Coastal Oceanography (L9-17)

Question 1

What are the different types of coastal environments and the main influences that occur there?

Answer 1

Sandy and rocky nearshore - sediment and breaking waves
Inlets, canyons, headlands - coastline and bathymetric variation
Coral reefs - shallow platforms, steep slopes
Estuaries and fiords - freshwater inputs

Question 2

What is a wave?

Answer 2

A signal or disturbance transferred from one part of a medium to another with a recognisable speed of propagation
It transfers energy through a medium with minimal material transport

Question 3

How are deep water waves measured?

Answer 3

h/L ≥ 1/2
Cₚ varies with wavelength
Longer waves travel faster (dispersive)
Cg = Cₚ/2

Question 4

How are shallow water waves measured?

Answer 4

h/L ≤ 1/20 (or 1/0.05)
Cₚ depends only on depth
All waves travel at the same speed (non-dispersive)
Cg = Cₚ

Question 5

Why is wave energy important to understand?

Answer 5

For companies that harness wave energy for electricity, people who look at the residence time of pollutants in an environment

Question 6

What is wave shoaling?

Answer 6

Where the wave speed slows down and the depth gets shallower
Wave height must increase to conserve the energy flux
Wave length decreases as wave speed decreases

Question 7

What is wave refraction?

Answer 7

Where waves are approaching a beach at an angle, and one part slows down and the other stays at the original speed
Eventually the waves become nearly shore parallel
Strong currents can also cause refraction

Question 8

How does wave refraction influence an embayment or headlands?

Answer 8

Waves refract to follow the bathymetry of the shore or bottom
Convergence of energy around headlands making larger waves and converge wave energy
Divergence of energy around embayments making smaller waves and divert wave energy

Question 9

What happens as waves approach the shore?

Answer 9

Deep water - waves are unaffected by the bottom and energy flux is conserved
Shoaling - waves slow down, height increases, wavelength shortens, and energy flux is conserved
Surfzone - waves slow down, height decreases, and energy flux is not conserved

Wave breaking energy goes into making noise, aeration/foam, mixing water, suspending sediment, and producing heat

Question 10

What is wave set down?

Answer 10

A small decrease (cm) in mean water level in the shoaling zone

Question 11

What is wave set up?

Answer 11

A rapid increase in mean water level in the surfzone
It is larger with larger waves

Question 12

What do wave set down and set up do?

Answer 12

These processes set up pressure gradients, creating alongshore currents

Question 13

What happens if there are alongshore currents converging?

Answer 13

Offshore flow or rip currents are formed, occurring where the wave breaking is the smallest

Question 14

What would happen if there were no differences in alongshore gradients?

Answer 14

There would be no horizontal wave-driven currents

Question 15

How is stratification quantified?

Answer 15

By the strength of the density difference between layers
The bigger the difference, the stronger the stratification

Question 16

What are the sources of stratification?

Answer 16

The sun - increases stratification by decreasing the density in the surface layer
River inputs - increases stratification by decreasing density in the surface layer
River plumes (local) - increases stratification by decreasing density in the surface layer, dependent on the size of the plume and coastal currents

Question 17

What are the implications of stratification?

Answer 17

Disconnect between layers, inhibiting transfer of properties
Internal waves, influencing thermal and nutrient environments (act as upwelling)

Question 18

What is the impact of internal waves on coral reefs?

Answer 18

They have the potential to create and support thermal refuges in which heat stress and coral bleaching risk may be mitigated

Question 19

What destroys stratification?

Answer 19

Ocean turbulence and mixing

Question 20

What are the different types of turbulence?

Answer 20

Convection - fluid heated from below and/or cooled from above (occurs in Autumn)
Shear - the difference in velocity between layers, wind-driven and bottom-generated

Question 21

What is the cycle of stratification and turbulence in winter?

Answer 21

Storms with strong winds and heat loss occurs at the ocean surface
Offshore - stratification found at depth
On shelf - well mixed by storms and tides
Nearshore - always mixed due to shallow water and wave action

Question 22

What is the cycle of stratification and turbulence in summer?

Answer 22

Weak winds and substantial heating occurs at the ocean surface
Offshore - well stratified
On shelf - some stratification, some tidal mixing due to strong tides close to shore
Nearshore - always mixed due to shallow water and wave action

Question 23

What is a tidal mixing front?

Answer 23

The boundary between stratified offshore and well-mixed onshore creates a tidal mixing front
This occurs on wide continental shelves
Has a large impact on coastal productivity

Question 24

How do tidal mixing fronts vary temporally?

Answer 24

Variability is seasonal (winter and summer) and twice monthly (move offshore during spring tides when mixing is stronger, are normal during neap tides)

Question 25

What are the common features of Eastern Boundary Currents?

Answer 25

Wind forcing, upwelling, and coastal circulation Winds are alongshore and blow towards the equator on the eastern side of ocean basins This wind forcing drives offshore Ekman transport and upwelling along the coastlines due to divergence at the coastline, setting up coastal circulation (Eastern Boundary Current)

Question 26

Why is there persistently high chlorophyll concentrations along Eastern Boundary Currents?

Answer 26

Due to upwelling bring cold, nutrient rich water from depth

Question 27

What are the processes of upwelling along Eastern Boundaries?

Answer 27

Nearshore band (10km wide) of upwelling driven by offshore Ekman transport Broader region of upwelling (>100km wide) that extends offshore driven by divergence

Question 28

What causes the divergence of water along Eastern Boundaries?

Answer 28

Equatorial wind stress It is not necessary for Ekman transports to be in opposite directions for convergence and divergence to occur, only that the wind stress (and Ekman transport) varies spatially

Question 29

What is curl-driven downwelling?

Answer 29

Alongshore wind stress decreases with distance offshore Ekman transport offshore is convergent, it is becoming weaker as you move offshore, more Ekman transport arriving than leaving Waters offshore are downwelling

Question 30

What is curl-driven upwelling?

Answer 30

Alongshore wind stress increases with distance offshore as the land topography slows the wind down Ekman transport offshore is divergent, it is becoming stronger as you move offshore, more Ekman transport leaving than arriving Upwelling extends offshore This is more likely to occur than curl-driven downwelling

Question 31

What is coastal upwelling?

Answer 31

Alongshore winds drive an offshore Ekman transport This produces a narrow band of upwelling near the coast

Question 32

What are the impacts of alongshore winds in Eastern Boundary Current regions?

Answer 32

1. Wind drives offshore Ekman transport in the top 10s of metres (coastal upwelling) 2. Offshore Ekman transport is compensated by coastal upwelling 3. Ekman transport divergence creates an expanded area of upwelling offshore (curl-drivel upwelling) 4. The sea surface is lowered at the coast, driving an equatorward geostrophic current (Eastern Boundary Current) 5. Upwelling causes isopycnals to curve upwards towards the sea surface, reducing geostrophic velocities with depth

Question 33

What is vorticity?

Answer 33

A measure of the spin of a column of fluid around its own axis, consisting of relative vorticity and planetary vorticity

Question 34

How is potential vorticity (PV) conserved?

Answer 34

When a column of fluid in the interior ocean is squashed or stretched due to the ocean bathymetry, it must change its planetary vorticity by changing latitude If H increases, f increases (moves poleward) If H decreases, f decreases (moves equatorward)

Question 35

What is Sverdrup balance?

Answer 35

The relationship between wind stress exerted at the surface of the open-ocean and the net movement of water in the interior ocean Sverdrup transport is the flow associated with this relationship

Question 36

Where does equatorward flowing water return poleward?

Answer 36

In the narrow western boundary currents, where flow shears and relative vorticity are large, and Sverdrup balance breaks down

Question 37

What drives subtropical ocean gyres?

Answer 37

Zonal (west to east) surface wind stress drives meridional (south to north) Ekman transport, convergence in the Ekman layer and downwelling Downwelling "squashes" fluid and this moves equatorward via Sverdrup transport to conserve potential vorticity The interior flow is returned poleward in the western boundary current

Question 38

Why is turbulence important?

Answer 38

It creates a well mixed layer 10-100 metres deep where temperature, salinity, and density are nearly uniform with depth

Question 39

What can deepen the mixed layer?

Answer 39

Wind - drives turbulence that vertically mixes surface water with cooler, deeper fluid, homogenising mixed layer temperatures Heat loss - creates parcels of cool, dense fluid at the sea surface that sink to a depth determined by the stratification (convection)

Question 40

What can make the mixed layer shallower?

Answer 40

Solar radiation - heats the surface waters leading to more stable thermal stratification and a shallower mixed layer

Question 41

How can we use the blooms of phytoplankton to see changes in the depth of the mixed layer?

Answer 41

The mixed layer holds phytoplankton When the mixed layer is deeper, there are less phytoplankton due to a lack of light When the mixed layer is shallower, there are more phytoplankton (blooms) due to an increase in light

Question 42

What are the seasonal variations in the mixed layer?

Answer 42

Mixed layers are shallowest in the late summer when winds are weak, and sunlight has warmed the surface layer During autumn, storms mix the water column deepening the mixed later In winter, heat is lost and the mixed layer continues to deepen, becoming deepest in later winter In spring, winds weaken, sunlight increases, gradually adding successive warm layers, causing the mixed layer to become shallower

Question 43

How does depth of the mixed layer regulate the seasonal growth of phytoplankton?

Answer 43

Spring bloom - decreasing mixed layer depth and increased light availability drives phytoplankton growth in the nutrient dense mixed layer from the winter Autumn bloom - mixed layer deepening allows the entrainment of nutrients into the nutrient depleted summer mixed layer, triggering a secondary smaller bloom

Question 44

How does the depth of the mixed layer affect the heating of the ocean?

Answer 44

When processes lead the mixed layer to be shallow, extreme warming of the upper ocean can result, causing a marine heatwave

Question 45

How does the temperature of the mixed layer change?

Answer 45

Through exchanges of heat with the atmosphere (surface fluxes), exchanges of heat with the interior ocean (entrainment), and lateral transport of heat within the mixed layer (advection)

Question 46

What is the AMOC?

Answer 46

The Atlantic Meridional Overturning Circulation It is characterised by a northward flow of warm, salty waters in the upper layers, and a southward flow of colder, deeper waters It is a part of the global vertical overturning circulation (thermohaline circulation) that involves all ocean basins

Question 47

How are water masses named?

Answer 47

Their formation site (eg. NA - North Atlantic, NP - North Pacific) and depth where they settle

Question 48

What main water masses make up the oceanic meridional overturning circulation?

Answer 48

Antarctic Bottom Water (AABW), Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW)

Question 49

How does pressure affect temperature?

Answer 49

As pressure increases, so does the in-situ temperature

Question 50

How do potential measurements allow for better measurement than in-situ?

Answer 50

Potential temperature removes the effect of pressure on in-situ temperature Potential density removes the effect of pressure on in-situ density Both allow for an easy comparison of temperatures of water parcels at different depths

Question 51

What are tracers and what can they be used for?

Answer 51

Oceanic properties or "tracers" that can be used to follow the movement of water masses, determine the pathways of circulation, and the age of the water These include nutrients, dissolved oxygen, radioactive elements and chemicals

Question 52

What are conservative and non-conservative properties?

Answer 52

Conservative properties can only change via mixing with other water masses (potential temperature) Non-conservative properties can change via mixing between water masses and other processes such as biological activity or decay (dissolved oxygen)