Lecture 2 - Waves, Tides and Currents

Question 1

What causes the build-up of waves?

Answer 1

Wind friction against the surface of the ocean.

Question 2

What happens as wind interacts with the ocean surface?

Answer 2

A pressure differential develops between the trough and peak, causing more energy to be transferred to the system.

Question 3

What are the four different parameters of waves?

Answer 3

• Wave height
• Wave length
• Wave frequency
• Wave period

Question 4

What is wave height?

Answer 4

The difference between the peak and trough.

Question 5

What is wave length?

Answer 5

The distance between two consecutive wave peaks.

Question 6

What is wave frequency?

Answer 6

How many waves pass a single point at any given time.

Question 7

How is wave frequency measured?

Answer 7

Using wave buoys.

Question 8

What is wave period?

Answer 8

The time it takes for a single wave to move from point A to B.

Question 9

What is the Beaufort scale?

Answer 9

A scale used to predict wave height based on the wind.

Question 10

Give the five factors affecting wave height.

Answer 10

• Wind speed
• Fetch
• Width of area affected by fetch
• Wind duration
• Water depth

Question 11

What is fetch?

Answer 11

The distance over which the wind can interact uninterrupted.

Question 12

What happens at half the wavelength in depth?

Answer 12

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.

Question 13

Describe wave energy at the coast.

Answer 13

• Concentrated on headlands

- Dissipated in bays

Question 14

What are tides mostly caused by?

Answer 14

The gravitational pull of the moon.

Question 15

What is an ebb?

Answer 15

Receding tide

Question 16

What is a spring tide?

Answer 16

• 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

Question 17

What is a neat tide?

Answer 17

• Gravitational pull of the moon and sun are working against each other
• Results in the highest low tides and lowest high tides

Question 18

What is semi-diurnal tide?

Answer 18

An area experiences two low tides and two high tides every day of the same magnitude.

Question 19

What is diurnal tide?

Answer 19

An area experiences a single high tide and single low tide each day.

Question 20

What is mixed semi-diurnal tide?

Answer 20

An area experiences two low tides and two high tides each day at different magnitudes.

Question 21

Give five factors influencing the amplitude of wave tides.

Answer 21

• Gravitational pull of the moon and sun
• Depth
• Near short bathymetry
• Shape of the coast line
• Amphidromic points

Question 22

What are amphidromic points?

Answer 22

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.

Question 23

Give the three different categories of tide range.

Answer 23

• Microtidal, less than 2m
• Mesotidal, 2-4m
• Macrotidal, more than 4m

Question 24

What is the world’s largest tidal range?

Answer 24

17m, at Bay of Fundy

110 billion tonnes of water flow in and out in 12 hours.

Question 25

When does a storm surge occur?

Answer 25

When waves and tides conspire.

Question 26

What was the largest storm surge?

Answer 26

21m at the Bay of Fundy

Question 27

What three things drive surface currents?

Answer 27

• Wind
• Land masses
• Coriolis effect

Question 28

What is the Coriolis effect?

Answer 28

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.

Question 29

Which direction is deflection in the Southern hemisphere?

Answer 29

Anti-clockwise, due to the Earth’s west-to-east rotation

Question 30

What is the Ekman spiral?

Answer 30

Causes a deflection of the water motion by 45 degrees at each subsequent depth down to 100 metres.

Question 31

How many ocean gyres are there?

Answer 31

Eleven

Question 32

What are gyres?

Answer 32

Water motion around the ocean basin, mostly caused by equatorial winds.

Question 33

What is the largest circulation around the planet?

Answer 33

The meridional overturning circulation

Question 34

What are the four key features of circulation?

Answer 34

1) Upwelling of deep water
2) Surface currents transport light water towards high latitude
3) Deep water formation
4) Spreading of deep water

Question 35

What drives deep water formation?

Answer 35

Ice formation

Question 36

Were does deep water formation occur?

Answer 36

At polar latitudes

Question 37

What is upwelling?

Answer 37

The movement of nutrient rich water.

Question 38

What are the two forms of upwelling?

Answer 38

• Offshore/longshore winds - direction of water movement at 90 degrees apparent to the wind.
• Divergent current - two bodies of water moving apart.

Question 39

What is deep water formation?

Answer 39

Takes cold water from the surface down to the deep, driven by ice production.

Question 40

Give the steps of deep water formation.

Answer 40

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.