TOPIC 5a - OCEAN WAVES AND TIDES

I. Ocean/Atmosphere Interactions

A. The atmosphere and oceans are two critical elements in the global climate engine. As part of the same system, they transfer energy and mass between them all the time. The transfer of energy or mass between them (and with the solid Earth and interplanetary space) can be seen as a budget that must be balanced.

B. We have already discussed the Global Heat Budget. This budget describes heat energy arriving from the Sun, traveling through the atmosphere, and reaching the oceans (and land). The heat energy reaches the ocean surface as radiation that is absorbed by water molecules. It is re-radiated into the atmosphere directly or wind interacting with the ocean surface may directly advect some of the heat and carry it away. Another way heat energy gets back into the atmosphere is through evaporation of water.

C. We have also briefly discussed the transfer of atmospheric gases into the upper ocean. Ocean currents then redistribute the gases and heat around the planet.

D. Another form of energy that is passed from the atmosphere to the oceans is direct kinetic energy which is advected as air blows across surface water. This energy transfer established important surface ocean currents in most of the world's oceans.

 

II. Water Motion at the Ocean Surface

A. Waves at sea (as well as in lakes and other standing bodies of water) are generated by wind shear at the sea/air interface.

1. The waves start as ripples. After the initial ripple is formed, the wind has an easier time building larger waves because it has a surface (of the wave) to push on.

2. The process of building up large swells depends on three factors:

a. the fetch, or open water distance, over which the wind moves.

b. the strength (speed) of the wind.

c. the persistence (duration) of the wind.

3. Wind-generated wave radiate away from source regions like radio waves from a broadcasting station. A spectrum of wave periods (different wavelengths) is transmitted.

B. Water particles within waves move in circles.

1. The direction of their circular motion indicates the overall direction of wave movement.

2. As one goes down into the water column from the sea surface, the amplitude of particle motion associated with a wave diminishes until there is no motion at all. Thus sea waves damp out at depth.

3. The depth at which a sea wave damps out depends on the amplitude and wavelength of the wave. Larger waves go deeper before they damp out.

4. Wave wavelength is the distance from wave crest to wave crest. Wave height is the distance from wave crest to wave trough. The wave period is the time required between one wave crest passing an observer and the next wave crest arriving.

 

III. Wave Motion in Shallow Water

A. In deep water, the speed of waves is a function of their wavelength. Larger waves move faster the smaller waves. This creates a condition called dispersion (separation of waves by their wavelength).

B. As waves approach the shore -

1. water particles moving in the deepest circles start running into the shallowing ocean bottom. That slows down the deepest part of the wave, but the top of the wave keeps moving along.

2. the waves slow down, wavelength decreases, wave height increases, but wave period stays the same.

3. finally the waves are high enough that gravity forces become important and breakers form. There are two types of breakers: plunging breakers and spilling breakers. Plunging breakers form when the wave is slowed suddenly (where the bottom slope is steep, for example). Spilling breakers form when the wave is slowed gradually (where the bottom slope is gentle, for example).

C. Waves do not usually approach the beach head-on with wave crests parallel to the beach, rather the lines of wave crests typically come in at an angle.

1. As the waves come in at an angle, they tend to straighten out because as the end of the wave closest to the beach hits shallow bottom and slows down, the rest of the wave keeps rolling through deeper water. This bending is called wave refraction.

2. Refraction does not completely straighten the wave out, so waves strike the beach at a bit of an angle. Then the water runs down the beach face.

3. Orthogonals are lines drawn perpendicular to wave crests. Where orthogonals converge, there is concentrated wave energy; where orthogonals diverge, there is diminished wave energy.

4. Sometimes, part of the wave is reflected back out into deeper water.

5. When waves pass through a restricted opening such a harbor entrance, they spread out inside the opening like ripples on a pond. This process is called diffraction.

D. Longshore Currents - If waves come onto a beach at an angle (not head on), the water will travel up the beach face and then slide back at a different place slightly down the beach from where it started. This is easy to see on a beach by watching the incoming waves carry swimmers and surfers and seaweed and paper cups and what-have-you with it. This movement of water along the beach is called longshore current (or littoral current), and the sand it transports along the beach is called littoral drift.

E. A rip current is a flow of water from the beach out toward the ocean. Waves move water toward the beach and the littoral current moves water along the beach. Water cannot build up against the beach indefinitely; there are rip currents at intervals along the beach that move water back out to the ocean.

 

IV. Tsunamis

A. Tsunamis are long-wavelength shock waves generated by sudden changes in sea floor level in coastal areas. This occurs as the result of

1) earthquakes; especially in the region of trenches.

2) landslides; either landslides into the ocean or submarine slumps.

3) volcanic eruptions; especially phreatic eruptions (remember Krakatoa or Tambora).

B. Many tsunamis are generated in the coastal Pacific Ocean basin.

C. Tsunamis are not true tide waves although they are often called tidal waves in newspapers.

D. Tsunamis may have wave heights of only a meter or so in the deep ocean but their wavelength may be hundreds of miles with wave periods of a few minutes to several hours.

E. Tsunamis behave like other ocean surface waves when they reach a shoreline - the waves steepen, shorten, and markedly increase in height, in some cases several 10's of meters when they break.

F. Tsunami warning systems have been organized in the Pacific basin to warn coastal regions of potential dangers after major earthquakes or other geologic events.

 

V. Tides

A. The tides represent a periodic rise and fall of local sea level over a 25-hour cycle. (25-hour period reflects period of Earth's rotation (24 hours) plus time needed to catch up with continuing monthly rotation of moon.)

1. The tides are driven by the gravitational attraction of the Sun and the Moon with respect to the Earth. Because the atmosphere and oceans are fluids, they respond more easily to the slightly varying gravity which occurs as the three bodies move with respect to each other. The gravitation attraction causes water in the Earth's oceans to constantly move toward those objects. This creates high tides as the objects pass overhead each day.

2. At the same time, the Earth's rotation wants to spin things off the Earth - this is called centrifugal force. It is the same force you feel on a marry-go-round. This creates high tides at night in places opposite the Sun (and to a lesser extent the Moon).

3. The tides can be considered sea waves that are very stable and have wavelengths that a global in scale.

B. Some Definitions

1. Tidal range - the height difference between high and low tide levels. May be as much as 15 meters.

2. Semidiurnal tides have two high tides and two low tides each day. Semidiurnal tides occur because of the varying declination between the moon and the sun.

3. Diurnal tides have one high tide and one low tide each day.

4. Tide waves are essentially 1/2 the Earth's circumference in length. Therefore tide waves are affected by deep ocean floor topography. Tide waves can also be reflected, diffracted, and refracted like normal waves.

C. Forces affecting the tides.

1. gravitational attraction between to bodies is directly proportional to their masses and inversely proportional to the square of the distance between them. All objects with mass attract all other objects with mass, but for the Earth, most objects are too small and far away to be important as contributing influences on the tides. The Sun exerts about half the gravitational force as the Moon on the Earth tides.

2. As the Earth spins, the mass of the Earth wants to spin off into space (centrifugal force - another mythical force). It is kept close to the Earth by gravitational attraction, but the Earth is not perfectly rigid. That means that the Earth's mass near the equator tends to bulge out further from the center of the Earth than does the mass near the poles (and the axis of rotation). This creates a permanent bulge in both the Earth's solid diameter and thickness of the water column near the equator.

D. Tides in Shallow Water

1. In coastal water bodies, tides move water into and out of lagoons and estuaries.

2. Create tidal currents which will produce lunar monthly varying current directions.

3. Tidal currents may be tapped as an energy source if the tidal range is large enough.

 

Gravitational attraction by the Moon creates a bulge in ocean thickness in the direction of the moon. Centrifugal forces create a bulge in the water column all around the equator. The bulge appears as a high tide and intervening troughs appear as low tides.

When the Moon and the Sun act together, tides are quite strong and are called 'spring' tides. There are two spring tides each month (full moon and new moon).

 

When the Sun and the Moon act opposite to one another, the tides are quite weak and are called 'neap' tides. There are two neap tides each month (first and last quarter of the moon).

 

RETURN TO INDEX