Chapter 9: Tides

Physical Oceanography

Last chapter you learned about waves

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

What is the period of a wave?

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Tides are also important physical forces in our ocean

Caused by gravitational force 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).

Are tides deep-water or shallow-water waves?

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What is a tide?

Shallow water! Recall that shallow-water waves have Depth < 1/20

Wavelength and deep-water waves have Depth > 1/2 Wavelength

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

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Tidal patterns vary around the world

Some places have one low tide and one high tide per day called diurnal tides.

Other places have two high and low tides per day approximately equal in size called semidiurnal tides.

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Figure 1: Tide data for Virginia Beach

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Tidal patterns vary around the world

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

In the US examples of tides include– semidiurnal tides Atlantic/East Coast– diurnal tides Some areas of the Gulf of Mexico– mixed tides Pacific/West Coast

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Tidal patterns vary around the world

The incoming tide (low to high) is called a flood tide, while the outgoing tide (high to low) is called an ebb tide.

The period between a high/low or low/high when the water is not rising or falling is called a slack tide.

Surfing a tidal bore…

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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 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 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

Tides in Virginia Beach

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

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Water Level (ft)

Time

Tidal extremes: The Bay of Fundy

The Bay of Fundy in Canada is known for its large tidal range.

Tidal range is the difference between high and low tide.

The Bay of Fundy has recorded a tidal range of up to 50 feet.

High tideLow tide

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: N

AS

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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 tides

Sun

Earth

FullMoon

Spring tides occur when the sun and moon are in a straight line during the full and new moon phases.

The tidal range is typically highest during spring tides

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NewMoon

Different types of tide

Neap tides occur when the sun and moon form a right angle with Earth at 1st and 3rd Quarter phase.

Low tides are typically higher and high tides are lower

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

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Earth

1st Qtr

Sun

3rd Qtr

Spring and Neap Tide cycle

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Sun

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

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Water Level (ft)

Time

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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