Coastal Oceanography (L9-17) Flashcards
What are the different types of coastal environments and the main influences that occur there?
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
What is a wave?
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
How are deep water waves measured?
h/L ≥ 1/2
Cₚ varies with wavelength
Longer waves travel faster (dispersive)
Cg = Cₚ/2
How are shallow water waves measured?
h/L ≤ 1/20 (or 1/0.05)
Cₚ depends only on depth
All waves travel at the same speed (non-dispersive)
Cg = Cₚ
Why is wave energy important to understand?
For companies that harness wave energy for electricity, people who look at the residence time of pollutants in an environment
What is wave shoaling?
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
What is wave refraction?
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
How does wave refraction influence an embayment or headlands?
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
What happens as waves approach the shore?
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
What is wave set down?
A small decrease (cm) in mean water level in the shoaling zone
What is wave set up?
A rapid increase in mean water level in the surfzone
It is larger with larger waves
What do wave set down and set up do?
These processes set up pressure gradients, creating alongshore currents
What happens if there are alongshore currents converging?
Offshore flow or rip currents are formed, occurring where the wave breaking is the smallest
What would happen if there were no differences in alongshore gradients?
There would be no horizontal wave-driven currents
How is stratification quantified?
By the strength of the density difference between layers
The bigger the difference, the stronger the stratification
What are the sources of stratification?
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
What are the implications of stratification?
Disconnect between layers, inhibiting transfer of properties
Internal waves, influencing thermal and nutrient environments (act as upwelling)
What is the impact of internal waves on coral reefs?
They have the potential to create and support thermal refuges in which heat stress and coral bleaching risk may be mitigated
What destroys stratification?
Ocean turbulence and mixing
What are the different types of turbulence?
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
What is the cycle of stratification and turbulence in winter?
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
What is the cycle of stratification and turbulence in summer?
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
What is a tidal mixing front?
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
How do tidal mixing fronts vary temporally?
Variability is seasonal (winter and summer) and twice monthly (move offshore during spring tides when mixing is stronger, are normal during neap tides)
What are the common features of Eastern Boundary Currents?
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)
Why is there persistently high chlorophyll concentrations along Eastern Boundary Currents?
Due to upwelling bring cold, nutrient rich water from depth
What are the processes of upwelling along Eastern Boundaries?
Nearshore band (10km wide) of upwelling driven by offshore Ekman transport
Broader region of upwelling (>100km wide) that extends offshore driven by divergence
What causes the divergence of water along Eastern Boundaries?
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
What is curl-driven downwelling?
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
What is curl-driven upwelling?
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
What is coastal upwelling?
Alongshore winds drive an offshore Ekman transport
This produces a narrow band of upwelling near the coast
What are the impacts of alongshore winds in Eastern Boundary Current regions?
- Wind drives offshore Ekman transport in the top 10s of metres (coastal upwelling)
- Offshore Ekman transport is compensated by coastal upwelling
- Ekman transport divergence creates an expanded area of upwelling offshore (curl-drivel upwelling)
- The sea surface is lowered at the coast, driving an equatorward geostrophic current (Eastern Boundary Current)
- Upwelling causes isopycnals to curve upwards towards the sea surface, reducing geostrophic velocities with depth
What is vorticity?
A measure of the spin of a column of fluid around its own axis, consisting of relative vorticity and planetary vorticity
How is potential vorticity (PV) conserved?
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)
What is Sverdrup balance?
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
Where does equatorward flowing water return poleward?
In the narrow western boundary currents, where flow shears and relative vorticity are large, and Sverdrup balance breaks down
What drives subtropical ocean gyres?
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
Why is turbulence important?
It creates a well mixed layer 10-100 metres deep where temperature, salinity, and density are nearly uniform with depth
What can deepen the mixed layer?
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)
What can make the mixed layer shallower?
Solar radiation - heats the surface waters leading to more stable thermal stratification and a shallower mixed layer
How can we use the blooms of phytoplankton to see changes in the depth of the mixed layer?
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
What are the seasonal variations in the mixed layer?
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
How does depth of the mixed layer regulate the seasonal growth of phytoplankton?
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
How does the depth of the mixed layer affect the heating of the ocean?
When processes lead the mixed layer to be shallow, extreme warming of the upper ocean can result, causing a marine heatwave
How does the temperature of the mixed layer change?
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)
What is the AMOC?
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
How are water masses named?
Their formation site (eg. NA - North Atlantic, NP - North Pacific) and depth where they settle
What main water masses make up the oceanic meridional overturning circulation?
Antarctic Bottom Water (AABW), Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW)
How does pressure affect temperature?
As pressure increases, so does the in-situ temperature
How do potential measurements allow for better measurement than in-situ?
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
What are tracers and what can they be used for?
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
What are conservative and non-conservative properties?
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)
