8: Ocean Currents

currents
1. what drives ocean currents?
  1. wind
    1. friction between air and water molecules drives surface curents
      1. friction on surface is poor
      2. up to ~200m depth
        currents get slower as they get deeper
    2. ocean currents are 1-3% of wind speed
2. density differences
  1. differences in density between water masses causes rising and sinking of these waters
    1. water evaporatese, salinity is increased
      1. dense water mass sinks down
        surface waters flow toward the are
    2. displaced deep waters come up
  2. additional causes
    1. ice formation
      1. salt is left behind
        sinks under ice into deeper water
    2. cooling of surface waters
3. momentum
  1. once water molecules are in motion, even if wind dies, the molecules will continue to flow for great distance
  2. steering forces
    1. coriolis effect
    2. horizontal pressure gradients
    3. shape of coast
      1. deflection of currents by landmasses
4. pressure gradients
  1. bulge of water sits at angle
    1. water tries to level itself out
      1. water flows back to where it came from
  2. water flows form high to low pressure
5. major surface currents
  1. coriolis effect northern turn clockwise
  2. gyre: sicrulation system of water
    1. moving heat
      1. gulf stream warm water keeps eastern coast of n. america warm at all times
        warmer relative to other places in latitude
        waters right next to coast more controlled by air temp and etc
      2. gulf stream brings mild, wet seasons to western europe
    2. 5 major gyres except by india
    3. northern/southern/etc boundary current
      1. names: memorize: gulfstream, antilles, etc.
ekman spiral
1. water mass of infinite depth
  1. as wind blows along surface, pushes surface water molecules toward wind
    1. net motion of this layer deflects to the right
      1. coriolis deflection
  2. layer beneath is pushed along same line as above (imposed force)
    1. but slightly deflected to right
  3. continues down to certain depth
    1. bottom of spiral ~4% of surface current speed
      1. each layer down is smaller due to lost energy
        water moving down: ekman transport
2. average net motion of the water-- the direction of spiral are 90 deg from the wind
  1. deeper waters are floating away from the coast
3. requirements
  1. uniform water density (rare)
  2. sufficient water depth
  3. ~2-day constant wind speed
thermohaline overturn engine
1. oceanic conveyor belt
  1. how all of the global oceans are connected
  2. gulf stream brings warm waters into actic
    1. cools, more dense
      1. turns into ice, water is salty and denser
        water sinking and thus overturning
        at that depth moves south then into indian ocean
        upwelling and water becomes warm
  3. brings warm water into western europe
    1. strength in trade due to the current
      1. coast is completely ice free
  4. embayments in arctic due to warm water that wont sink
2. warm surface waters cool and sink
  1. go down toward equator
    1. process of "overturn" drives atlantic ocean circulation
3. how might global warming affect this circulation?
  1. increase in arctic surface water temp reduces "ability" of surface waters to cool and sink
  2. increased freshwater input from melting arctice glacial ice reduces density of cool arctic waters and its "ability" to sink
  3. shutting down may have happened in the past
    1. younger dryas cooling event
      1. reconstruction of temperatures
      2. occurred between 12700 and 11500 ya at the end of the last ice age
      3. debate whether northern hemi only
        or north atlantic only (no pacific?)
      4. hypothesis
        massive ice sheet several km thick
        covering allmost all of canada and greenlad=nd
        begins melting, raises sea level several levels
        a lot of the water pooled in lakes
        ice pushed crust down
        filled lower lined crust with water
        lake agassiz from laurentide ice sheet
        this held more freshwater than all fresh lakes combined today
        this lake drained into north atlantic as result of break up of laurentide
        caused reduction of atlantic meridional overturning circulation for at least 1000 years
        freshwater floating on top as a lens initially
        extremely low density, than cool saltwater
        pushes gulf stream south, so it doesn't touch northern europe

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8: Ocean Currents

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