Geography and Oceanography - Chapter 20 pptx

Origin of the Tides◆ Unlike wind-driven surface waves and unlike tsunamis, tides are caused by two principal factors: – Gravitational attraction – Centrifugal force... ◆ The moon becaus

Tides

Origin of the Tides

◆ Unlike wind-driven surface waves and unlike tsunamis, tides are

caused by two principal factors:

– Gravitational attraction

– Centrifugal force

Gravitational Attraction

◆ All masses are drawn to each other.

◆ The moon because of its closeness

to the Earth exerts a greater

gravitational effect on the Earth than the Sun, despite the fact that the Sun

is much more massive than the

Moon.

Gravitational 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 Force

◆ Earth and Moon revolving around one another.

◆ unequal masses of Earth and Moon

– center of rotation lies beneath the

Earth’s surface.

◆ second tidal bulge

– ocean shifts away from center of

rotation

Centrifugal Forces: Center of Rotation

2 Bulges from Gravitational Attraction

& Centrifugal Force

Animation

Dynamic Model of the Tides

True Tidal Waves (not tsunamis!)

◆ The motion of water around the basin

is a rotary wave.

– The crest is at one side of the basin.

– The trough is at the other side of the

basin.

◆ Tides have characteristics of both

progressive and standing waves

Standing Waves

◆ Sea, swell, and surf are progressive

waves

◆ Standing waves (seiches)…

– Oscillate back and forth about a node,

i.e., a fixed point.

Node = Fixed Point Antinode = maximum displacement

Dynamic Model of theTides

Earth rotates from

Pressure Gradient Coriolis

cut-away

Dynamic Model

◆ equilibrium model of the tides is good, BUT

– not totally detailed and accurate

– oceans are separated by continents – oceans 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 Tides

◆ High tide : Wave crest

◆ Low tide : Wave trough

◆ Tidal range : Wave height

◆ Tidal periods depending on location:

Wave Parameters

Tidal Periods

day.

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

Classification by Daily Record

◆ Diurnal: have one high tide and one low tide daily (high lat).

◆ Semidiurnal: have two high tides

and two low tides daily (low lat).

◆ Mixed: there are two high tides and two low tides daily, but of unequal shape (mid lat).

Classification by Monthly Record

◆ Spring tide: phase when tidal range

Spring & Neap Tides

◆ Spring tides occur when the Earth, Sun, and Moon are aligned.

Sun/Moon/Earth - Spring & Neap

Can you read a tide table?

Marine Biorhythms

◆ Cyclical behaviors associated with tidal rhythms

– The diurnal activity of fiddler crabs.

– The spawning behavior of the grunion

fish.

Diurnal Activity of Fiddler Crabs

Natural

in burrows Searching for

food

Movie Trivia