Chapter 13 — Movements of Ocean Water

The three types are waves (horizontal energy movement across the surface), tides (periodic vertical rise and fall of sea level), and ocean currents (continuous horizontal flow of large water volumes in a definite direction).

Wind provides energy to waves and causes them to travel across the ocean. The energy is eventually released on shorelines. Waves are actually energy moving across the surface — water particles themselves only travel in small circles as a wave passes.

Wave height is the vertical distance from the bottom of a trough to the top of a crest. Wave amplitude is one-half of the wave height.

Tides are caused mainly by the moon's gravitational pull and, to a lesser extent, the sun's gravitational pull. Centrifugal force also plays a role. Together, gravitational attraction and centrifugal force create two tidal bulges on opposite sides of Earth.

Spring tides occur when the sun, moon, and Earth are in a straight line, producing higher-than-normal tides. They occur twice a month — once during the full moon period and once during the new moon period.

Neap tides occur when the sun and moon are at right angles to each other, so their gravitational forces counteract one another. There is normally a seven-day interval between spring tides and neap tides, and tidal ranges during neap tides are lower than average.

Semi-diurnal tides feature two high tides and two low tides each day of approximately the same height. Diurnal tides have only one high tide and one low tide per day. Mixed tides show variations in height and generally occur along the west coast of North America and Pacific islands.

The primary forces are: heating by solar energy (which causes water near the equator to expand and creates a slight gradient), wind (which pushes surface water), gravity (which pulls water downslope), and the Coriolis force (which deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere).

Cold currents bring cold water into warm water areas and are usually found on the west coasts of continents in low and middle latitudes. Warm currents bring warm water into cold water areas and are usually found on the east coasts of continents in low and middle latitudes.

Tides can be predicted well in advance, helping navigators and fishermen plan their activities. Tidal heights are especially important for harbours near rivers and estuaries with shallow bars at the entrance. Tides also help in desilting sediments, removing polluted water from estuaries, and generating electrical power.

Gyres are large accumulations of water and the circular flow around them, produced when the Coriolis force deflects wind-driven surface water. They produce large circular current patterns in all ocean basins.

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