Lesson 10: Tides

Physical Oceanography

Last lesson we learned about waves

How do you differentiate between a deepwater and shallow-water wave?

What is the period of a wave?

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Figure 1: Tide predictions for Dauphin Island, 3/1/11

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Water Level (ft)

Time

Tides are also important physical forces in our ocean

Gravitational interactions between the sun, moon and earth are the primary causes of tides

Different positions of the sun and moon create two different types of tides: spring tides and neap tides

Tides play an important role in the life cycle of many marine organisms

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What is a tide?

A tide is the periodic rise and fall of a body of water due to gravitational interactions between the sun, moon and Earth

The wavelength of an average tide can be up to 17,000 km (over 10,500 miles). Do you think tides are deepwater or shallow water waves?

Shallow water! Recall that shallow-water waves have Depth < 1/20 Wavelength and deepwater waves have Depth > 1/2 Wavelength

The ocean would have to be deeper than 5,250 miles for an ocean tide to be a deepwater wave!

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Tidal patterns vary around the world

Some places have one low tide and one high tide per day (diurnal tides)

Other places have two high and low tides per day approximately equal in size (semidiurnal tides)

If the two high and low tides of a semidiurnal tide are unequal in size, they are call mixed semidiurnal tides

The US has examples of semidiurnal tides (East Coast), diurnal tides (some areas of the Gulf of Mexico) and mixed tides (Pacific Coast)

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Tidal extremes: The Bay of Fundy

The Bay of Fundy in Canada is known for its large tidal range (differences of over 50ft have been recorded!)

High tideLow tide

Ph

oto

: N

AS

A

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“Bulge” of Earth

What causes tides?

The gravitational pull of the sun and the moon causes “bulges” on Earth that move as we rotate

Sun

Earth

Moon

Gravitational pull

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What causes tides?

The tide rises when coastline enters the bulge and falls when it rotates out

High tide

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Different types of tide

Sun

Earth

Moon

Spring tides occur when the sun and moon are in a straight line

The tidal range is typically highest during spring tides10

Different types of tide

Neap tides occur when the sun and moon form a right angle with Earth

Low tides are typically higher and high tides are lower

Why is the moon’s pull greater than the sun’s?

Earth

Moon

Sun

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Tides affect marine life

Organisms that live near the shore have adapted to the rise and fall of the tide

We’ll meet some of these creatures in today’s activity!

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Tide exercise: Can you identify the different types of tides?

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You will be presented with three figures and corresponding questions

See if you can determine which figures represent diurnal, semidiurnal and mixed semidiurnal tides.

Good luck!

Figure 1: Tide predictions for Dauphin Island, 3/1/11

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Water Level (ft)

Time

Figure 1 shows a diurnal tide: one high and one low tide

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Water Level (ft)

Time

First low tide~8am

First hightide~8pm

Figure 2: Tide predictions for Woods Hole, MA, 8/25/10

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Water Level (ft)

Time

Figure 2 shows a semidiurnal tide: two high and low tides of equal heights

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Water Level (ft)

Time

First low tide~4am

First high tide~9am

Secondhigh tide~9pm

Second low tide~4pm

Tidal range2ft

Tidal range2ft

Figure 3: Tide predictions for Seattle, WA (Puget Sound), 5/1/11

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Water Level (ft)

Time

Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights

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Water Level (ft)

Time

First high tide~4am

Second high tide~6pm

First low tide~11am

Secondlow tide~11pm

Tidal range10ft

Tidal range4ft

Student activity

In today’s activity, you will meet some marine organisms that live in the intertidal zone: the area between the low tide water mark and high tide water mark

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