Chapter 9: Tides Physical Oceanography
Jan 21, 2016
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
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 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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