Oceanography Lecture Notes Outline

Ocean Waves

I. Contents -  Topics Covered

What are Ocean Waves?

Classification of Ocean Waves

Deep Water Versus Shallow Water Waves

Wind-Generated Waves

Rogue Waves

When Wind Waves Meet the Shoreline – Surf’s Up!

Internal Waves

 

II. What is an Ocean Wave?

            A.  Defined

                        1. In general, an ocean wave is any periodic, circular

                                    displacement of the ocean surface or subsurface

                                     interface

 

                        2. Ocean waves are water mass disturbances expressed as

                            a ribbon of kinetic energy that is moving at the speed of

                            the traveling wave form

·        Energy is moving, but the water itself moves very little

 

            B. How Are They Generated?    

                        1. Waves are produced by a generating (water disturbing)

                                     force and a restoring (water calming) force

 

                        2.  Ocean waves come in many sizes, ranging from capillary

                                     waves (ripples) to tides.

·        For capillary waves, the generating force is wind and the restoring force is the surface tension of the water

 

·        For wind waves and sea swell, the generating force is the wind and the restoring force is the Earth's gravity.

 

·        Storm surges are partly due to variations in atmospheric pressure over the sea surface, often augmented by winds and tides.

 

·        For tsunami, the generating force is usually a vertical movement of the sea floor and the restoring force is the Earth's gravity.

 

·        For tides, the generating force is the gravity of the moon and sun.

 

                        3. Waves are also generated within the ocean

·        Termed internal waves

 

·        Mainly along pycnoclines and near the surface

 

C. What are the Physical Aspects of an Ocean Wave?

                        1.  An ocean wave transfers energy from water particle to

                                     water particle in the direction of travel (forward)

·        Causes water particles to move in circles or ellipses, called orbits

 

·        The lateral propogation of orbiting water particles causing other water particles to orbit allows the transmittal of energy across the ocean surface – causes the waveform to move

 

·        This type of wave is called an orbital wave

 

·        Orbital waves occur at the ocean-atmosphere and

      within the ocean along density boundaries

 

·        Because orbital ocean wave forms move in the direction of the energy propagation, they are termed progressive waves

 

2. Ocean waves have distinctive characteristics

·        Wave crest - highest part of wave (peak)

 

·        Wave trough - lowest part of wave (valley)

 

·        Wave height (H) - vertical distance between crest

                                     and trough

 

·        Wavelength (L) - horizontal distance between two

 successive waves

 

·        Wave period (T) - Time it takes for one wave to

move a distance of one wavelength

 

·        Wave frequency - Number of waves passing fixed

 point per one second

 

·        See Figure 10.2 for wave anatomy

 

3.  Wavelength determines size of the water particle orbits

 

4. Progressive decrease in the diameter of orbits with depth

·        Size of orbit at surface = wave height

 

·        Wave motion (orbit) is virtually absent at a depth corresponding to about ½ of the wavelength

Ø          Termed the wave base (Figure 10.3)

 

5. Difference in particle speed within a single orbit causes

 the upper portion to move slightly more forward than

 bottom portion (see Fig. 10.4)

Ø          Orbits traces are not completely closed

 

Ø          Small net forward motion of water column

 

Ø          Forward mass transfer of water termed Stokes Drift  

 

III. Classification of Ocean Waves

            A.  Classified by the Disturbing Force that Creates Them

1. Disturbing Forces

·        Energy that causes waves to form

·        Several different types of disturbing forces

 

2. Wind over the ocean causes wind waves

 

3. Storm surges, seismic sea wave, or pressure change

            causes seiches

 

4. Underwater earthquakes. landslides, or meteor impacts

            causes tsunami

 

5. Nearby celestial body gravity causes tides

 

B. The Difference Between Free and Forced Waves

            1. Free waves continue to propagate away from the region

   of generation after the disturbing force has been removed

·        Examples are wind swell and tsunamis

 

2. Forced waves will cease to propagate soon after the

    disturbing force has been removed

·        Example is the tides

 

C. Overcompensation of Restoring Force Allows Ocean

     Waves to Propagate

            1. Cohesion restoring force for capillary waves

 

            2. Gravity is the restoring force for wind waves, seiches,

tsunamis, and tides

 

            3. The overcompensation of the restoring force is what

                         causes the circular orbit motion in waves

 

D. The Amount of Wave Energy in the Ocean Varies

 According to Wave Type

1. Most wave energy is in the form of wind waves

 

2. See Figure 10.5

 

IV. Deep Water- Versus Shallow-Water Waves

A. Water Depth Controlling Factor for Orbit Shape

1.  Waves traveling in water depths that are greater than 1/2

       the wave's wavelength have circular orbits

·        Waves with circular shaped orbits are termed deep water waves

 

·        Wind waves are the only shallow water type

 

2. Waves traveling in water depths shallower than 1/20th

of their original (deep water) wavelength have elliptical orbits 

·        Waves with elliptical shaped orbits are termed shallow water waves

 

·        Seiches, tsunamis and tides are shallow water types

 

3. Waves traveling in depths between 1/2 and 1/20 of their original wavelength are termed transitional waves

 

                4.  See Figure 10.6

 

B. Wave Velocity is a Function of Wavelength

      1. The longer the wave, the faster the wave energy propagates

 

     2.  Deep water wave velocity formula -  C = L/T

·        C is speed (in meters/second), L is wavelength, and T is time or period (in seconds)

 

                    3.  Shallow water wave velocity formula -  C = sq root 3.1d

·        C is speed (meters/second) and d is the period (in seconds)

 

              4. When deep water waves enter shallow depths:

·         The wavelength shortens (bunches up)

·         The wave slows down

·         The circular orbits become elliptical

·        Indian Ocean

 

           

V. Generation of Wind Waves

A. Wind Waves Defined

1. Wind waves are gravity waves generated by the transfer

      of wind energy into the surface waters

 

2. Wind waves range in size from between 2 cm to 3 meters

 

            3. Wavelengths range from 60 to 150 meters

 

B.  Wind Generates Wind Waves, Swell, and Currents

1. Friction coupling between wind and ocean surface causes

   surface water to get piled up perpendicular to direction of

   wind

 

2. Higher pressure on upwind side of piling up water – lower

      pressure on downwind side

 

3. Piled-up water flows “downhill” (under influence of

    gravity) toward low pressure side of pile (wave form)

 

·        Net water current flow in “downwind” direction

 

·        Capillary size waves first form

Ø      Smaller than 2 centimeters

Ø      Controlled by surface tension

 

·        Capillary waves build into wind waves

 

·        The region of rough ocean surface where wind

waves are being generated is termed “seas”

 

Ø      Made up of chaotic variety of wind waves of

different height, wavelength, period, and speed

                                   

C. Swell Formation and Dispersion

4. The process of wave separation, termed dispersion,  

        creates “clean” ocean swell out of stormy seas

 

·        Smooth undulation of sea surface

 

·        Swell can travel thousands of kilometers across

ocean basins to break as surf on a distant shore

 

·        Progressive groups of swell of similar origin traveling together across the ocean are called

wave trains

Ø      Surfers call it ocean “corduroy”

 

5. The shape of wind waves is different than that of swell

·        Wind waves have sharp crests

 

·        Swell have more rounded crests

 

5. Factors affecting wind wave and swell development

·        Wind persistence (duration)

 

·        Wind strength (intensity)

 

·        Size of fetch - Length of continuous stretch of

     ocean surface under a a persistent  wind current

 

Ø      Illustration of a fetch in Figure 10.11

 

6. Global-scale winds responsible for wind waves and swell

·        Powerful mid-latitude Westerlies

Ø      Winter storm systems

 

·        Persistent Trades (Easterlies)

Ø      year-round

 

·        Tropical hurricane

Ø      Summer/fall season

 

7. Once generated, the movement and direction of ocean

   swell will Not  be much affected by the Coriolis effect

·        Remember it is the translation of wave energy –

Not water particles across the ocean

 

            D. Classifying Fully Developed Seas

                        1. A fully developed sea is defined as the maximum wave

                     size theoretically possible for a wind of specific strength,

                     duration, and fetch

 

2. “Whitecaps” or “combers” form when wave steepness

       becomes unstable

 

·        Wave height to wavelength exceeds 1:7 ratio

 

2. See Table 10.2 for the conditions necessary for a given

                                    wind speed

 

                        3. Theoretical maximum sized fully developed seas with

                       waves heights averaging 15 meters (50 feet)

 

                        4.  Largest measured open ocean swell is 112 feet!

 

VI. Rogue Waves

      A. Interference Between Wave Trains Makes Rogue Waves

1.      Attributed to constructive interference

 

2.      Occurs both out at sea and along shorelines

·        Called surf beat along shorelines

 

3.      Very unpredictable and dangerous to ships

·        Huge Supertankers and Cargo Ships are even

     susceptible to these waves

 

4.      Surface currents can help generation of rogue waves

 

 

VII. Wind waves Meeting the Shoreline

A. Wind Waves and Swell Approaching Shore Will

   Change Character

 

            1. “Feel” bottom at ½ the wave’s wavelength

 

            2. Wave will slow down

 

            3. Wavelength will decrease, but period remains the same

 

            4. Wave height will increase

 

            5. Wave crest will become peaked

 

            6. Water particle orbits begin to change from to circular to

            flattened ellipses

 

B. Wind Waves and Swell Lines Approaching Shore May

             Change Shape and/or Direction

 

            1. Refraction – Wave lines approaching the shoreline at

                        an angle will experience slowing and bending

 

·        Wave lines bend towards the nearest point of shore

 

·        Refracting waves attempt to line up with shoreline

 

·        Refracted wave lines focus (wrap around) wave energy at headlands

 

·        Refracted waves lines diffuse wave energy in bays

 

            2. Diffraction – Wave lines that change shape and direction

               due to interruption of wave train as it moves around an

               object

 

·        Smaller secondary diffraction waves are generated

     on by the wave train interruption

 

            3. Reflection – Waves lines that bounce off vertical barriers

                        and propagate back in the direction form which it came

 

·        Barriers such as seawalls, breakwaters, and bluffs

 

·        Standing waves can result within enclosed waters

          such as harbors and bays

 

C. Wind Waves and Swell Meeting Shore Will Break

 

            1.  As the wavelength shortens, the height to wavelength

             ratio approaches the 1:7 instability ratio

 

            2.  Water particles in the top of wave begins to move faster

               than the actual wave energy form

 

            3. The crest of the wave moves ahead past its supporting

             base and the wave breaks

 

            4. A wave breaks when the wave height to water depth

                        approaches a 3:4 ratio

 

·        A three foot waves breaks in four feet of water

 

5. Wave energy dissipated as heat, sound and work

 

6. Waves break against the shore in different ways.

·        Plunging waves – Steep and hollow

 

·        Spilling waves – gradual-sloped, and mushy

 

VIII. Internal Ocean Waves

A. Defined:

1. Waves formed between ocean layers of different

 denisties

 

 

XIII. Vocabulary Terms