Understanding Tides in Physical Oceanography

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?

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

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!

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)

Low tide High tide 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!) Photo: NASA

Gravitational pull “Bulge” of Earth Sun Moon Earth What causes tides? The gravitational pull of the sun and the moon causes “bulges” on Earth that move as we rotate

High tide What causes tides? The tide rises when coastline enters the bulge and falls when it rotates out

Sun Moon Earth Different types of tide • Spring tides occur when the sun and moon are in a straight line • The tidal range is typically highest during spring tides

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? Different types of tide Earth Sun Moon

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!

Tide exercise: Can you identify the different types of tides? • 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 Water Level (ft) Time

Figure 1 shows a diurnal tide: one high and one low tide First hightide~8pm Water Level (ft) First low tide~8am Time

Figure 2: Tide predictions for Woods Hole, MA, 8/25/10 Water Level (ft) Time

Figure 2 shows a semidiurnal tide: two high and low tides of equal heights First high tide~9am Secondhigh tide~9pm Tidal range2ft Tidal range2ft Water Level (ft) Second low tide~4pm First low tide~4am Time

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

Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights Second high tide~6pm First high tide~4am Tidal range4ft Tidal range10ft Secondlow tide~11pm Water Level (ft) First low tide~11am 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

Understanding Tides in Physical Oceanography