08 - Tides are the longest of all ocean waves What are...


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Unformatted text preview: Tides are the longest of all ocean waves What are characteristics & causes of tides? Tides are caused by the gravitational force of the moon and sun and the motion of earth. Tides are the longest of all waves- with a waves- wavelength ~half the circumference of Earth. Tides are forced waves because they are never free of the forces that cause them. 1 2 Low tide dictine abbey of Mont-Saintel off the coast of Normandy, ce High tide 3 4 Intertidal Zone 5 6 1 TIDES- Characteristics 2 High and 2 Low per day Magnitudes of each pair are not equal Tide today ~50 minutes later than tide yesterday. yesterday. Maximum amplitude ("Spring" tide) occurs 14 days (" Spring" apart ("fortnight"). (" fortnight" Minimum range is the "Neap" tide. Neap" Spring tides coincide with New and Full Moons 7 tide charts 8 TIDES - Why 2 pairs per day? Earth has 2 tidal bulges with Moon: Gravity-caused bulge toward Moon Gravity Inertia bulge away from Moon Earth-Moon system rotates about a Center-ofEarthCenter- ofMass shifted about 3/4th Earth radii from Earth's core. Earth' Centrifugal force, as Earth rotates about this Center, flings water to opposite side Center, Tides are NOT caused by Gravity alone! 9 Gravity- Earth "feels" sun and moon Gravityfeels" Gravity inertia Gravity + inertia A Planet orbits the sun in balance between gravity and inertia. (a) If the planet is not moving, gravity will pull it into the sun. (b) If the planet is moving, the inertia of the planet will keep it moving in a straight line. (c) In a stable orbit, gravity and inertia together cause the planet to travel in a fixed path around the sun. 10 Earth-moon attraction Earth- Moon's gravity- pulls ocean water Moon' gravity- The moon does not rotate around the center of Earth. Earth and moon together the Earth moon system rotate around a common center of mass about 1,650 kilometers (1,023 miles) beneath Earth's surface. Earth' 11 The moon's gravity attracts the ocean toward it. moon' The motion of Earth around the center of mass of the Earth moon system throws up a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges. 12 2 The movement of the moon generates strong tractive forces Forces influencing tides The action of gravity and inertia on particles at five different locations on Earth. At points (1) and (2), the gravitations attraction of the moon slightly exceeds the outward-moving outwardtendency of inertia; the imbalance of forces causes water to move along Earth's surface, converging at a Earth' point toward the moon. At points (3) and (4), inertia exceeds gravitational force, so water moves along Earth's surface to Earth' converge at a point opposite the moon. Forces are balanced only at the center of Earth (point CE). 13 14 Earth rotates beneath tidal bluges Semi-diurnal tidal cycle (2 / day) Semi- Earth's rotation beneath the tidal bulges produces high Earth' and low tides. 15 Notice that the tidal cycle is 24 hrs 50 minutes long because the moon rises 50 minutes later each day. 16 Why are tides later each day? Lunar day vs solar day A lunar day is longer than a solar day. Moon revolves Counter-clockwise Counter--clockwise)) in 28--29 days around Earth (rotating CounterCounter clockwise 28 Rises 0.8 Hour later each day A lunar day is the time that elapses between the time the e moon is highest in the sky and the next time it is highest in the th sky. the moon moves In a. 24--hour solar day,another 12.2 - 50eastwardabout 24 12.2 Earth must rotate minutes to again 12.2 12.2 place the moon at the highest position overhead. A lunar day is therefore 24 hours 50 minutes long. Because Earth must turn an additional 50 minutes for the same tidal alignment, lunar tides usually arrive 50 minutes later later each day. 17 18 3 The Movement of the Moon Generates Strong Tractive Forces Tidal bulges follow the moon. When the moon's moon' position is north of the equator, the gravitational bulge toward the moon is also located north of the equator and the opposite inertia bulge is below the equator. 19 The Movement of the Moon Generates Strong Tractive Forces How the changing position of the moon relative to Earth's Earth' equator produces higher and lower high tides. Sometimes the moon is below the equator, and sometimes it is above. 20 TIDES - Moon AND Sun Sun's pull on Earth = 46% of Moon's Sun' Moon' Solar Tides are 1/2 Lunar amplitude Solar tides at same time each day When are the Solar HIGH tides? Sun-Earth-Moon align, then Solar and Lunar Sun- Earthtides combine. Spring Tide = Full and New Moon Neap Tide when Solar LOW tide partially cancels Lunar HIGH tide. 21 Sun, Earth, moon allignment 22 Spring tides Neap tides Relative positions of the sun, moon, and Earth during spring tides At the new and full moons, the solar and lunar tides reinforce each other, making spring tides, the tides, highest high and lowest low tides. 23 Relative positions of the sun, moon, and Earth during neap tides At the first-and third-quarter moons, the sun, Earth, and firstthirdmoon form a right angle, creating neap tides, the tides, lowest high and the highest low tides. 24 4 The Dynamic Theory of Tides The dynamic theory of tides explains the Tidal patterns: mixed, diurnal, semidiurnal... semidiurnal... Tide curves for the three common types of tides. (a) A mixed tide pattern at Los Angeles, California. (b) A diurnal tide pattern at Mobile, Alabama. (c) A semidiurnal tide pattern at Cape Cod, Massachusetts. (d) The worldwide geographical distribution of the three tidal patterns. Most of the world's ocean coasts have semidiurnal tides. 26 characteristics of ocean tides based on celestial mechanics (the gravity of the sun and moon acting on Earth) and the characteristics of fluid motion. Semidiurnal tides occur twice in a lunar day Diurnal tides occur once each lunar day Mixed tides describe a tidal pattern of significantly different heights through the cycle Amphidromic points are nodes at the center of ocean basins; these are no-tide points. 25 TIDES - amphidromic points Why are there these differences in tidal patterns? "Slosh" of tidal "wave" around pivot point ("amphidromic point") Tidal ranges generally increase with increasing distance from amphidromic points. The colors indicate where tides are most extreme (highest highs, lowest lows), with blues being least extreme. White lines radiating from the points indicate tide waves moving around these points. In almost a dozen places on this map the lines converge. Notice how at each of these places the surrounding color-- the tidal force for that region--is blue, indicating little or no apparent tide. These convergent areas are called amphidromic points. Tide waves move around these points, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. 27 Amphidromic points in the world ocean 28 Fig. 11-15, p. 308 Amphidromic points Tidal patterns center on amphidromic points A tide wave crest enters an ocean basin in the Northern Hemisphere. The wave trends to the right because of the Coriolis effect... effect... ...causing a high tide on the basin's eastern shore. basin' Unable to continue turning to the right because of the interference of the shore, the crest moves northward, following the shoreline ...and causing a high tide on the basin's northern basin' shore. The wave continues its progress around the basin in a counterclockwise direction ...forming a high tide on the western shore and completing the circuit. The point around which the crest moves is an amphidromic point (AP). 29 30 5 Tidal patterns vary with ocean basin shape and size: closed basins How do tides behave in confined basins? The tidal range is determined by basin configuration. (a) An imaginary imaginary amphidromic system in a broad, shallow basin. The numbers indicate the hourly hourly positions of tide crests as a cycle progresses. (b) The amphidromic system for the Gulf of St. Lawrence between New Brunswick and Newfoundland, southeastern Canada. Dashed lines show the tide heights when the tide crest is passing. Tidal patterns vary with ocean basin shape and size: narrow basins Tides in a narrow basin. (a) True amphidromic systems do not develop in narrow basins because there is no space for rotation. (b) Tides in the Bay of Fundy, Nova Scotia, are extreme because water in the bay naturally resonates (seiches) at the same frequency as the lunar tide. 31 32 Low tide: Bay of Fundy High tide: Bay of Fundy tidal range= 15 m water rises 1 m in 23 min! 33 Fig. 11-18a, p. 309 34 Grunion run: Pacific coast Biological ties to tides During spring and summer months these small fish (genus Leuresthes) swim ashore at night in large numbers just after the highest spring tides, deposit and fertilize their eggs below the sand surface, and return to the sea. Nearly two weeks later, when spring tides return, the eggs hatch. No one is certain how grunion time their reproductive behavior so precisely to the tidal cycle. The grunion are found only along the Pacific coast of North America and in the Gulf of California. Unlike the Pacific coast species, Gulf of California grunion spawn during daylight. 35 36 6 Tides in Indiana??? The Hindostan whetstone beds are siltstones that can be found in Orange County, Indiana. These rocks are 300-million-years old tidal rhytmites 300- millionFrom the early 1800s, these deposits were mined as a "whetstone" -- stones used to sharpen knives. In fact, by the end of the 1800s Indiana led the world in the production of whetstones. Prior to the Civil War, the Hindostan whetstone was also used for headstones in graveyards. These whetstone tombstones can be found in many old cemeteries in southwestern Indiana (including Bloomington) and along the Wabash and Ohio Rivers in Illinois. In all cases, the "tidal signature" can be seen in the tombstones. In most cases, the person whose grave is marked with the whetstone lived a longer period of time than it took for the material in the headstone to be 37 deposited... Tidal rhythmites- records of rhythmitesancient tides Tidal rhythmites are packages of laterally and/or vertically accreted laminated sandstone, siltstone and mudstone of tidal origin that exhibit rhythmic change in lamina/bed thickness and grain-size unequivocal evidence of marine setting provide ancient record of moon-Earth astronomical cycles Hindostan whetstone: Indiana 38 Paleoceanography The study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology, and patterns of sedimentation (ex: tidal rhythmites) rhythmites) Uses proxies- substrates that record proxiesenvironmental conditions in some way 39 tidally deposited sediments, Portugal 40 Questions addressed by paleoceanographic records How have ocean properties (circulation, chemistry, basin shape, How have Earth surface temperature, climate, the hydrologic cycle, and global biogeochemical cycles changed during the past and why? What is the ocean's role in natural climate change, and how ocean' are the oceans likely to respond to future changes in Earth's Earth' climate, both natural and anthropogenic? Paleoceanographers use natural archives--sediment cores, archives-- corals, glacial ice--and analytical tools to understand ice-- climate-linked changes in the circulation, biology, and climatechemistry of the oceans 41 Paleozoic marine record, Alaska 42 7 Tides: summary Tides have the longest wavelengths of the ocean's waves They are caused by a combination of the gravitational force of the moon and the sun, the motion of the Earth, and the tendency of water in enclosed ocean basins to rock at a specific frequency. Unlike the other waves, these huge shallow-water waves are shallownever free of the forces that cause them and so act in unusual but but generally predictable ways. Amphidromic points, basin resonances, and other factors points, combine to cause different tidal patterns on different coasts. Tides are important in creating unique tidal zone biological niches The rise and fall of the tides can leave detailed paeoceanographic records 43 44 8 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue.

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