TidesTides

Tidal CharacteristicsTidal Characteristics single waves - stretch across entire single waves - stretch across entire

ocean basins. ocean basins. shallow-watershallow-water waves waves

– wavelengths greatly exceed the depth of wavelengths greatly exceed the depth of the ocean.the ocean.

complex interactions of complex interactions of moon and moon and sunsun

Origin of the TidesOrigin of the Tides

Unlike wind-driven surface waves Unlike wind-driven surface waves and unlike tsunamis, tides are and unlike tsunamis, tides are caused by two principal factors:caused by two principal factors:–Gravitational attractionGravitational attraction

–Centrifugal forceCentrifugal force

Gravitational AttractionGravitational Attraction

All masses are drawn to each other.All masses are drawn to each other. The The moonmoon because of its closeness because of its closeness

to the Earth to the Earth exerts a greater exerts a greater gravitational effect on the Earth than gravitational effect on the Earth than the Sunthe Sun, despite the fact that the Sun , despite the fact that the Sun is much more massive than the is much more massive than the Moon.Moon.

Gravitational Effect of MoonGravitational Effect of Moon

Animation courtesy of NASA Goddard Space Flight Center: The Moon's gravity tugs at the Earth, causing ocean water to slosh back and forth in predictable waves called tides. We can visibly observe some of that energy dissipate at the beach, with waves rolling across coastal shallows and shoals. Most of the energy dissipates due to friction between the water and the shallow floor beneath it

Centrifugal ForceCentrifugal Force Earth and Moon revolving around Earth and Moon revolving around

one another.one another. unequal massesunequal masses of Earth and Moon of Earth and Moon

– center of rotation lies beneath the center of rotation lies beneath the Earth’s surface.Earth’s surface.

second tidal bulgesecond tidal bulge– ocean shifts ocean shifts awayaway from center of from center of

rotationrotation

Centrifugal Forces: Center of RotationCentrifugal Forces: Center of Rotation

2 Bulges from Gravitational Attraction 2 Bulges from Gravitational Attraction & Centrifugal Force& Centrifugal Force

AnimationAnimation

Dynamic Model of the TidesDynamic Model of the Tides

True Tidal Waves True Tidal Waves ((notnot tsunamis!) tsunamis!)

The motion of water around the basin The motion of water around the basin is a is a rotary waverotary wave..– The crest is at one side of the basin.The crest is at one side of the basin.

– The trough is at the other side of the The trough is at the other side of the basin.basin.

Tides have characteristics of both Tides have characteristics of both progressive and standing wavesprogressive and standing waves..

StandingStanding Waves Waves Sea, swell, and surf are progressive Sea, swell, and surf are progressive

waves. waves. Standing waves (seiches)…Standing waves (seiches)…

–Oscillate back and forth about a Oscillate back and forth about a nodenode,, i.e., a fixed point.i.e., a fixed point.

Node = Fixed Point Node = Fixed Point Antinode = maximum displacementAntinode = maximum displacement

Dynamic Model of theTidesDynamic Model of theTides

Dynamic Model of theTidesDynamic Model of theTides

Earth rotates from W to E

Tidal bulge to W

cut-away

Pressure GradientCoriolis

Pressure GradientCoriolisPressure Gradient

Corioliscut-away

Dynamic ModelDynamic Model

equilibrium model of the tides is good, equilibrium model of the tides is good, BUT...BUT...

– not totally detailed and accuratenot totally detailed and accurate

– oceans are separated by continents oceans are separated by continents

– oceans not infinitely deepoceans not infinitely deep

Animation courtesy of NASA Goddard Space Flight Center: Using 6 years of data from TOPEX/Poseidon satellite altimeter, they derived a 16-day set of predictive data, showing a synthetic view of how the tides move around the world's oceans.

Key Features of TidesKey Features of Tides High tide : Wave crestHigh tide : Wave crest Low tide : Wave troughLow tide : Wave trough Tidal range : Wave heightTidal range : Wave height Tidal periods depending on location:Tidal periods depending on location:

– 12 hours, 25 minutes12 hours, 25 minutes– 24 hours, 30 minutes24 hours, 30 minutes

Wave ParametersWave Parameters

Tidal PeriodsTidal Periods

why not exactly 24 hours or 12 hours?why not exactly 24 hours or 12 hours? Moon moves forward in it’s orbit each Moon moves forward in it’s orbit each

day.day.– Takes 50 additional minutes for a spot on Takes 50 additional minutes for a spot on

the Earth’s surface to regain it’s position the Earth’s surface to regain it’s position relative to the Moon.relative to the Moon.

Classification by Classification by DailyDaily Record Record DiurnalDiurnal: have one high tide and one : have one high tide and one

low tide daily low tide daily (high lat).(high lat).

SemidiurnalSemidiurnal: have two high tides : have two high tides and two low tides daily and two low tides daily (low lat).(low lat).

MixedMixed: there are two high tides and : there are two high tides and two low tides daily, but of two low tides daily, but of unequal unequal shapeshape (mid lat).(mid lat).

Classification by Classification by MonthlyMonthly Record Record

Spring tideSpring tide: phase when tidal range : phase when tidal range is is maximalmaximal..

Neap tideNeap tide: phase when tidal range is : phase when tidal range is minimalminimal..

There are 2 spring and 2 neap tides There are 2 spring and 2 neap tides each montheach month

Spring & Neap TidesSpring & Neap Tides SpringSpring tides occur when the Earth, Sun, tides occur when the Earth, Sun,

and Moon are aligned.and Moon are aligned.– New Moon and Full Moon phasesNew Moon and Full Moon phases– Constructive interferenceConstructive interference

NeapNeap tides occur when the Sun and Moon tides occur when the Sun and Moon are aligned at right angles to one another.are aligned at right angles to one another.– Quarter Moon phasesQuarter Moon phases– Destructive interferenceDestructive interference

Sun/Moon/Earth - Spring & NeapSun/Moon/Earth - Spring & Neap

Can you read a tide table?Can you read a tide table?

Marine BiorhythmsMarine Biorhythms

Cyclical behaviors associated with Cyclical behaviors associated with tidal rhythms. tidal rhythms. – The diurnal activity of fiddler crabs.The diurnal activity of fiddler crabs.

– The spawning behavior of the grunion The spawning behavior of the grunion fish.fish.

Diurnal Activity of Fiddler CrabsDiurnal Activity of Fiddler CrabsNatural

in burrows

Searching for food

Movie TriviaJeopardy, 1953, Barbara Stanwyck