10: Tides
- Page ID
- 45480
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- CC12 The Coriolis Effect: Water masses move freely over the Earth’s surface while the solid Earth itself is constrained to move with the Earth’s rotation. This causes moving water masses, including long wavelength waves that comprise the global tide wave motion, to appear to follow curving paths across the Earth’s surface. The apparent deflection is called the Coriolis effect. Coriolis deflection can create a rotary motion of the tide wave, called amphidromic systems, within certain ocean basins. Amphidromic systems rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Most of us are familiar with the water movements that slowly expose and then cover the seaward part of the shore during a day. This rise and fall of sea level is called the tide. If we observe tidal motions long enough at one location, we will see that they are periodic. At some locations, the tide rises and falls twice during a day; at others, it rises and falls only once. The times of high and low tide vary predictably, and the height of the tide also changes somewhat from day to day. Indeed, the tidal range is often very different for the two tides on a single day. In addition, we may observe that the sea-level change between high and low tide is large in some places, while in other places there appears to be no tide at all.
Tides are important to mariners because many harbors and channels are not deep enough for vessels to navigate at low tide. Tides are an energy source that has been harnessed for electricity generation in some parts of the world. They are also important to many marine creatures, especially species that live in the intertidal zone between the high-tide line and low-tide line and must cope with alternate periods of immersion in water and exposure to air.
Tides cause tidal currents that can be very swift in coastal waters and within harbors and estuaries. In estuaries, tidal currents reverse direction as the tide rises and falls. When the current flows in from the sea, it is a flood current. When it flows out, it is an ebb current. The reversals between flood and ebb currents do not necessarily occur at high or low tide. However, where only one tide occurs each day, there is one flood and one ebb; and where two tides occur each day, there are two floods and two ebbs.
In the deep ocean, tides produce generally weaker currents than in coastal waters. However, throughout the ocean depths, tidal motions are responsible for generating currents that interact with density differences between water layers and with seafloor topography to cause internal waves, eddies, and turbulence that is a major contributor to the vertical mixing of deep layer water upwards as part of the Meridional Overturning Circulation (MOC) discussed in Chapter 8.
It has been known for more than 2000 years that tides are related to the movements of the sun and moon. However, the relationship was not fully explained until 1686, when Sir Isaac Newton published his theory of gravity. Tides are a phenomenon that are controlled entirely by a few simple physical principles, yet they can be extraordinarily complex.