What is a Wave?brosenhe/Oceanography/15_Ocean_Waves.pdfWave Speed • Speed is equal to wavelength...

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Ocean Waves

What is a Wave?

• A response to a generating force (in this case a pebble thrown into a pond)

• System returns to normal through restoring force (in this case, surface tension of the water)

Ripple on Great Pond, Eastham, MassachusettsLaura Perkins

Where’re the waves?!

• No good waves in Louisiana• Great waves in California• Winter waves are best in

some regions• Waves are better at certain

parts of the beach than at others

• Why?

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Waves

• Large waves in certain regions– http://www.youtube.com/watch?v=LhKFTqxn6

qs• Seasonal waves in other places

– http://www.youtube.com/watch?v=zfJrINTs7wI&feature=related

• Louisiana surfing:– http://www.youtube.com/watch?v=Yd3RlVRZ

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Wave Formation

Anatomy of a Wave

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Wind Waves• Dimensions are astonishingly alike,

no matter the size of the storm

Wave Speed• Speed is equal to wavelength divided by

period– C = L/T

» “C” stands for celerity, another term for wave speed

• A wave’s period always remains the same– If wavelength becomes shorter, it slows down– If it speeds up, the wavelength has to increase

Wave Formation

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Wave Travel

• Storm waves are generated by winds and pressure differences within the storm area

• Within the storm, waves are a jumbled mess of different wavelengths and heights

• Away from the storm, waves traveling at different speeds separate from the others

• This results in distinct groups of waves having similar size characteristics

Wave Travel

Group Speed• Group speed is half of the individual wave

speed– V = C/2

Ripple on Great Pond, Eastham, MassachusettsLaura Perkins

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Interference

• Constructive• Destructive• After point of

interference, waves remain unchanged

Interference

Movement of Waves and Groups

Carleton College Oceanography Course

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Wave Motion

• Orbital Path• No net movement

Wave Motion

• Motion diminishes with depth

• Depth of wave is equal to half of the wave length– D = L/2

Wave Height

• Wind speed• Wind duration• Fetch distance

– Not enough fetch distance in Miami for big, consistent waves

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How big can Southern LA’s waves get?• Assume:

– A tropical storm force wind (58mph)– 60 miles between here and the Bahamas

• Then:

How big can Southern LA’s waves get?• Assume:

– A tropical storm force wind (58mph)– 800 miles between here and the Bahamas

• Then:

Shallow vs. Deep Waves

• Wave height, steepness, and speed change as a wave comes to shore

• Interaction with the bottom causes these changes

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Deep Waves

• Depth of wave (D = L/2) is less than the depth of the water

• Speed (C) and period (P) are defined by the storm

– Wind Strength

–– Wind DurationWind Duration–– Fetch DistanceFetch Distance

• Deep Wave does not “feel bottom.”

Shallow vs. Deep Waves

• Depth of wave is equal to half of the wave length– D = L/2

• If the depth of the water < L/2 of the wave, it is no longer a deep wave.

Shallow Waves

• Waves “feel bottom.”– D < L/2

• Wave slows down (Cs < Cd)• Wave height increases (hs > hd)• Wavelength decreases (Ls < Ld)• Speed solely dependent on water depth

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Shallow Wave Group Movement

Carleton College Oceanography Course

Shallow Waves

Wave Steepness

• Steepness is the wave’s height divided by the wavelength– S = H/L

• Steepness is what causes a wave to break

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When does a wave break?

• When the height to wavelength ratio is greater than one to seven– H/L > 1/7

Waves Coming to Shore

• Waves can approach coasts at an angle

• However, breaking waves almost always seem to be parallel with the coast

Curving of Wave Trains

• The part of the wave that “feels bottom”first is slowed.

• The entire wave begins to curve around this slow point.

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Curving of Wave Trains

• Energy is concentrated on land points

Non-Wind Waves

• Tides– Generated and restored by force of gravity

• Internal waves– In case of ENSO, cause by relaxation of wind-

driven Ekman mixing of the surface layer (Kelvin waves)

• Tsunamis– Generated by seismic events (earthquakes,

landslides, etc.) and restored by gravity

Tsunamis

• Carry huge amounts of energy• As deep water waves, H is usually lower

than waves generated by storms– Hard to detect in the open ocean

• Large amounts of energy result in a tremendous increase in wave height (H) as the tsunami “feels bottom”

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Chilean Landslide

Chilean Landslide

Chilean Landslide

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Chilean Landslide

Chilean Landslide

Chilean Landslide

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Chilean Landslide

Chilean Landslide

Chilean Landslide

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Chilean Landslide

Chilean Landslide

Chilean Landslide

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Chilean Landslide

Chilean Landslide

Chilean Landslide

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Are tsunami’s surfable?

Japan Tsunami

Japan Tsunami

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Japan Tsunami

Japan Tsunami

Hawaiian Tsunami

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Hawaiian TsunamiHawaiian Tsunami

Hawaiian Tsunami

Hawaiian Tsunami

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Hawaiian Tsunami

Summary

• Causes of Waves– Wind– Gravity– Surface Tension

• Anatomy of Waves– H, L, D, C, v, S

Summary

• Group Travel• Interference• Matter Transport or Energy Transport?• Deep Waves vs. Shallow Waves• Wave Height

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Summary

• Different types of waves– Storm (Wind)– Internal– Tides– Tsunamis