Waves in the Ocean

Waves are the undulatory motion of a water surface.

• Parts of a wave are, Wave crest,Wave trough, Wave height (H), Wave Amplitude, Wave length (L),and Wave period (T).

• Wave period provides a basis for the wave classifications: Capillary waves, Chop, Swell, Tsunamis, Seiches.

Sketch of a harmonic or ideal (sinosoidal) wave. H is exaggerated

against l for clarity. The narrow vertical bar gives the wave

amplitude A (the distance between mean water level and wave

crest); the heavy vertical bar gives the wave height H (the difference

between wave trough and wave crest, or twice the wave amplitude).

Most of the waves present on the ocean’s surface are wind-generated waves.

• Size and type of wind-generated waves are controlled by: Wind velocity, Wind duration, Fetch, and Original state of sea surface.

• As wind velocity increases wave length, period and height increase, but only if wind duration and fetch are sufficient.

• Fully developed sea is when the waves generated by the wind are as large as they can be under current conditions of wind velocity and fetch.

• Significant wave height is the average wave height of the highest 1/3 of the waves present and is a good indicator of potential for wave damage.

Progressive waves are waves that move forward across the surface.

• As waves pass, wave form and wave energy move rapidly forward, not the water.

• Water molecules move in an orbital motion as the wave passes.

• Diameter of orbit increases with increasing wave size and decreases with decreasing water depth.

• Wave base is the depth to which a wave can move water.

• If the water is deeper than wave base, orbits are circular and there is no interaction between the bottom and the wave, but if the water is shallower than wave base, orbits are elliptical and become increasingly flattened towards the bottom.

•There are three types of waves defined by water depth: Deep-water wave, Intermediate-water wave, and Shallow-water wave.

•Celerity (speed) is the velocity of the wave form, not the water, C= L/T.

7-2 Wave Motions

Particle movement in deep and

shallow water waves

The diagrams use vertical

exaggeration for clarity. First diagram:

deep water waves. Second diagram:

shallow water waves.

In deep water waves particles move on

circles, in shallow water waves

particles move on very flat ellipses.

Particle movement decreases rapidly

(exponentially) with depth in deep

water waves but remains essentially

the same over the entire water depth in

shallow water waves.

In both cases, particles under wave

crests move in the direction of wave

propagation; particles under wave

troughs move against the direction of

wave propagation.

Fetch is the area of contact between the wind and the water and is where wind-generated waves begin.

• Seas is the term applied when the fetch has a chaotic jumble of new waves.

• Waves continue to grow until the sea is fully developed or becomes limited by fetch restriction or wind duration.

• Wave interference is the momentary interaction between waves as they pass through each other. Wave interference can be constructive or destructive.

• Because celerity increases as wave length increases, longer waves travel faster than short waves.

7-3 Life History of Ocean Waves

Great

Lakes

Open

Ocean

Sketch of the relative amounts of energy as a function of wave frequency in ocean waves. The top line (green) gives the classification based on period, the line below (gold) the classification based on the wave-generating force, and the bottom line (blue) the classification based on the restoring force.

The shallower the water, the greater the interaction between the wave and the bottom alters the wave properties, eventually causing the wave to collapse.

• Celerity decreases as depth decreases.

• Wave length decreases as depth decreases.

• Wave height increases as depth decreases.

• Troughs become flattened and wave profile becomes extremely asymmetrical.

• Period remains unchanged. Period is a fundamental property of a wave

• Refraction is the bending of a wave into an area where it travels more slowly.

7-3 Life History of Ocean Waves

Wave steepness (stability) is a ratio of wave height divided by wave length (= H/L).

• In shallow water, wave height increases and wave length decreases.

• When H/L is larger than or equals 1/7 (H/L 1/7), the wave becomes unstable.

• There are three types of breakers:, Spilling breakers, Plunging breakers, and Surging breakers.

7-3 Life History of Ocean Waves

Storm surge is the rise in sea level resulting from low atmospheric pressure associated with storms and the accumulation of water driven shoreward by the winds.

• Water is deeper at the shore area, allowing waves to progress farther inland.

• Storm surge is especially severe when superimposed upon a high tide.

7-3 Life History of Ocean Waves

Refraction along California

Gibraltar

Standing waves or seiches consist of a water surface “seesawing” back and forth.• A node is an imaginary line across the

surface which experiences no change in elevation as the standing wave oscillates. It is the line about which the surface oscillates.

• Antinodes are where there is the maximum displacement of the surface as it oscillates and are usually located at the edge of the basin.

• Geometry of the basin controls the period of the standing wave. A basin can be closed or open.

• Standing waves can be generated by storm surges.

7-4 Standing Waves

• Resonance amplifies the displacement at the nodes and occurs when the period of the basin is similar to the period of the force producing the standing wave.

7-4 Standing Waves

A first order seiche. The undisturbed sea level is indicated by the broken yellow line. Three water particles are shown as an indication of water movement in the seiche. Note the node in the centre and that water under the node moves only horizontally, while water at both ends of the basin moves vertically.

A first order seiche in a basin open to the deep ocean. The undisturbed sea level is indicated by the broken yellow line. Two water particles are shown as an indication of water movement in the seiche. Note the node at the connection of the basin to the deep ocean, and that water under the node moves only horizontally, while water at the end of the basin moves vertically

Internal waves form within the water column on the pycnocline.

• Because of the small density difference between the water masses above and below the pycnocline, wave properties are different compared to surface waves.

• Internal waves display all the properties of surface progressive waves including reflection, refraction, interference, breaking, etc.

• Any disturbance to the pycnocline can generate internal waves, including: Flow of water related to the tides., Flow of water masses past each other, Storms, or Submarine landslides.

7-5 Other Types of Progressive

Waves

An internal wave propagating on the interface between two layers. The undisturbed sea level is indicated by the yellow line. Water particles are shown as yellow and magenta dots. Yellow dots sit in the middle of the water column and move only up and down. Magenta dots sit at the top and bottom of the water column and move only in the horizontal.

By watching a yellow dot you can see how a water particle in the middle of the water column moves up and down, but does not move horizontally, as the wave passes through.

The most common internal waves are of tidal period and manifest themselves in a periodic lifting and sinking of the seasonal and permanent thermocline at tidal rythm. In some ocean regions their surface expressions, produced by convergence over the wave troughs, is visible in satellite

images

An internal wave traveling up the

Derwent River estuary in Hobart,

Tasmania. The effect of the wave is

visible by streaks of smooth water

produced by the convergences above

the wave troughs

Internal Waves - Indian Ocean

Tsunamis were previously called tidal waves, but are unrelated to tides.

• Tsunamis consist of a series of long-period waves characterized by very long wave length (up to 100 km) and high speed (up to 760 km/hr) in the deep ocean.

• Because of their large wave length, tsunamis are shallow-water to intermediate-water waves as they travel across the ocean basin.

• They only become a danger when reaching coastal areas where wave height can reach 10 m.

• Tsunamis originate from earthquakes, volcanic explosions, or submarine landslides.

7-5 Other Types of Progressive

Waves

An example of tsunami propagation, derived from a numerical model. The example simulates a tsunami that occurred on the 17th of July 1998 off the coast of north eastern Papua New Guinea. The earthquake epicentre that created it was located less than 50 km from the coast. The close proximity to the coast ruled out any warning for the coastal inhabitants; one village was destroyed, nearly 3000 villagers lost their lives. The animation shows the tsunami propagation for the first two hours after the earthquake.

Deadly 100' Rogue Waves Destroy World's Giant ShipsScientists Baffled by Giant Walls Of Water

What Is a Rogue Wave?

A "rogue wave" is typically used to describe a wave that is

significantly larger than other waves during a given time, at a

given location. Several factors can create rogue waves. Multiple

waves can intersect to create a much larger wave- up to the sum of

the heights of the waves that combine. Additionally, currents can

contribute to the formation of rogue waves. And rogue waves can

result as a normal part of the wave spectrum. In other words,

waves are not created individually but rather, in large groups. And

a wave(s) within this group can be significantly larger than the

rest.

Rogue waves have been witnessed all around the world. The

North Sea, Gulf of Alaska, S. African coasts are infamous for

producing rogue waves- some that have approached 100 feet in

height.

Rogue Wave in the Agulas Current

• Strong currents or the combination of two other large waves can produce these giants, but Miguel Onorato of the University of Torino in Italy and his colleagues have been investigating another cause. Taking the Schrödinger equation from the realm of quantum mechanics, the researchers have found that monstrous waves can grow from tiny, random changes in wave height, speed or direction.

• The team examined initial sea states typical of those in the North Sea with the Schrödinger equation. Waves usually form a broad, shallow spectrum of wave heights, but the researchers tweaked this spectrum to produce waves ten times higher than usual.

Coastal

Rogue

Wave

East Mediterranean Eddies

Gulf Stream Eddies