1. Waves, Currents, and Tides
MOTION IN THE OCEAN
Waves, Currents, and Tides

2. Waves A disturbance which moves through or over the surface of a fluid
Mostly caused by winds
(Also earthquakes, volcanoes, the pull of gravity).
Form of great energy

3. Wave Characteristics Parts of a Wave Crest = high point
Trough = low point
Height = vertical distance from crest to trough
Amplitude = ½ of height. Crest to midline or trough to midline
Wavelength = Horizontal distance between crest to crest or trough to trough
Midline or Still Water Level =
No waves or mid point of wave
height between the crest and
The trough

4. Breaking Waves; Breakers
Waves break near the shore because the water becomes shallow. Close to the shore the ground beneath the water interferes with the circular motions of the water molecules as they participate in a passing wave. Sandbars further off shore can have the same effect, much to the delight of surfing enthusiasts like Bart.

5. Wave period : time for 2 crests to pass fixed point (T) sec
Wave speed (C) : C = wavelength / T (m/s)
Wave steepness : H / wavelength
When H / wavelength = 1/7 or angle at crest 120 or less = Breaker

6. Size of Wind Generated Waves
Depends on 3 things:
Wind Speed
Wind Duration (length of time wind blows)
“Fetch” Extent of open water across which the wind can blow

7. Water Motion in Waves Water travels in vertical circular orbits
Wave moves, particles don’t!

8. Wave from deep water reaching shallow water.

9. Waves reaching a sandbar or reef underwater

10. Breakers at the Beach

11. Importance of Waves Shaping Coastlines Ecology Erode cliffs
Grind rock into sand
Ecology
Returns O2 to water
Stir up food for filter feeders

12. Types of Waves CHOP – Short period (back bays)
SWELL – Long period (boat rolls; seasickness)
SWASH – water up beach BACKWASH – back down

13. TSUNAMI “TIDAL WAVE” Caused by undersea quake or volcano
Wavelength = ~150 mi. Wave height = 6” – 1’
Can NOT perceive in boat Speed > 500 mph
Slows down to ~25 mph at shore; water builds up to ~65+ ft

14. Tsunami Waves

15. Creation of a Tsunami

16. Receding Ocean

17. Types of Tides Continued
Diurnal Tides
1 high & 1 low / day
Parts of Gulf of Mexico and Asia
Semi-Diurnal Tides
2 high & 2 low / day
Atlantic coasts of North America and Europe
Mixed
2 high & 2 low / day (height varies)
Pacific coast

18. Currents What are currents? Causes - “Rivers” of circulating water
- Wind
- Rotating Earth
Density Changes
Topography of the ocean floor
Layout of the
Continents

20. Surface Ocean Currents
Broad, slow drifts; never cross equator
Wind generated; circular gyres
The Ocean’s Conveyor Belt

21. Coriolis Effect
- N. Hemis – clockwise; Right
- S. Hemis – counterclockwise; Left

22. Gulf Stream - N. Atlantic
- Brings warm water from equator north along east coast of N. A.
Sometimes form eddies – circulating water that pinches off from the current

23. IMPORTANCE OF SURF. CURRENTS
NAVIGATION
MIGRATION
WEATHER

24. Localized Surface Currents
Longshore Current.
Flows parallel to shore; move sediment

25. RIP CURRENT
- Caused by converging longshore currents
- Very dangerous ; Red Flag
- DO NOT fight rip current; swim parallel to shore to get out of channel

27. Deep Ocean Currents Flow beneath surface; cross equator
Separated from surface currents by boundary called a “Thermohaline” (diff in densities)
Move North to South

28. Importance Of Deep Currents
Upwelling
Brings deep water to surf.
Circulates nutrients up
Moves plankton & larvae

29. Tides

30. Tides The rhythmic rise and fall of the ocean’s water
High tide = rising, incoming tide, flow
Low tide = receding, outgoing tide, ebb
Slack tide = vertical movement stops

31. Tides are very long, slow waves
They have a wave period of 12 hours 25 min
Tidal day is 24 hours 50 min
NC has 2 high and 2 low tides daily

32. What Causes Tides? 1. Gravitational pull of sun & moon on Earth
Moon closer, therefore > effect
Like magnet, pulls water away from surface = TIDAL BULGE

33. 2. Centrifugal Forces
Bulge on opposite side because centr. force > pull of moon
Produced by motions of Earth, sun, & moon

34. Types of Tides Spring Tide
- Moon and sun are in direct line with one another
- Results in unusually high tidal range
Tidal Range = vertical distance between high & low tides
2x’s/month

35. Neap Tide sun and moon are at right angles
Pulls cancel each other out – causes a weak pull
unusually low tidal range
2 x’s / month

36. Spring vs. Neap Tides

37. Distance bet. Moon & Earth
Perigee Tides
Moon closest to earth, very high tides (causes flooding)
Apogee Tides
Moon farthest away from earth, very low tides

39. Importance of Tides Expose & submerge orgs
Circulate water in bays & estuaries
Circulates food, wastes, etc
Trigger spawning (grunion, horseshoe crab)