Chapter3 summery

Question 1

Waves form due to: (apart from waves generated by tsunamis)

Answer 1

• The action of the wind blowing over the sea surface. which generate from
  
  • wind sea: local wind, in real time
  • swell or groundswell: wind that blew over the ocean surface thousands of km away up to several days before

Question 2

Swell:

Answer 2

• AKA surface gravity waves
• Series of mechanical waves that propagate along the interface between water and air
• Long wavelength (rarely more than 150m in Mediterranean)
  wavelength varies due to
  
  • Weather systems
    
    • Size,
    • Strength and
    • Duration
  • Size of water body
• Wavelength varies from event to event (longer than 700m are a result of severe storms like tropical cyclones)

Question 3

Swells and locally generated wind

Answer 3

• Narrower range of frequency and directions than locally generated wind because
  
  • Swell waves have dispersed from their generation area.
    they have a more defined
    
    • Shape and direction

Question 4

Swell direction

Answer 4

The direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in general directions, such as a NNW or SW swell.

Question 5

Causes of swell formation

Answer 5

• Wind speed or strength relative to wave speed — the wind must be moving faster than the wave crest for energy transfer, stronger prolonged winds create larger waves
• The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
• Width of area affected by fetch
• Wind duration — the time over which the wind has blown over a given area
• Water depth

Question 6

The causes of swell formation determine

Answer 6

• The size of the wind waves
  
  • Wave height (trough to crest)
  • Wave length (crest to crest)
  • Wave period (time interval between arrival of consecutive crests at a stationary point)
  • Wave propagating direction

Question 7

Fully developed sea has the maximum wave size possible for a wind of a

Answer 7

• Specific strength
• Duration
• Fetch

Question 8

Exposure to that wind can cause

Answer 8

• Loss of energy due to the breaking of wave tops
• Formation of whitecaps

Question 9

Wave in specific area have a

Answer 9

range of heights

Question 10

Sea water waves is generated by

Answer 10

disturbances such as

• Seismic events
• Gravity
• Crossing wind

Question 11

Generation of wind wave is initiated by

Answer 11

• Disturbances of cross wind field on the surface of the sea water

Question 12

Mechanisms that explain how wind generate waves and perturbation on the surface ocean

Answer 12

• Kelvin-Helmholtz instability (KHI)
• Miles-phillips mechanism

Question 13

Kelvin-Helmholtz instability (KHI)

Answer 13

• Relevant to the generation of large wavelength purturbations

Question 14

Kelvin-Helmholtz instability (KHI)

• Relevant to the generation of large wavelength purturbations

Can occur when

Answer 14

• There is velocity shear in a single continuous fluid or
• There is a velocity difference across the interface between two fluids

Question 15

Kelvin-Helmholtz instability (KHI)

• Relevant to the generation of large wavelength purturbations

Can occur when

• There is velocity shear in a single continuous fluid or
• There is a velocity difference across the interface between two fluids

Example is:

Answer 15

Wind blowing over the water

• The instability manifests in waves on the water surface. More generally
  
  • Clouds
  • Ocean
  • Saturn’s bands
  • Jupiter’s red spot
  • Sun corona

Question 16

KHI require

Answer 16

• Minimum wind speed of 6 m/s to make waves grow against the competing effect of
  
  • Gravity and
  • Surface tension

Question 17

Miles phillips mechanism

Answer 17

• Describe how waves are generated from a flat sea using two mechanisms
  
  • First: produce small ripples “capillary waves”
  • Second: produce bigger waves “gravity waves”
• Relevant to low wind speeds and short wave length perturbations
• Involved a resonant interaction between the surface of the water and turbulent fluctuations in the air

Question 18

How waves are produced in general:

Answer 18

Air moving over the surface of the water transmits its energy to the water.

Question 19

Capillary and gravity waves:

Answer 19

• Capillary grow from flat sea
• Gravity wave form from sea containing capillary waves

Question 20

Capillary and gravity waves:

• Capillary grow from flat sea
• Gravity wave form from sea containing capillary waves

Reason behind the naming:

Answer 20

Because of the restoring force

Question 21

Restoring forces of capillary and gravity waves:

Answer 21

• Gravity in case of gravity and capillary
• Surface tension in case of capillary

Question 22

Swell dissipation for short waves

Answer 22

• Stronger than long waves, this is why swells from distant storms are only long waves
• Dissipation of waves with periods >13s is weak but significant at the pacific ocean scale
• Long swells lose half of their energy over 20 thousand km to over 2 thousand km
  
  • This variation is a systematic function of swell steepness (due to friction at the air-sea interface)
    
    • Ratio of the swell height to the wavelength

Question 23

Swells are often created by

Answer 23

Storms thousands of nautical miles away from the beach where they break

Question 24

The propagation of the longest swells is limited by

Answer 24

The shoreline

Question 25

The propagation of the longest swells is limited by The shoreline Example:

Answer 25

Swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip. This distance allows the waves comprising the swells to be * Better sorted * Free of chop as they travel toward the coast

Question 26

Characteristic of waves that are generated together by storm winds

Answer 26

* Have the same speed * Group together and travel with each other * Others with slower speed will lag behind arriving late due to distance covered

Question 27

The time of propagation from the source t is proportional to

Answer 27

the distance X divided by the wave period T. In deep water it is t=4P_ix/(gT) where g is the acceleration of gravity

Question 28

How can you tell the distance at which swells were generated

Answer 28

* This dispersive arrivals of swells, * long periods first with a reduction in the peak wave period over time

Question 29

Sea state

Answer 29

The effect that the local winds have on sea conditions

Question 30

Sea state is independent of

Answer 30

Travelling swell waves generated by winds outside of the local area

Question 31

Sea state is related to

Answer 31

The Beaufort scale which describes the state of the sea. The Beaufort wind force scale is an empirical measure that relates wind speed to observed conditions at sea

Question 32

Wind wave heights are significantly affected by

Answer 32

Local conditions, such as whether the wind is * onshore * offshore * fetch and its duration