Let’s Review Currents! Entrance “Ticket” • Currents that run along the western coast of the United States are _________________. • Currents that run along the eastern coast of the United States are __________________. • Currents that only effect the upper 100 m of water are called ________________________. • What two things cause the water to become more dense? • _________________________________________ • __________________________________________
Chapter 18: Ocean Motion Section 3: Ocean Waves and Tides
Waves • A rhythmic movement that carries energy through matter or space. • Crest – highest point of the wave. • Trough – the lowest point of the wave. • Wavelength – the horizontal distance between the crests or between the troughs of two adjacent waves. • Wave Height – the vertical distance between crest and trough • Amplitude – Half the distance of the wave height. • Amplitude squared is proportional to the amount of energy the wave carries • EX) Calm day = small amplitude Stormy day = increased amplitude (increased wave energy)
Wave Movement When you watch a wave in a lake or ocean it looks as though the water is moving forward. Unless the wave is breaking onto shore, the water does not move forward. Each molecule of water returns to near its original position after the wave passes. Water molecules in a wave move around in circles. ONLY energy moves forward. Below a depth equal to about ½ the wavelength, water movement stops as it’s not affected by waves.
Breakers A wave changes shape in the shallow area near shore. Friction with the ocean or lake bottom slows water at the bottom of the wave. As the wave slows, its crest and trough come closer together and the height increases. The top of a wave, not slowed by friction, moves faster than the bottom. The top of the wave outruns the bottom and it collapses and the wave crest falls as water tumbles over on itself. The wave breaks onto shore…BREAKER. After the wave breaks onto shore, gravity pulls the water back into the sea.
How Water Waves Form • When wind blows across a body of water, wind energy is transferred to the water. • If the wind speed is great enough, the water begins to pile up, forming a wave. • As the wind continues to blow, the wave increases in height. • More than 30 m high – TALLER than a 6-STORY BUILDING! • Height depends on the speed of the wind, the distance over which the wind blows, AND the length of time the wind blows. • When the wind stops so does the forming waves, but once set in motion, waves continue moving.
Tides The rise and fall in sea level. Caused by a giant wave produced by the gravitational pull of the Sun and the Moon. Height – 1m to 2m Wavelength is thousands of kilometers long. As the crest of this wave approaches the shore, sea level appears to rise (high tide) and when there is a drop in sea level is called low tide.
Tidal Range As the Earth rotates, different locations on Earth pass through high and low positions. Daily cycle of 2 high tides and 2 low tides. One low-tide/high-tide cycle takes 12 hours, 25 minutes. Daily cycle takes of 2 high tides and 2 low tides makes 24 hours, 50 minutes. Because ocean basins vary in size and shape, some coastal locations, like the Gulf of Mexico, have one 1 high and 1 low tides each day. Tidal Range is the difference between the level of the ocean at high tide and low tide.
Extreme Tidal Range http://www.youtube.com/watch?v=u3LtEF9WPt4 Affected by the shape of the seacoast and the shape of the ocean floor. Smooth, wide beaches allow the water to spread over a large area and the water level might only rise a few centimeters during a high tide. Narrow gulfs or bays might have the water rise many meters during a high tide. Average shorelines have a tidal range between 1 to 2 meters. Mediterranean Sea has a tidal range of 30 cm. The Bay of Fundy – extremely narrow bay and has the largest tidal range (15 meters between the high and low tides)
Tidal Bores When a rising tide enters a shallow, narrow river from a wide area of the sea. Can have breaking crest or a smooth wave. Typically found in areas that have large tidal ranges. Examples: Amazon River in Brazil, Bay of Fundy in Nova Scotia. Causes the river waters to reverse their flow.
The Gravitational Effect of the Moon • Tides are typically cased by the interaction of gravity in the Earth-Moon system. • Earth and the water in Earth’s oceans respond to the moons pull. • Water bulges outward as Earth and the Moon revolve around a common center of mass. • 2 bulges of water form, one on the side of Earth closest to the Moon and one on the opposite side of Earth. • The one closest to the Moon is caused by the gravitational attraction of the Moon and Earth. • The one on the opposite side of the Earth is caused by the same opposing force that, here, is greater than the force of gravity
The Gravitational Effect of the Sun • The Sun also affects tides. • The Sun can strengthen or weaken the Moon’s effects. • SPRING TIDES: When the Moon, Earth, and the Sun are line up together. • High tides are higher and low tides are lower than normal. • Has NOTHING to do with the season of Spring. • NEAP TIDES: When the Sun, Earth, and the Moon form a right angle. • High tides are lower and low tides are higher than normal.
Tidal Videos http://www.youtube.com/watch?v=XO6UkjbC2BI awesome song http://www.youtube.com/watch?v=lfehEHwys2o NASA and tides