Ocean Tides

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This presentation from the U of Hawaii outlines the origins and effects of tides in our oceans.

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Tides & Waves

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Basically, tides are very long waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate near the middles of oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range. Tides High tide Low tide

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Causes of the Tides Tides are created by the imbalance between two forces: 1. Gravitational force of the Moon and Sun on Earth a. If mass increases (  ), then gravitational force increases (  ) b. If distance increases (  ), then gravitational force greatly decreases (  ) 2. Centripetal (center-seeking) force required to keep bodies in nearly circular orbits Our sun is 27 million times larger than our moon. Based on its mass, the sun’s gravitational attraction to the Earth is more than 177 times greater than that of the moon to the Earth. If tidal forces were based solely on comparative masses, the sun should have a tide-generating force that is 27 million times greater than that of the moon. However, the sun is further from the Earth than the moon. Thus, its tide-genera- ting force is about 59 million times less than the moon. Because of these conditions, the sun’s tide-generating force is about half that of the moon.

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Centripetal forces on Earth • Moon’s Force is the same everywhere on Earth • Force is directed perpendicular to Earth’s center everywhere on Earth Gravitational forces on Earth due to the Moon • Force decreases with increasing distance • Force is directed toward the Moon’s center of mass

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Centripetal Force

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Tidal bulges Tide-generating forces produce 2 bulges: • Away from Moon on side of Earth opposite Moon • Toward Moon on side of Earth facing Moon Earth rotates into and out of tidal bulges, creating high and low tides

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Figure 9-7 • Tidal bulges follow the Moon as it rotates around Earth • A Lunar day is 50 minutes longer than a solar day because the Moon is moving in its orbit around Earth The Lunar day

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The Monthly Tidal Cycle About every 7 days, Earth alternates between: Spring tide Alignment of Earth-Moon-Sun System in line with one another(syzygy) • Lunar and solar bulges constructively interfere • Large tidal range 2. Neap tide Alignment of Earth-Moon-Sun System at right angles (quadrature) • Lunar and solar bulges destructively interfere • Small tidal range Spring tide Neap tide

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Earth-Moon-Sun positions and the monthly tidal cycle

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Tidal patterns Diurnal One high and one low tide each lunar day Semidiurnal Two high and two low tides of about the same height daily Mixed Characteristics of both diurnal and semidiurnal with successive high and/or low tides having significantly different heights Datum is the permanently established surface from which soundings or tide heights are referenced. MLLW is the most commonly used

Tidal patterns in the U.S.:

Tidal patterns in the U.S. Figure 9-15

The Bay of Fundy: Site of the world’s largest tidal range :

The Bay of Fundy: Site of the world’s largest tidal range Tidal Range Tidal Range is the difference between high tide and low tide. The Bay of Fundy in Canada has the greatest tidal range on the planet. Tidal energy is focused by shape and shallowness of bay Maximum spring tidal range in Minas Basin 53 feet Nova Scotia bends when the tide comes in! As 14 billion tons (14 cubic kilometers) of sea water flow into Minas Basin twice daily, the Nova Scotia countryside actually tilts slightly under the immense load! Figure 9-17 Bay of Fundy tidal bore

Tidal Currents:

Tidal Currents A horizontal movement of water often accompanies the rising and falling of the tide. This is called the tidal current. The incoming tide along the coast and into the bays and estuaries is called a flood tide; the outgoing tide is called an ebb current. The strongest flood and ebb currents usually occur before or near the time of the high and low tides. The weakest currents occur between the flood and ebb currents and are called slack tides Tidal currents occur in some bays and rivers due to a change in tides • Ebb currents produced by outgoing tides • Flood currents produced by incoming tide • LSW means Lower Slack Water - HSW means Higher Slack Water Figure 9-18

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Tidal Bores The wave on the incoming tide is known as a Tidal Bore. Tidal bores occur in about 100 rivers throughout the world. Three of the better known bores are the Qiantang Bore in China. This bore reaches heights of 15 ft. and travels 15 mph. The Pororoca, in the Amazon River, forms waves 12 ft. high and can reach speeds of 20 mph. The Severn Bore in England is a popular one to surf as shown below. Qiantang Bore Surfing the Severn Bore

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Where do the tides start? Tides start in the middles of the world’s oceans. Look at the diagrams below and think about how water in a bowl moves if you move the bowl in a circle. The “node” in Fig. 1 never rises or falls. It is the point around which water moves. Fig. 1

Tides in the ocean:

Tides in the ocean Cotidal map shows tides rotate around amphidromic points. There are 140 amphidromic points in the world’s oceans. More realistic pattern of tides in the ocean Figure 9-14 Amphidromic Points

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“If you don’t understand amphidromic points I’m gonna bite you ! SPIDEY SAYS,

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Since we are becoming We will move on to waves TIDAL BORES

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Waves

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Waves Properties of Ocean Waves - An ocean wave is an undulation of the sea surface Waves Period - The time it takes one wave to pass a given point Wave frequency - The number of waves that pass a given point in a given time Waves may be progressive or standing • Progressive waves move across the sea surface. • Standing waves oscillate about a fixed point.

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Progressive Waves • Wind-generated waves are progressive waves because they travel across the sea surface. What Causes Waves? • Wind • Gravitational attraction of sun and moon • Submarine disturbance Wind Generation of Waves The type of wave generated by wind is determined by: - • Wind strength - • Wind duration • Fetch (distance over which the wind blows)

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Wind velocity Wind velocity Wind velocity can be determined by dividing: LENGTH (in Feet or Meters) _ PERIOD (in seconds) Wave windows Swells approach our local coastline from two directions: the south or northwest . These directions are referred to as wave windows (unobstructed approaches to the coast). All other directions are blocked by offshore channel islands (i.e. Santa Catalina, San Clemente) or the coastline itself. Velocity of the wind

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As the storm subsides, the waves move out of the storm zone and carry a lot of energy. Swells, - more regular waves, beyond area of generation, form and the progressive wave move across the ocean. These waves may travel hundreds of miles across the ocean,only to break on some distant shore.

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Wave Motions Two basic motions associated with an ocean wave: • The forward movement of the wave form (not the water). • The orbital motion of water particles beneath the wave. It is wave energy, not water molecules, that moves across the sea surface. A floating seagull demonstrates that it is the wave form that travels, not the water. As the wave moves from left to right the gull, and the water in which it is resting, rotates in an imaginary circle, moving slightly to the left up the front of the approaching wave, then to the crest, then sliding to the right down the back of the wave. Orbits within a given wave will become smaller with depth until, at one half wave length the orbits no longer exist. The period of the wave is the time it takes to complete one orbit.

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Wave Motion with Depth Motion of Water Particles Beneath Waves • Deep Water Waves In deep water most waves do not interact with the sea bottom and are called deep-water waves. The orbits of the water molecules are circular. • Shallow Water Waves Waves which interact with the sea floor are known as shallow-water waves.

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Wave Refraction & Wave Diffraction Bending of waves around an obstacle. This is what causes the “scallop” effect along our coast. Diffraction Refraction Bending of waves as they approach a shore at an angle. As drag along the bottom and differential speed along the crest, waves refract It is this action that allows surfers to surf.

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Wave Reflection Wave Reflection occurs when incoming waves do not break, expending energy, but are reflected back only to interfere with incoming waves. This occurs at the surf spot known as the “Wedge” in Newport, See the diagram below. The Wedge

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Shore breakers (surf) are collapsing waves. Breaking is determined by wave steepness. When the wave height/wave length ratio is about 1/7, waves begin to break. Breakers

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Tsunamis Tsunamis are incorrectly called “tidal waves” because they have nothing to do with tides. They are caused by movement of the sea floor (earthquakes, landslides). As they move out from their source, the waves formed end up having very small heights (1 - 2 ft.), very long wave lengths (100+ miles long) and they move at high speed (hundreds of miles per hour). When they approach shallow water, the water “piles up” and a very high wave may form, and when it breaks, thousands of people living near the shore could be killed.

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Cowabunga dude, I’m outta here !

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