Physical Oceanography Flashcards
What is the Coriolis Force
it is a change of direction.
- In Northern Hemisphere, wind is deflected to the right
- In the Southern Hemisphere, wind is deflected to the left
What is Frictional Coupling
when wind stress is transferred down into the water column due to internal friction
Why is the Coriolis force greater at higher latitudes
Due to a larger circumference at the equator, the equator spins faster than the poles do. This is why any water that drifts north of the equator is moving faster as it carries the speed from the equator with it. The Earth underneath is moving slower than the ocean and causes the water to spiral/spin/fan out.
What is a driver of oceanic motion
Solar radiation drives oceanic motion by heating and cooling the oceans.
What type of radiation is incoming radiation?
Shortwave radiation
What type of radiation is outgoing radiation
Longwave radiation
What is the radiation distribution on Earth?
(eg why poles get less concentrated radiation)
the closer to the equator you are, the more radiation you are exposed to.
at higher latitudes, the angle if the suns rays are deflected/scattered as the rays cover a larger surface area. Due to the curvature of the Earth, the rays are more concentrated at the equator and less concentrated towards the poles.
What are the 3 Milankovitch cycles
- Obliquity
- Orbital Precession
- Eccentricity
What is Obliquity? (Milankovitch Cycle)
Obliquity related to the tilt of the Earth
- moves in ~ 41,000 year cycles
- more tilt means more extreme seasons (eg closer/further away from the sun)
- less tilt means less extreme seasons (Earth is less crooked)
What is Orbital Precession? (Milankovitch Cycle)
Orbital precession is the wobble of the Earth
- moves in ~26,000 year cycles
- it makes seasonal contrasts more extreme in one hemisphere and less extreme in the other
- currently, the southern hemisphere experiences hotter summers while the northern hemisphere seasonal variations are more mild
What is Eccentricity? (Milankovitch Cycle)
the orbit around the sun changed to be more elliptical or more circular
- moves in ~ 100,000 year cycles
- more elliptical means more fluctuations
- more round means more equitable
Why is the northern hemisphere more sensitive to changes in climate?
The Northern hemisphere consists of a lot more landmass than the Souther hem. Land heats up faster than the oceans which is why is it more sensitive to changes in the climate and solar radiation.
How/why does the Earth distribute heat to the poles
Because there is an imbalance in the net surface heat flux, global circulation distributes heat from the equator to the poles through:
- wind
- deep ocean circulation
- surface currents
there is a surplus of heat energy at the equator and a net radiative loss at the poles.
think atmosphere and insolation
Why are there higher temperatures over landmasses than oceans?
This is due to:
Insolation
- land mass heats up more and faster in comparison to oceans
Atmosphere
- atmosphere over oceans has more water content so more clouds form over the water which disperse more of the incoming radiation
- water vapour absorbs about 20% of incoming radiation
State the mechanism of how ocean currents disperse heat
Surface waters at lower latitudes (equator) carry the heat polewards. The currents then get cooled in polar areas and get carried back to the equator
What is the albedo effect
It is the ability of a surface to reflect light.
darker surfaces absorb more heat energy, lighter surfaces reflect more heat energy
eg snow has a high albedo
What is the albedo feedback loop
Global warming causes more polar ice to melt. This means more light surfaces that reflect heat are being lost. this means the albedo keeps decreasing until a tipping point is reached and (in our lifetime) the Earth won’t be recovered
What is the layer in the water column called when there is a change in density
Pycnocline
How is density impacted by temperature and salinity?
Colder water is denser
Saltier water is denser
Why are colder waters more salty?
Warm surface waters move to the poles get colder and increase in salinity due to sea ice formation
cooler and saltier water is more dense so it sinks an pushes the bottom waters south
Where does rising of water occur?
the rising of water occurs in the Indian and Pacific Ocean because it warms up
this movement of warm water keeps Iceland and Greenland ice free in the summer
Facts about Broeckers Great Ocean Conveyor Belt
Wallace Broecker came up with it in 1980s
it was suggested that fluxes between heat and freshwater could cause a conveyor belt
What was the Young Dryas and what caused it?
type of circulation caused it…
an abrupt turn to glacial conditions and back to interglacial conditions
it was thought to be caused by a temporary shift in the thermohaline circulation
the temperature decrease and decreased by 10ºC in a decade
What is the Atlantic Meridional Overturning Current (AMOC)?
conveyor belt…
it carries warm water north in the Gulf Stream.
While water warms it evaporates and cools further north
- this increases the density and salinity of the seawater which drives the oceanic conveyor belt
Why might the AMOC slow down?
An increase in greenhouse gases warms the atmosphere which causes the surface of the ocean to retain more heat.
This leads to an increase in rainfall. A more active hydrological cycle increases the melting of ice, leading to an increase in freshwater inputs which impacts the density of water. The water will become less dense which leads to a weaker sinking mechanism which impacts the whole conveyor belt.
what happens if the AMOC weakens or collapses
The North will receive less warm water and Europe will get colder
What is Ekman Transport?
It is a wind driven current that shows that water transport from the wind travels at 90º to each other
this is due to friction and the Coriolis force
What are Gyres?
They are rotating current systems compromised of the Earths rotation (Coriolis effect) couples with currents and the prevailing winds
There is one big ‘whirlpool’ in the N. Hem. and one in the S. Hem which are called GEOSTROPHIC GYRES as they are a balance between the coriolis and the horizontal pressure gradient
What are the 6 main gyres?
- North Pacific Sub Tropical Gyre
- South Pacific Sub Tropical Gyre
- North Atlantic Sub Tropical Gyre
- South Atlantic Sub Tropical Gyre
- Indian Ocean Sub Tropical Gyre
- Antarctic circumpolar current (ACC)
What direction does the wind blow in the polar regions?
The winds blow west to east (50-70º N/S)
- this is the same direction in which the Earth spin
Why is circulation better in the south pole than it is in the north pole?
South pole circulation works well as it is a landmass surrounded by an ocean meaning nothing is obstructing the currents
North pole circulation doesn’t as the currents bump into the landmasses as it is an ocean surrounded by a landmass
What are the 4 types of currents?
- geostrophic currents
- western boundary currents
- eastern boundary currents
- transverse currents
Name the 5 Western Boundary Currents
- The Gulf Stream
- Brazil Current
- Agulhas current (Indian ocean)
- Kuroshio current (by Japan)
- East Australia current
What are the properties of WBCs
They are the
- fastest
- narrowest
- deepest
they form in the western side of an oceanic basin
they transfer warm water polewards due to the Coriolis force
Properties of the Gulf stream
- largest WBC
- moves with 2m/s
- 450m depth
- shifts the water 160km per day
- 70km wide and merges with the NA gyre and Sargasso Sea
- transports 55 sverdrups (sv)
- it meanders and loops and creates eddies
- because it is narrow it has distinct boundaries
What is a Sverdrup?
A unit of volume for the transport of 1 million cubic meters of water per second
- eg Gulf stream is 55sv
Why do eddies form?
They form due to the natural tendency of water to be turbulent and chaotic
this happens due to spatial variations in temperature, density and velocity
what is an eddie?
a circular current of water
How do eddies form?
small scale answer…. think what is happening
if one water mass is moving faster than another, horizontal and vertical shear forces can occur. This makes water turbulent and eddies can form.
They begin as small disturbances but will grow and develop into eddies which break off
Eddies need a rigid boundary for the shear force to occur
What direction do warm core eddies spin?
They spin clockwise and break off above the gulf stream
what direction do cold core eddies spin
They spin anti-clockwise and break off below the gulf stream
How big are eddies and how long can they last?
They can range from 10-500km in diameter and can last for more than 3 years.
They can also reach the seafloor
What are the properties of Eastern Boundary Currents?
- carry cold water to the equator
- form in the east of an ocean basin
- wider (~1000km)
- shallower (~2-4km deep)
- slower (eg Canary current only 16sv)
- they have no defined boundary due to width so no eddies form
What are the 5 EBCs?
- Canary current
- California current
- Peru/Humboldt current
- Benguela current
- Western Australia Boundary Current
What are Transverse currents?
they link Western and Eastern boundary currents to form big gyers
ie location and how they form
Properties of Transverse currents
- they move transversely
- they are derived from the stress of the wind with the spin of the earth
- found at the fringes of the tropics and at mid latitudes
- at the tropics, they go WEST
- at the mid latitudes they go EAST
What are the main transverse currents?
- North Pacific current
- North Equatorial Pacific Current
- South Equatorial Pacific Current
- North Equatorial Atlantic Current
- South Equatorial Atlantic Current
- North Atlantic Current
- ACC (also called West Drift)
Properties of the Transverse North and South equatorial current
The North and South equatorial currents are broad and shallow
they transport 30sv westward
Properties of the Transverse mid-latitude currents
they are broader and shallower than the equatorial currents due to the westerly winds (blowing east)
What is the Western Intensification?
An increase in the strength/intensity of western currents
- WBC are much more concentrated
- EBC diffuse
what causes coastal upwelling?
When wind blows OFFSHORE (in N.Hem, when wind blow S, in S.Hem. when wind blows N) due to the Ekman spiral the water moves at an angle to the wind. This pushes the surface water away from the coast. The removal of water causes the replenishment from deep waters
What weather conditions and biological activity are associated with coastal upwelling?
deeper waters have more nutrients and so upwellings cause these deep waters to come into the photic zone which increases the biological productivity.
lots of upwelling causes lots of cold water to rise from the depth which can cause lots of mist and clouds to form as the overlying air becomes cold
what causes coastal downwelling?
When winds blow ONSHORE (in N.Hem, when wind blow N in S.Hem. when wind blows S), due to the Ekman sprial the water moves at an angle to the wind and pushes the surface water towards the coast. A slight hill will form and cause the water to get pushed down, resulting in the deepwater moving away
What do downwellings redistribute
They redistribute organisms from the photic zone as well as dissolved gases such as O2 and CO2
what is a water mass and how can you identify it?
A body of water that can be identified by its temperature and salinity properties.
What are the 3 types of water masses
- Upper
- Intermediate
- Deep/Bottom
Properties of the Mediterranean water mass
- Formed in the Med
- high in salinity as it is a warm area with lots of evaporation and very little freshwater input (dry and saline)
Explain the process of how the Mediterranean Water Mass is formed and how it ends up in the the Atlantic
Mistral Winds occur (South of France) which are cold and dry which cool down the surface of the water. This increases the density of the surface water which drives vertical mixing called CONVECTION in the Med.
This convection goes all the way to the seafloor, moving the Western Mediteranean deep waters (WMDW) over the Straits of Gibraltar and into the Atlantic
Because the water is saline, it sinks until Neutral Buoyancy is reached in relation to its surroundings (~1000m depth)
It then spreads out horizontally but remains recognisable due to its high temperature and salinity
What is an upper water mass?
An upper water mass refers to the surface and upper parts of the thermocline
the upper water mass can vary in thickness depending where it is
- eg Pacific Equatorial and Indian Equatorial water have low salinity due to large freshwater input (rain) and are quite stable so are SHALLOW upper water masses
Properties of central water masses?
where do you find them, are they stable, thicker or thinner than upper
- they occur in the sub-tropical gyre region
- they are a lot thicker due to circulation systems (convergence and divergence) where water gets pushed down making it thicker
- they are unstable as there is more cooling in winter. This leads to mixing and as colder water gets denser it sinks causing it to mix. THIS MAKES CENTRAL WATERS MORE HOMOGENOUS (7-20ºC)
what is an example of a central water mass?
Sargasso Sea
Where do you find/What is an intermediate water mass?
They flow between the upper and deep/bottom water masses
Name some Intermediate water masses
- Mediterranean water
- Western Atlantic Sub-Arctic Water (WASW)
- Antarctic Intermediate Water (AAIW)
List some properties of Antarctic Intermediate Water (AAIW)
- AAIW is a MODE water mass
- the water temp doesn’t vary very much
- it forms in the Antarctic polar frontal zone where it is very cold and has low salinity due to melting ice
- approx 2-4ºC and 34ppt
What is a deep water mass?
cold and dense water mass that flows at the seabed
what is the main deep water mass?
North Atlantic Deep Water (originates in the Greenland Sea)
How does North Atlantic Deep Water (NADW) form?
Cold polar water is carried south through the Fram Strait by the East Greenland Current.
in the winter, the water becomes denser due to ice formation and lower temps so it sinks (34.9ppt & temps below 0ºC)
It gets stuck behind a submarine ridge between Greenland and Scotland
it occasionally spills over into the Atlantic through the Denmark Strait and between the Faroe Islands and Iceland
This leads to turbulent mixing with the overlying water
the dense water from the basin mixes with the North Atlantic Central Water to form NADW
Characteristics of NADW
- warm
- nutrient rich
- saline
What causes the stratification of Polar Waters?
think about main differences in the layers and ice formation
The contrast between high salinity warmer waters underneath the cold surface waters leads to turbulent mixing.
This mixing causes a sinking mechanism where in the North it causes a mixture of cold winds, precipitation and ice formation. In the South it causes more ice formation and drives the mixing
What is the difference between an Ice Sheet, Ice Shelf and Sea Ice
Ice Sheet = on land
Ice shelf = on water but connected to a continent
Sea Ice = not connected and just floating around
What are Polynyas and how do they occur?
They are large, persistent regions of open water and thin ice that occur within much thicker pack ice
They occur when ice is blown away from the land or when you have an upwelling of warmer waters
Coastal polynyas occur due to winds and open ocean polynyas due to topography
Characteristics/Properties of Antarctic Bottom Water
Found in 3 ocean basins
- Pacific
- Indian
- Antarctic
It is the most widespread water mass in the ocean
high salinity and very cold (-5ºC)
Extremely dense
moves very slowly and takes about 1000 years to reach the eq.
Where does AABW form and migrate to?
Migrates north along the seafloor and is formed off the coast of Antarctica near South America and skims along the antarctic continental shelf.
What are teleconnections?
_ and _ anomalies…..
They are spatial and temporal large scale anomalies that influence the variability of atmospheric circulation
they provide the link between weather phenomena and regions very far apart
something that happens in one place impacts another place
List the 5 examples of teleconnections
- Atlantic Oscillation (AO)
- North Atlantic Oscillation (NAO)
- El Niño Southern Oscillation (ENSO)
- Pacific Decadal Oscillation (PDO)
- Pacific North American Index (PNA)
What is the North Atlantic Oscillation (NAO)
high and lows… where does it impact
It is a large scale mode of climate variability that directly impacts UK weather systems and North Europe
It is a pressure index which describes the average pressure difference between the Icelandic Low and Azores High
Name of the Pressure systems in the North Atlantic (Links to NAO) and what they influence
High pressure system over the Azores Island Group
- called Sub-Tropical Azores High
Low pressure system over Iceland
- called sub-polar/Icelandic Low
these are responsible for the weather patterns in the North Atlantic. they influence
- rainfall
- temperature
- wind direction and speed
Describe/Explain the Positive NAO phase
- occurs when there is a higher than average pressure difference (eg high is quite high and low is quite low)
- westerly winds dominate (W–>E)
- the strength of the winds correlate to the strength of the pressure difference
- brings warm air to Northern Europe
- the Gulf Stream is positioned further North due to stronger winds giving N Europe warmer air
What are the positive and negative NAO phases associated with?
the intensity and the location of the jet stream
associated with the storm track across the Atlantic
- heat and moisture transport
- changes in wind and temperature
- wind patterns in US and Europe
What does the positive NAO phase lead to?
more heat and moisture is transferred over to NW Europe
warmer and wetter winters in the North US and North Europe and colder in Greeenland
warmer air carries more moisture so there is a higher than average precipitation in Northern Europe and lower than average precipitation in Southern and Central Europe (because jet stream is shifted north)
Describe the Negative NAO phase
- lower than average pressure difference
- westerly winds are weaker so there is a change for easterly winds to coming in which brings cool and dry polar siberian air to Europe
- cold and dry winters in N Europe and East US
- Jetstream is positioned further south and is weaker
- less frequent storms across the atlantic
- mild and wet winters in Greenland and in South and Central Europe
What is El Niño?
a climatic phenomena that causes increased warming in the Eastern tropical Pacific (PERU)
What happens in el niño?
how often… how long… where
every 3-8 years the pressure centres across South Pacific flip, causing the high to be in the West (N Aus) and the low to be in the East (Peru)
this causes the trade winds to weaken or change direction
- warm water flows from W-E causing less upwelling in the East to occur meaning there is less nutrient rich water
it can last for 1-3 years and affect any region in a similar latitude (~30º) from equator
What are the consequences of El Niño?
sea level rises by about 20cm in the Pacific
water temps can rise up to 7ºC warmer
- warmer water = more evaporation = more precipitation
occurs about ~2000km W of Peru
This causes Peru to have lots of rainfall inland when it is usually a cool and dry climate.
it impacts both marine and terrestrial eviornments and habitats (including humans)
What does a normal year look like in comparison to ENSO?
surface winds are E-W (trade winds) and follow the high to low pressure
Occurs in the south pacific sub tropical gyre
- south pacific equatorial current, east australian current, AACC and Peru current
usually cold upwelling by Peru which is nutrient rich and the moves west due to trade winds
Previous ENSO 1982 year
- strongest and most devastating
- winds reversed
- water temp raised by 7ºC and were still detected 12 years later
- caused weather disasters on every continent (droughts, fire, storms, floods)
- increase in mosquito illnesses in US due to increase moisture
- angular momentum of the Earth changed making the day 0.2milliseconds longer (slowed the earth down)
Previous ENSO 1998 year
- series of tornadoes that killed 40 people
- tornados occurred due to warm air meeting a polar front
- Hawaii and Papa New guinea has drought
- Peru has extreme winter leaving 250,000 people homeless
- US had lots of storms that led to lots of natural disasters (ie mudslides, landslides etc)
Previous ENSO 2015 year
- drought, flooding, tropical cyclones occured in pacific
- global temp increased from 0.13-0.18ºC
- raised levels in CO2 due to more precipitation causing more weathering which speeds up the carbon cycle
- 8 million people recieved food aid
what happens in a La Niña year?
after El Niño, it goes back to normal or can go into over drive
La niña the normal, just intensified.
- stronger currents and lots of up welling
- dry coast along south America
- east is cooler but west warms rapidly
can last 1 year
What is Ekman Motion?
to do with depth in the water column and the forces in the water column
the speed of wind driven currents decreases with depth and this happens in a spiral
the top layer is subject to wind stress and each layer below is subject to friction/shear from water layer above it
the surface layer of water deviates at 45º from the wind and is deflected down into the water column
How does the ekman layer form?
there is a 45º deviation from wind to water due to the drag between the coriolis and the surface of the water
this leads to the ekman spiral and when the spiral disappears you have the ekman layer
What is Divergence?
if wind blows accross the surface, water moves away at an angle (ekman) causing divergence of water which creates space which water from below will fill
causes upwelling
the horizontal movement of water causes vertical movement of water
what is Convergence?
when water comes together, it bulges up but cant go anywhere so it has to go down, causing DOWNWELLING
both convergence and divergence happen ~6m from the waters surface
what are cyclonic winds?
cyclonic winds causes water to be pushed away from the centre of the cycle
it causes surface divergence, causing upwelling
water is removed form the centre and so water from below comes up
KNOWN AS EKMAN PUMP
what are anti-cyclonic winds?
anticyclonic winds cause water to be pushed towards the centre of the cycle
it causes surface convergence and downwelling as water forms a hill at the centre and has to go down
what causes the thermocline to rise and the sea surface to lower?
upwelling
what causes the thermocline to lower and the sea surface to rise?
downwelling
how are waves generated?
generated by the transfer of energy from the wind to the surface ocean
what displaces water particles from equilibrium?
The wind which is a disturbing force
how to water particles regain equilibrium?
they regain equilibrium by restoring forces such as gravity and surface tension
3 factors that affect wave growth
- wind speed (velocity (U))
- fetch length (distance which the wind blows (F))
- duration of the wind event (D))
how are surface waves generated?
they are wind generated.
if 2 fluid layers (eg water and air) have different speeds in contact, frictional stress is generated
how does air pressure impact waves
most of the air pressure on the windward side of the wave which pushes the water down
minimum air pressure in contact with leeward side of the wave, allowing the water to move up (which drives wave motion forward)
what is laminar flow?
the velocity of the liquid……
the velocity of the liquid is controlled by the molecular make up
smooth flow
if the water isnt mixed, energy isnt transferred down as easily
what is turbulent flow?
eddie viscosity
if the wind moves over the surface, due to eddie viscosity there is downward momentum from the energy causing the water to be disturbed
if the water is already disturbed, it is easier to transfer the energy
what is wave length and what tells us the speed of the wave?
crest to crest/trough to trough (L)
full length of the wave tells us the speed of the wave
what is wave height?
trough to crest (H)
what is significant height?
taking the average of the 3rd biggest wave height (Hs)
what is amplitude?
half the wave height to crest (a)
What is wave steepness?
wave height / wave length
what is wave period?
time it takes for one full wave to pass
measured in seconds (s) (T)
what is wave frequency?
the inverse of the wave period
1 / wave period = wave frequency
as wave period increases, frequency decreases
the number of waves to pass a point each second
what are the wave types?
capillary waves
ultra gravity waves
ordinary gravity waves
infra gravity waves
long period waves
ordinary tide waves
trans-tidal waves
these increase in size and period
What is dispersion relation?
pebble example
it is the separation of waves due to different speeds
- they form groups and sort/filter themselves
this biggest waves arrive first
What is a tsunami and what are its characteristics?
caused by tectonic activity (either uplift, shifting or earthquakes)
it creates a big wave and can impact land
wavelength is between 200-250km
are tsunamis deep water or shallow water waves?
they are created in deep water but have very long wavelengths so in relation of their wavelength to depth of the water, they are shallow waves
Deep Water Approximation
- measuring wavelength formula
Lo = gT^2/2π
g is gravity
T is wave period
Deep Water Approximation
- measuring wave speed/celerity formula
Co = gT/2π
g is gravity
T is wave period
Deep Water Approximation
- only valid for deep water waves when…
h / Lo > 0.5
it is a deep water wave when the wave depth is more than half of the wavelength
Shallow Water Approximation
- measuring wavelength formula
Ls = T √gh
wavelength = wave period √gravity x water depth
Shallow Water Approximation
- measuring wave speed/celerity formula
Ls = √gh
wavelength = √gravity x water depth
Shallow Water Approximation
- only valid for shallow water waves when…
h / Lo < 0.05
only valid when the water depth is less than a 20th (1/20) of the wave length
AMOC PAPER CITATION
(Oltmans et al, 2020)
ENSO PAPER CITATION
(Wang et al, 2022)
