Lecture 2 - Waves, Tides and Currents Flashcards
What causes the build-up of waves?
Wind friction against the surface of the ocean.
What happens as wind interacts with the ocean surface?
A pressure differential develops between the trough and peak, causing more energy to be transferred to the system.
What are the four different parameters of waves?
- Wave height
- Wave length
- Wave frequency
- Wave period
What is wave height?
The difference between the peak and trough.
What is wave length?
The distance between two consecutive wave peaks.
What is wave frequency?
How many waves pass a single point at any given time.
How is wave frequency measured?
Using wave buoys.
What is wave period?
The time it takes for a single wave to move from point A to B.
What is the Beaufort scale?
A scale used to predict wave height based on the wind.
Give the five factors affecting wave height.
- Wind speed
- Fetch
- Width of area affected by fetch
- Wind duration
- Water depth
What is fetch?
The distance over which the wind can interact uninterrupted.
What happens at half the wavelength in depth?
The wave interacts with the seabed, slowing down the base of the wave and pushing the water up.
Causes the wave to increase in steepness, eventually resulting in a breaking wave.
Describe wave energy at the coast.
- Concentrated on headlands
- Dissipated in bays
What are tides mostly caused by?
The gravitational pull of the moon.
What is an ebb?
Receding tide
What is a spring tide?
- The moon and sun are parallel to one another
- Their gravitational pulls are combined
- Results in the highest high tides and the lowest low tides
What is a neat tide?
- Gravitational pull of the moon and sun are working against each other
- Results in the highest low tides and lowest high tides
What is semi-diurnal tide?
An area experiences two low tides and two high tides every day of the same magnitude.
What is diurnal tide?
An area experiences a single high tide and single low tide each day.
What is mixed semi-diurnal tide?
An area experiences two low tides and two high tides each day at different magnitudes.
Give five factors influencing the amplitude of wave tides.
- Gravitational pull of the moon and sun
- Depth
- Near short bathymetry
- Shape of the coast line
- Amphidromic points
What are amphidromic points?
Positions on the planet where there is no rise and fall of the tide, leading to a build-up of water and a rotational pattern as the moon rotates.
Give the three different categories of tide range.
- Microtidal, less than 2m
- Mesotidal, 2-4m
- Macrotidal, more than 4m
What is the world’s largest tidal range?
17m, at Bay of Fundy
110 billion tonnes of water flow in and out in 12 hours.
When does a storm surge occur?
When waves and tides conspire.
What was the largest storm surge?
21m at the Bay of Fundy
What three things drive surface currents?
- Wind
- Land masses
- Coriolis effect
What is the Coriolis effect?
An apparent force at 90 degree direction compared to Earth’s movement.
Causes a displacement of movement of air due to the rotation of the Earth.
Which direction is deflection in the Southern hemisphere?
Anti-clockwise, due to the Earth’s west-to-east rotation
What is the Ekman spiral?
Causes a deflection of the water motion by 45 degrees at each subsequent depth down to 100 metres.
How many ocean gyres are there?
Eleven
What are gyres?
Water motion around the ocean basin, mostly caused by equatorial winds.
What is the largest circulation around the planet?
The meridional overturning circulation
What are the four key features of circulation?
1) Upwelling of deep water
2) Surface currents transport light water towards high latitude
3) Deep water formation
4) Spreading of deep water
What drives deep water formation?
Ice formation
Were does deep water formation occur?
At polar latitudes
What is upwelling?
The movement of nutrient rich water.
What are the two forms of upwelling?
- Offshore/longshore winds - direction of water movement at 90 degrees apparent to the wind.
- Divergent current - two bodies of water moving apart.
What is deep water formation?
Takes cold water from the surface down to the deep, driven by ice production.
Give the steps of deep water formation.
1) Fresh water in the ice drives out brine by brine rejection, as it has a lower freezing point.
2) Brine sinks because it is salty.
3) Brine spreads out across the seabed, and is very nutrient-rich.
