Ocean tides
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Ocean Tides. Why tides occur. Refers to cyclic rise and fall of sea water. Variations in gravitational attraction between the Earth, moon and sun. The Moon. Primary factor that controls the rhythm and height of the tides 2 tidal bulges due to gravitational attraction

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Why tides occur

  • Refers to cyclic rise and fall of sea water.

  • Variations in gravitational attraction between the Earth, moon and sun.


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The Moon

  • Primary factor that controls the rhythm and height of the tides

  • 2 tidal bulges due to gravitational attraction

  • Side of Earth closest to moon - seawater drawn towards moon.

  • Opposite side - bulge produced away from the moon.

  • Timing of tides is related to the Earth’s rotation, and the rotation of the moon around the Earth.


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The Sun

  • The second factor controlling the tides is the sun’s gravity.

  • Average solar tide = half average lunar tide.

  • At certain times, the direction of the moon’s gravitational attraction is aligned with the sun’s. The two act together to produce highest and lowest tides of the year.


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SPRING TIDES

  • Highest and lowest tides of the year. (sun and moon aligned)

  • Occur every 14-15 days during full and new moon.


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PROXIGEAN SPRING TIDES

  • Rare, unusually high tide.

  • Occurs when the moon is both unusually close to the Earth, and in the new moon phase (between the sun and the Earth).

  • Occurs at most once every 1.5 years.


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NEAP TIDES

  • The gravitational pull of the sun and moon are at right angles to each other.

  • Daily tidal variations on Earth are at their least.

  • Occur during first and last quarter of the moon.


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TYPES OF TIDES

  • Geometric relationship of sun and moon to locations on the Earth’s surface creates three different types of tides.

  • DIURNAL TIDES: one high and one low water tidal per day. Northern Gulf of Mexico and SE Asia.


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2. SEMI-DIURNAL TIDE:

  • Two high and two low waters per tidal day.

  • Common on Atlantic coasts of the United States and Europe.


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3. MIXED TIDES:

  • Successive high-water and low-water stands differ appreciably.

  • Higher high water and lower high water, as well as higher low water and lower low water.

  • West coast of Canada and the US.


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Renewed Interest in Ocean Tides

  • The Moon's gravity imparts tremendous energy to the Earth, raising tides throughout the global oceans. What happens to all this energy? This question has been pondered by scientists for over 200 years, and has consequences ranging from the history of the moon to the mixing of the oceans.

  • Amount of tidal energy dissipates in shallow water due to friction between the water and the floor - waves on the beach.

  • June 2000 - Richard Ray and Gary Egbert: According to their report, about 1 trillion watts, or 25 to 30 percent of the total tidal energy dissipation, occurs in the deep ocean. The remainder occurs in shallow seas, such as the continental shelf off the southeast coast of South America

  • Using altimeter data from the TOPEX/Poseidon satellite we can empirically map the tidal energy dissipation.

  • Deep-water tidal dissipation occurs near rugged bottom topography.


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(continued)

  • Important implication: energy sources needed to maintain the ocean’s large scale “conveyor-belt” circulation.

  • Requires 2 terawatts (2 trillion watts)

  • Winds supply ~ 1 terawatt.

  • Speculate that the tides provide the remainder.


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TOPEX/Poseidon

  • Accurate global maps of tides, can now be predicated with an accuracy of 2cm.

  • Data used for study of ocean circulation.

  • Level of accuracy necessary to understand many oceanic processes.


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Gary D. Egbert, Richard D. Ray and Bruce G. Bills, “Numerical modeling of the global semidiurnal tide in the present day and in the last glacial maximum”. Journal of Geophysical Research, Vol. 109, March 2004.

  • By developing a hydrodynamic model, they explored tidal energies during the last glacial maximum.

  • Conclusion: sea level was about 100m lower then than present day - would have changed the tides, with some amplitudes increasing by a factor of 2.

  • Drop in sea level would also have affected energy dissipation - present day increase by about 50% globally, almost triple in the deep ocean.

  • Problem: IT drag dependent on ocean stratification, and this is poorly known during the LGM.


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