Ocean water is constantly in motion: north-south, east-west, alongshore, and vertically. Seawater motions are the result of waves, tides, and currents (Figure below). Ocean movements are the consequence of many separate factors: wind, tides, Coriolis effect, water density differences, and the shape of the ocean basins. Water movements and their causes will be discussed in this lesson.
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Ocean waves transfer energy through the water over great distances.
Waves have been discussed in previous chapters in several contexts: seismic waves traveling through the planet, sound waves traveling through seawater, and ocean waves eroding beaches. Waves transfer energy and the size of a wave and the distance it travels depends on the amount of energy that it carries.
This lesson studies the most familiar waves, those on the ocean’s surface. Ocean waves originate from wind blowing – steady winds or high storm winds – over the water. Sometimes these winds are far from where the ocean waves are seen. What factors create the largest ocean waves?
The largest wind waves form when the windis very strongblows steadily for a long timeblows over a long distance
The wind could be strong, but if it gusts for just a short time, large waves won’t form.
Wind blowing across the water transfers energy to that water. The energy first creates tiny ripples that create an uneven surface for the wind to catch so that it may create larger waves. These waves travel across the ocean out of the area where the wind is blowing.
Remember that a wave is a transfer of energy. Do you think the same molecules of water that starts out in a wave in the middle of the ocean later arrive at the shore?
Water molecules in waves make circles or ellipses (Figure below). Energy transfers between molecules but the molecules themselves mostly bob up and down in place.
In this animation, a water bottle bobs in place like a water molecule: http://www.onr.navy.mil/focus/ocean/motion/waves1.htm
An animation of motion in wind waves from the Scripps Institution of Oceanography:http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_wind.html
The circles show the motion of a water molecule in a wind wave. Wave energy is greatest at the surface and decreases with depth. A shows that a water molecule travels in a circular motion in deep water. B shows that molecules in shallow water travel in an elliptical path because of the ocean bottom.
An animation of a deep water wave is seen here: http://en.wikipedia.org/wiki/File:Deep_water_wave.gif.
An animation of a shallow water wave is seen here: http://commons.wikimedia.org/wiki/File:Shallow_water_wave.gif.
When does a wave break? Do waves only break when they reach shore? Waves break when they become too tall to be supported by their base. This can happen at sea but happens predictably as a wave moves up a shore. The energy at the bottom of the wave is lost by friction with the ground so that the bottom of the wave slows down but the top of the wave continues at the same speed. The crest falls over and crashes down.
Local Surface Currents
The surface currents described above are all large and unchanging. Local surface currents are also found along shorelines (Figure below). Two are longshore currents and rip currents.
Longshore currents move water and sediment parallel to the shore in the direction of the prevailing local winds.
Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Look at the rip-current animation to determine what to do if you are caught in a rip current: http://www.onr.navy.mil/focus/ocean/motion/currents2.htm. Each summer in the United States at least a few people die when they are caught in rip currents.
This animation shows the surface currents in the Caribbean, the Gulf of Mexico, and the Atlantic Ocean off of the southeastern United States:http://polar.ncep.noaa.gov/ofs/viewer.shtml?-gulfmex-cur-0-large-rundate=latest.
Wave Action and Erosion
Have you ever been to visit a beach? Some beaches have large, strong rolling waves that rise up and collapse as they crash into the shore. All waves are energy traveling through some type of material (Figure 10.13). The waves that we are most familiar with travel through water. Most of these waves form from wind blowing over the water; sometimes steady winds that blow and sometimes from a storm that forms over the water. The energy of waves does the work of erosion when a wave reaches the shore. When you find a piece of frosted glass along a beach, you have found some evidence of the work of waves. What other evidence might you find?
As wind blows over the surface of the water, it disturbs the water, producing the familiar shape of a wave. You can see this shape in Figure 10.14. The highest part of a wave is called the wave crest. The lowest part is called the wave trough. The vertical distance from the highest part of a wave to the lowest is called the wave height. The horizontal distance between one wave crest and the next crest, is called the wavelength. Three things influence how big a wave might get. If the wind is very strong, and it blows steadily for a long time over a long distance, the very largest waves will form. The wind could be strong, but if it gusts for just a short time, large waves won’t form. Bigger waves do more work of erosion which changes our shorelines. Each day that waves break along the shore, they steadily erode away a minute bit of the shoreline. When one day, a really big storm like a hurricane arrives, it will do a lot of damage in just a very short time.
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As waves come into shore, they usually reach the shore at some angle. This means one part of the wave reaches shallow water sooner than the parts of the wave that are further out. As a wave comes into shore, the water ‘feels’ the bottom which slows down the wave. So the shallower parts of the wave slow down more than the parts that are further from the shore. This makes the wave ‘bend’, which is called refraction. The way that waves bend as they come into shore either concentrates wave energy or disperses it. In quiet water areas like bays, wave energy is dispersed and sand gets deposited. Areas like cliffs that stick out into the water, are eroded away by the strong wave energy that concentrates its power on the cliff (Figure 10.15).