Chapter3 summery Flashcards
Waves form due to: (apart from waves generated by tsunamis)
- 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
Swell:
- 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
- Weather systems
- Wavelength varies from event to event (longer than 700m are a result of severe storms like tropical cyclones)
Swells and locally generated wind
- 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
- Swell waves have dispersed from their generation area.
Swell direction
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.
Causes of swell formation
- 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
The causes of swell formation determine
- 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
Fully developed sea has the maximum wave size possible for a wind of a
- Specific strength
- Duration
- Fetch
Exposure to that wind can cause
- Loss of energy due to the breaking of wave tops
- Formation of whitecaps
Wave in specific area have a
range of heights
Sea water waves is generated by
disturbances such as
- Seismic events
- Gravity
- Crossing wind
Generation of wind wave is initiated by
- Disturbances of cross wind field on the surface of the sea water
Mechanisms that explain how wind generate waves and perturbation on the surface ocean
- Kelvin-Helmholtz instability (KHI)
- Miles-phillips mechanism
Kelvin-Helmholtz instability (KHI)
- Relevant to the generation of large wavelength purturbations
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
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:
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
KHI require
- Minimum wind speed of 6 m/s to make waves grow against the competing effect of
- Gravity and
- Surface tension
Miles phillips mechanism
- 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
How waves are produced in general:
Air moving over the surface of the water transmits its energy to the water.
Capillary and gravity waves:
- Capillary grow from flat sea
- Gravity wave form from sea containing capillary waves
Capillary and gravity waves:
- Capillary grow from flat sea
- Gravity wave form from sea containing capillary waves
Reason behind the naming:
Because of the restoring force
Restoring forces of capillary and gravity waves:
- Gravity in case of gravity and capillary
- Surface tension in case of capillary
Swell dissipation for short waves
- 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
- This variation is a systematic function of swell steepness (due to friction at the air-sea interface)
Swells are often created by
Storms thousands of nautical miles away from the beach where they break
The propagation of the longest swells is limited by
The shoreline
The propagation of the longest swells is limited by
The shoreline
Example:
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
Characteristic of waves that are generated together by storm winds
- Have the same speed
- Group together and travel with each other
- Others with slower speed will lag behind arriving late due to distance covered
The time of propagation from the source t is proportional to
the distance X divided by the wave period T. In deep water it is t=4Pix/(gT)
where g is the acceleration of gravity
How can you tell the distance at which swells were generated
- This dispersive arrivals of swells,
- long periods first with a reduction in the peak wave period over time
Sea state
The effect that the local winds have on sea conditions
Sea state is independent of
Travelling swell waves generated by winds outside of the local area
Sea state is related to
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
Wind wave heights are significantly affected by
Local conditions, such as whether the wind is
- onshore
- offshore
- fetch and its duration
