the tide is so small that it is completely masked
by water-level changes caused by wind and weather.
Lake Superior, for example, has a tide that rises and falls only about two inches. Only along coasts where oceans
and continents meet are tides great enough to be noticed.
Such effects are most pronounced where funnel-shaped estuaries and bays magnify tidal effects. In Canada's Bay
of Fundy, for instance, the difference between high and low tide is sometimes more than fifty feet.
The "perigean spring tides" which are very high can lead to coastal flooding, especially if they happen to be
accompanied by a storm with winds blowing water onshore.
If, for example, strong east-to-northeast winds occur along the East Coast of the United States on or near
December 22, widespread flooding can take place, especially in low-lying areas.
Tidal swelling occurs twice a day on both sides of the earth, once when the moon passes overhead, and once
when the moon is on the opposite side of the earth. Tidal forces have an appreciable affect only on large bodies--
such as oceans--and this explains why soup doesn't spill over the sides of the bowl when the moon is full.
moon are in a line (during full or new moon phase) the combined force produces the higher than normal "spring"
tides in certain areas. The effects are even more amplified when the moon is at perigee, as it will be this month.
(Although the moon's distance from apogee to perigee varies only from 9 to 14 percent, tidal influences can be 30
to 48 percent greater.
The resulting high tides (which usually peak one or two days after perigee because of "gravitation lag") can cause
coastal flooding, and some scientists have suggested that the chances of earthquakes and volcanic eruptions may
also be slightly increased.
The positions and distances the earth, moon and sun all have an effect on the magnitude and size of the two tidal
Scientists have found that the actual speed and height of tides are affected not only by the moon but also by land
masses, water depth, winds, and barometric pressure. Tides typically range from three to six feet, but some areas
show no tides at all, and others, such as the Bay of Fundy, have tides of more than thirty feet. If the barometer
drops by one inch, the seas can rise by a foot.
A storm can have an even larger effect; when strong winds are blowing ashore, water can pile up against the
coast, turning a high-tide-perigee coincidence into a disaster.
The tides do more than merely cause our coastal area authorities to post notices on the beaches. They also keep
our earth-moon system evolving. Long ago, when the moon and earth were closer, the earth's powerful tidal
effects gradually brought the moon's rotation into agreement with its orbital period, so that we never see its far
Partly because of tidal action, the rotation of the earth is gradually slowing down--by about one second every
50,000 years. This causes the moon to speed up its revolution about our planet, which in turn, causes the moon to
spiral slowly away from the earth--at a rate of about one and a half inches a year.
What drives ocean circulation? Mostly the moon, although the there is some effect from the sun, its great distance
reduces its gravitational force on the tides.
Tides Mix Ocean Waters
The oceans are in constant motion
Winds whip surface waters into major currents and the North Atlantic Ocean is like a wet conveyor belt, with cold
water constantly sinking in the polar regions and then traveling, deep in the ocean, back toward the tropics.
The ocean’s continual movement is essential to the life of its inhabitants. Colder, deeper water must mix with
warmer surface waters--otherwise, almost all the ocean would become cold and Earth's climate would be
What watery spoon stirs the deep sea--and how?
Scientists at the Scripps Institution of Oceanography in La Jolla, California began to search for an understanding of
the power behind ocean circulation.
researchers simply assume that ocean layers sort themselves out according to differences in their temperature and
density. With few actual measurements at their fingertips, however, researchers have been forced to crunch
equations and, basically, guess at what drives the deep.
That may now be starting to change. Armed with evolving models, finer satellite images, and a better
understanding of the topsy-turvy seas, scientists are finding some surprises in the seascape.
It is now believed that the moon's orbital energy causes the far-reaching ocean tides that mix the open ocean's
warm and cold layers.
This moony mix-up helps drive ocean circulation, making the moon's relevance to ocean circulation as central.
The orbiting moon dumps energy into the world's oceans, causing the telltale ocean tides that make beachgoers
inch backward as the day progresses. Most researchers had assumed that these tides only stir shallow seas. For
instance, tidal waves constantly crash against the continental shelf, losing energy and rolling back out to sea.
By contrast, the deep sea has always appeared calmer, with little tidal turbulence to mix the water. It is now
estimated that 25 to 30 percent of the total tidal energy deposited into the ocean is released into the deep sea.
Scientists used sea-surface data from a U.S.-French satellite to measure the height of daily tides by bouncing radar
beams off the ocean surface. Using 7 years of TOPEX/Poseidon data scientists mapped tidal currents, watching
where they scattered into underwater tides and mixing into deep water.
The mapping told scientists that open-ocean tides are spilling much energy--not all over but in hot spots, where
tidal currents smash into undersea mountains and midocean ridges that jut from the seafloor.
The stratified layers of ocean water--warm and light above, cold and heavy below--travel the oceans separately.
When tides in the open ocean crash into an undersea mountain, however, some of the cold, deep water lurches
into the upper ocean. The chilled water then mixes with surrounding warm water as it sinks back down again.
Repeated over and over in localized spots, this mixing might be enough to lighten and warm all that cold, deep
water flowing along the Atlantic's conveyor belt from the poles back toward the equator.
Some of the mixing in the deep sea is controlled by wind, but a good half of it is driven by the moon's motion.
The deep sea isn't just a workshop for oceanographers--it's also one of Earth's major tools for climate control.
Along with the atmosphere, oceans even out the climate, shuttling heat between the equator and the poles. Without
this circulation, temperatures would skyrocket in the tropics and plummet at higher latitudes like Europe.
Some researchers worry that global warming will throw this circulation off, changing local climates-- or even shut
it down entirely, jerking the globe into the next ice age.
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Ocean Tides by The Sun Maker Publishing House (Jul 7, 2012)
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