What Causes the Tides and How Do They Influence Marine Life


    Although the new moon may be invisible to us, lost in the blinding rays of the sun, its effects in December should
    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.
















    The sun's gravitational pull on the earth is roughly half that of the moon's, but when the sun, the earth, and the
    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
    bulges.  















    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
    side.

    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
    strikingly different.

    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.











    The deep sea makes an inconvenient workbench; it's tricky to track water circling the globe so far down. Some
    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.

    References
    Bowermaster, J. Oceans: The Threats to Our Seas and What You Can Do to Turn the Tide. 2010.  
    Ocean Tides by The Sun Maker Publishing House (Jul 7, 2012)
    Oba, D. Ocean Tide. 2011.
    Pedlosky, J. Waves in the Ocean and Atmosphere: Introduction to Wave Dynamics. 2010.
    Ricketts, E. Between Pacific Tides. 1992.