Waves - Introduction to Oceanography - Lecture Slides, Slides of Oceanography

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Waves

Waves

• When undisturbed by wind (or some other factor such as an earthquake), the sea surface is naturally smooth!
• Waves are moving energy and begin as a disturbance - Wind blowing across the surface of the ocean generates most waves - Tides, turbidity currents, coastal landslides, calving icebergs, and sea floor movement can also cause waves

• Imagine a seagull resting on the ocean surface
• The bird moves in circles – up and forward as the tops of the waves move toward its position, and down and backward as tops of the waves move past
• Energy in the waves flows past the bird, but the gull and it’s patch of water move only a short distance

Each circle is equal in diameter to the wave height

Orbital Waves

• The bigger the wave, the larger the size of the orbit
• The diameter of the orbit diminishes rapidly with depth

Wave motion is negligible when orbits reach a diameter that is 1/23 of those at the surface

Components of a Wave

• Ocean waves has distinct parts:
  • Wave crest: highest part of the wave above average water level
  • Wave trough: lowest part of the wave below average water level
  • Wave height: the vertical distance between a wave crest and its trough
  • Wavelength: the horizontal distance between 2 successive crests, or troughs

Making Waves

• Energy that causes waves to form is called a disturbing force
• Wind blowing across the ocean surface provides the disturbing force to generate capillary waves (waves <1.73 cm) and wind waves
• Landslides and tectonic processes (volcanic eruptions, faulting of the sea floor) are the disturbing forces for seismic sea waves, or tsunamis

Making Waves

• The restoring force seeks to return the water to flatness after a wave has formed in it; gravity provides the restoring force on all waves >1.73cm

Deep vs. shallow water waves

• Waves moving through water deeper than ½ their wavelength are deep water waves - Example: A wind wave with a 20m wavelength is considered to be a deep water wave so long as it is passing through water >10m deep
• Waves in water shallower than ½ their wavelength are shallow water waves - Example: A wave with a 20m wavelength will act as a shallow-water wave if the water is <10m deep

Shallow water waves

• When a wave approaches the shore, its proximity to the bottom flattens out the orbits of water molecules
• Causes the water at the bottom to move back and forth ; no longer in a circular pattern

Gee, that’s just swell…

• Generally speaking, the longer the wavelength, the faster the wave
• When waves move away from their area of origination, wind speeds diminish and they eventually move faster than the wind
• Mature waves from a storm sort themselves into groups of waves with similar wavelengths and speeds as they outrun their smaller ‘relatives’

When waves meet

• Because longer waves will outrun shorter waves, wind waves from different storm systems can interfere with one another
• When waves meet, they add to or subtract from one another
• Such interaction is called interference
  • Constructive : additive
  • Destructive: subtractive (cancellation)

Dude, constructive and destructive

waves rule

• Surfers depend on constructive and destructive waves to generate their ‘wave sets’
• Constructive interference between waves of different wavelengths create the sought-after big waves
• Destructive interference diminishes the waves and makes it easier for the surfer to swim back out