1. Chapter 9: Tides Physical Oceanography

2. 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? 2

3. 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 3

4. 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? 4

5. What is a tide? Shallow water! Recall that shallow-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! 5

6. 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. 6

7. Figure 1: Tide data for Virginia Beach 7

8. 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 8

9. 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… Surfing a tidal bore 9

10. Figure 1 shows a diurnal tide: one high and one low tide 10 Water Level (ft) Time First low tide ~8am First high tide ~8pm

11. Figure 2 shows a semidiurnal tide: two high and low tides of equal heights 11 Water Level (ft) Time First low tide ~4am First high tide ~9am Second high tide ~9pm Second low tide ~4pm Tidal range 2ft

12. Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights 12 Water Level (ft) Time First high tide ~4am Second high tide ~6pm First low tide ~11am Second low tide ~11pm Tidal range 10ft Tidal range 4ft

13. Tides in Virginia Beach 13

14. Figure 2: Tide predictions for Woods Hole, MA, 8/25/10 14 Water Level (ft) Time

15. 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 tide Low tide Photo: NASA 15

16. “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 16

17. What causes tides? The tide rises when coastline enters the bulge and falls when it rotates out High tide 17

18. Different types of tides Sun Earth Full Moon 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 18 New Moon

19. Different types of tide Neap tides occur when the sun and moon form a right angle with Earth at 1 st and 3 rd Quarter phase. Low tides are typically higher and high tides are lower Why is the moon’s pull greater than the sun’s? 19 Earth 1 st Qtr Sun 3 rd Qtr

20. Spring and Neap Tide cycle 20 Sun

21. 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! 21

22. Tide exercise: Can you identify the different types of tides? 22 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!

23. Figure 1: Tide predictions for Dauphin Island, 3/1/11 23 Water Level (ft) Time

24. Figure 3: Tide predictions for Seattle, WA (Puget Sound), 5/1/11 24 Water Level (ft) Time

25. 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 25