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Tides: Key Ideas

Caused by the gravitational force of the moon andsun and the motion of Earth

Longest of all waves; wavelength is about half thecircumference of Earth

Behave as shallow-water waves

Tides are forced waves because they are neverfree of the forces that cause them

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Tidal Curves

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Three basic types: diurnal, semidiurnal,and mixed tides

Over a month the daily tidal ranges varysystematically with the cycle of theMoon causing spring and neap tides.

Tidal range is also altered by the shapeof a basin and sea floor configuration

Types of Tides and Tidal Range

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Tidal Curvesfor ThreeCommonTypes of

Tides

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Figure 10.2

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Result from gravitational attraction andcentrifugal effect

Moon exerts twice the gravitational attractionand tide-generating force as Sun

Two tidal bulges form in the ocean (high tides)

Origin of Tides

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The Equilibrium Theory of Tides

Note the bulges that are aligned with the moon as Earth spins on its axis.The Earth turns beneath these bulges.

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Gravitational and Centrifugal Force

Gravitational attraction and centrifugal forceproduce two tidal bulges of water of aboutthe same size, positioned on oppositesides of the Earth.

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Moon’s declination varies +/- 5o from plane of Earth’s orbit.

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Tidal Day: 24 Hours and 50 Minutes

Earth and Moon both revolve eastward; causestides to occur 50 minutes later each day

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Spring and Neap Tides

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Top: The positions of the Sun, the moon and Earth during a spring tide.

Bottom: The positions of the Sun, the moon and Earth during a neap tide.

Sun and Moon Together

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Spring and Neap Tides

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Figure 10.8

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Rotary Tidal Motion

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Figure 10.13

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A rotary wave is part of an amphidromicsystem in which the wave progressesabout a node (no vertical displacement)with the antinode (maximum verticaldisplacement) rotating about the basin’sedges.

Rotary Tide

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Amphidromic System

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Amphidromic Systems

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Cotidal lines connect points on the rotarywave that experience high tide at thesame time.

Corange circles are lines connectingpoints which experience the same tidalrange. Tidal range increases outward from the node.

Amphidromic Tide

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Rotate clockwise in the southernhemisphere and counterclockwise in thenorthern hemisphere because of Coriolisdeflection.

Irregular coastlines distort the rotarymotion.

Actual tide at any location is a compositeof many different tidal components.

Amphidromic Systems

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Cannot rotateCurrents in these basins reversedirection flowing in with high tide and outwith low tide.

Cotidal and corange lines are nearlyparallel to each other.

Tidal ranges increase if a bay taperslandward because water is funneledtowards the basin’s narrow end.

Tides in Elongated Basins

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An imaginary amphidromic system in a broad, shallow basin(left) and a natural system (right).

Tides in Confined Basins

The tidal range is determined by basinconfiguration.

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Tides in Confined Basins

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Movement of water towards and away fromland with high and low tides

Offshore, the tidal currents inscribe a circularpath over a complete tidal cycle.

Nearshore, the tidal currents produce simplelandward and then seaward currents.

Tidal Currents: Flood and Ebb

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Figure 10.14

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The Worldwide Distributionof the Three Tidal Patterns

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Tides and Marine Organisms

Tides have a profound affect on coastal marine life.

Coastal life is sorted into zones and subzones,depending on the amount of emergence andsubmergence the organisms can tolerate.

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Electricity can be generated from tidal currentsif the tidal range is greater than 5 m in a largebay connected to the ocean by a narrowopening

A dam is constructed across the opening andwater is allowed to flow into and out of the baywhen sufficient hydraulic head exist to driveturbines and generate power

The first major tidal power station, in France, iscapable of generating 544 million kilowatthours of electricity annually

Power from Tides

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Power from the Tides

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The dynamic theory of tides explains the characteristics of ocean tidesbased on celestial mechanics (the gravity of the sun and moon acting onEarth) 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 heightsthrough the cycle

Amphidromic points are nodes at the center of ocean basins; these areno-tide points.

Summary