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What way do winds on a non-rotating earth flow?
- Non-rotating: North to South.
- East to west: Polar winds (easterlies), trade winds.
- West to east: Jet stream & Westerlies
What is main source of energy* that drives surface currents? deep currents?
What depth for surface currents?
- Surface currents: winds (100-200 m).
- Deep currents: thermohaline circulation
But essentially, sun. Sun is responsible for wind (density diff in atm due to solar heating & radiative/evaporative cooling); thermahaline circulation (evaporation/precipitation; solar heating & radiative cooling).
How does sun affect energy behind currents? (3, 4)
Sun is responsible for wind (density diff in atm due to solar heating & radiative/evaporative cooling)
- Thermahaline circulation (evaporation/precipitation; solar heating & radiative cooling).
What is frictional coupling (or lack of it) responsible for? (2 each)
- 1. Creation of currents
- 2. Ekman motion
Lack of it: Coriolis effect & why you find currents in places w/o wind.
How are currents generally generated? (3)
- 1. Wind blows over ocean surface setting water in motion thanks to friction (Ekman transport)
- 2. Water continues moving due to inertia
- 3. Coriolis effect deflects currents (bc frictional coupling bt surface water and earth's surface is small).
Describe how Ekman motion/Coriolis effect determine direction of current.
- 1. As surface water is set in motion by wind, kinetic energy is transferred down water column via friction.
- 2. This successively sets in motion a series of thin layers of water cum sole--> Ekman spiral
- 3. Additionally, each water is ALSO affected by Coriolis effect
- 4. Water will keep moving cum sole to wind stress/drag until forces balance and vector addition of Coriolis effect force + Wind force = 0.
- 5. When forces are balanced, velocity of water happens to be 45 degrees to right (N) or left (S) of wind direction and net transport of water is 90 degrees to wind direction. (for fully developed Ekman spiral. Otherwise, will be less than 90 degrees to right and less than 45 degrees to right.
What is the Ekman layer depth: quantitatively? Qualitatively?
Quantiatively: 50-100 m.
Qualitiatively: Depth underwhich there is no water motion due to winds.
Define the geostrophic current.
Cyclonic fluid motions (gyres) maintained as a result of near balance bt gravity induced HPG and Coriolis effect.
Basically, currents are able to move between isobaric areas of ocean due to earth's rotation.
What are forces to consider when looking at surface current patterns? (4) How far down can some extend?
Gravity, friction, Coriolis effect, horziontal pressure gradient force
To 1000 m
What are some key points about forces to understand? (2)
- 1. Forces: acceleration of water particles will occur unless balanced by another equal, opposite force
- 2. Large scale wind-driven circulation does not accelerate bc forces are in balance.
What is Ekman transport magnitude proportional to?
Magnitude of wind stress
What occurs at convergence points? (3) for example at horse latitudes? What about at divergence points? What are these caused by?
Due to Ekman transport bringing together water from diff directions, you see CONVERGENCE at which sea surface becomes elevated, depressing thermocline, and deepening nutricline.
Divergence, at which sea surface becomes depressed.
Define geostrophy. What does it generate?
Frictionless balance between pressure gradient and Coriolis force --> generates currents that move around a "hill" aka gyres.
What does geostrophic balance require of velocity of water?
That it be into page (along pressure lines) rather than across them.
How do you get wind driven currents as much as 200-500 m deep? (3)
Ekman motion (50-100m) --> creates resulting pressure gradient felt throughout water column --> geostrophic current (indirect wind friction) occurs at much greater depth (200-500 m).
Where are gyres most developed? How many gyres are there? In which currents do they occur and in what directions?
- Closer to equator.
- 1. Subpolar gyre - polar current (CCW)
- 2. Subtropical gyre - North equatorial current (CW)
- 3. Subtropical gyre - South equatorial current (CCW)
What is the standard unit of ocean current movement? Name and value
Sverdrups: 10^6 m^3/s
Draw currents (5 - includes counter current), gyres (3), west wind drifts (2) in comparison to winds: polar easterlies, cyclones, westerlies, anticyclones, westerlies, polar easterlies,
Remember: there are way mor ethan 5 currents, there are 7 gyres, etc.
At 36 degrees N:
1. What is effect of gyre? (2)
Bc of gyre, west is warmer than east. Also, extra pile up due to gyre means you have thicker and thinner layer of surface water meaning permanent thermocline
(depth in ocean w/ sig temp change) is much deeper on west vs east.
- Go in at west and come out at east. Also west has strong
- west: strong northward gulf stream; east: slow southward flow due to open gyre.
What is the difference between water density at equator vs arctic?
What about if you move vertically vs horizontally in a cross section of ocean? Why?
At equator low salinity and as you move towards higher latitudes.
Also, salinity increases mor edrastically in vertical than in horizontal direction bc of stratification.
What is useful about measuring salinity?
Can be used to trace water masses, can be measured with high precision so good way to track water masses
How do density gradients set fluids in motion?
Denser water slides under lighter water until interface between two water masses is horizontal. Results in sharp density transition with little mixing. movement is slow.
What are density gradients a function of? (2) Which water masses can mix freely?
Density gradients are a function of differences in S% and water masses.
Water masses that do not differ in salinity or temp can mix freely.
Where do we find higher salinity waters? (3)
In gyres, ocean basins, and coasts (red sea/med sea)
How do we explain patterns of salinity?
1. Evaporation vs precipitatoin and also location to freshwater inputs which would decrease salinity.
Which gyres have highest salinity? Why?
Bc surface salinity is greatest in subtropics and marginal seas
Where do we generally have low primary productivity?
How is primary productivity measured?
Using absorbance of light associated with chlorophyll: take average ratio of chlorophyll:total cellular carbon, then ships go out and measure how fast phytoplankton photosynthesize and use satellite pictures together in an algorithm.
What are characteristics of subtropical gyres? (2)
- 1. Low phytoplankton density (low primary productivity; nutrient sparse)
- 2. Seasurface elevation
What causes subtropical sealevel highs?
With what kinds of winds? (2)
Ekman transport leading to convergence --> buildup of surface water (50-100m) by trade winds and westerlies.
What are geostrophic currents caused by? Another name for geostrophic currents?
What sends water masses into motion?
Geostrophic currents around these sealevel highs result from pressure gradient (away from hill) & Coriolis deflection of downward flowing water masses (inwards).
Hill = high pressure.
Pressure gradients set water masses into motion.
At what angle does the inward Coriolis force of fluid balance the outward pressure gradient?
At 90 degrees.
Where do we find particularly dense waters?
1. Because of cold temp (3)
2. Because of high salinity (2)
Cold temp: Arctic Ocean (NADW), Weddell Sea & Ross Sea (AABW)
High salinity: Red/Med seas
What effect does Med/Red Sea have on Atlantic, Indian, in comparison to Pacific ocean?
Med/Red Sea increase salinity of Atlantic & Indian Ocean and increase the temperature of these two oceans too in comparison to Pacific.
How does Coriolis/Mid Ocean Ridge affect distribution of deep water?
Midocean ridge & Coriolis turns water to left and leaves antarctic bottom water pinned to South American continent.
Same with salinity.
Below pycnocline, what is the driving force for currents?
Density differences due to temperature and salinity (thermohaline circulation)
Where is surface water salinity greater than deep water salinity? Why?
Between 45 degrees N - 45 degrees S. Bc of high amount of evaporation, but its temperature is enough to offset density diff so it stays on top.
Where is there no pycnocline?
Where is it shallower? Why?
In higher latitudes bc heat lost from ocean > heat gained from solar radiation, so surface waters cool --> sink --> deep water.
Shallower at equator bc of upwelling.
How does surface layer salinity compare to deep water salinity? What does it depend on? What is the exception?
It's variable depending on latitude (precipitation:evaporation), but is generally higher, EXCEPT IN high latitudes (high precip, low evap).
How are water layers separated in deep water? In what directions can water masses spread? Why? (2)
By slight variations in density
Water masses can spread only horizontally, bc density diff inhibit vertical mixing & it requires A LOT more energy bc of gravity
List the deep waters in decreasing density
Antarctica Bottom Water > North Atlantic Deep Water/Med Water > Antarctic Intermediate Water
Where is Atlantic Bottom Water formed? Where does it move? Into which ocean basins? Where isn't it found and why?
Weddell Sea, moves NORTH.
Enters all 3 ocean basins (Atlantic, Pacific, Indian).
Not found in North Pacific (straits/sills) or Indian (bc doesn't extend to high latitudes).
What do topography and Coriolis maintain? (2) Against what?
They keep AABW & AAIW against South American continent.
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