Lec 10: Waves in the Open Ocean

• Nazaré, Portugal


  • Europe and the world's premier spot for colossal waves
  • Male record at 86 feet
  • Female record 73.5 feet
• Waves

  Energy transmitted through matter
• Waves

  • The medium (solid, liquid, gas) does not travel as the energy passes through, but particles in the medium oscillate (or cycle) back and forth, and up and down, as the energy passes through them

  
• Waves are generated by two competing forces: a disturbing force that modifies a 'un-disturbed' medium to create the wave, and a restoring force that tends to bring a system to its undisturbed state

  
• Disturbing force


  • Wind that blows over the surface and transfers energy to the ocean
• Restoring force


  For waves along liquid surfaces, the restoring force is gravity
• Waves can be created by movement of fluids with different densities

  
• Types of waves

  • Ocean waves: along air-water interface
  • Atmospheric waves: along air-air interface
  • Internal waves: along water-water interface

  
• Other factors that can generate waves

  • Gravitational pull of Moon and Sun (tides)
  • Turbidity currents
  • Coastal landslides
  • Large icebergs falling from coastal glaciers
  • Underwater volcanic eruptions
  • Uplift/downdroping of large areas of ocean floor, fault slippage (Tsunami)
  • Ships
  • Anything that releases energy to the ocean

  
• Much of the global wave energy are short period wind waves
• Standing (stationary) wave


  A wave that does not move or propagate

  
• Progressive wave


  A wave that oscillates uniformly and progresses without breaking

  
• Types of progressive waves

  • Longitudinal
  • Transverse
  • Orbital

  
• Crest


  Point of highest elevation
• Trough


  Lowest part between a pair of crests
• Wavelength (L or λ)


  Distance between each successive pair of crests or troughs
• Wave height (H)


  Vertical distance between crest and trough

  
• Amplitude


  Vertical distance between the crest (or trough) and mean water level

  
• Steepness


  Ratio of the wave height to the wavelength (H/L)

  
• Period (T)


  Time it takes for a full wave (one wavelength) to pass a fixed position
• Frequency (f)


  Number of crests (or troughs) that pass a point in a fixed unit of time
• Period and frequency


  T = 1/f
• Wave speed (C)


  Rate the wave form moves at, not the water

  
• Wave speed

  C = L/T

  
• When a wave moves across the ocean without breaking, there is almost no net forward motion of the water itself

  
• Circular orbital motion


  As the wave travels, the water passes energy along by moving in a circle

  
• There is some net forward motion of the water as a wave passes, because the particle speed decreases with depth

  
• Stokes Drift


  This type of forward motion of the water as a wave passes

  
• Wave development


  1. Wind deforms the ocean surface into small waves (ripples, L<1.74 cm) → CAPILLARY WAVES

  2. More energy is transferred to the ocean → GRAVITY WAVES develop (symmetric, L>1.74 cm)

  3. As additional energy is gained, wave height increases faster than wavelength → the crest becomes pointed and troughs rounded → trochoidal waveform

  
• When wave speed = wind speed, nothing changes (no net energy exchange)

  
• Winds never blow uniformly over the ocean surface → waves of many heights, wavelengths, periods, direction, etc.

  
• Sea


  This confused state of waves of many heights, wavelengths, periods, direction, etc.

  
• When waves get too steep (H/L > 1/7), they break
• Fetch


  Distance over which the wind blows in one direction
• Swell

  Waves generated in the "sea" that move to its margins, with wind speed decreasing, so the waves move faster than the wind and become long-crested, uniform, and symmetrical

  
• Deep-water waves


  Waves where the water depth (d) is greater than the wave base (d > L/2), so they have no interference with the ocean bottom

  
• Deep-water wave speed


  C = 1.25 √L or C = 1.56 √T

  
• Wave dispersion

  In the open ocean, longer waves travel faster than shorter waves, sorting the waves by their wavelength

  
• The swell outruns the storm that generates it and arrives first
• Wave train

  A group of waves