Deep Ocean Circulation

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Deep Ocean Circulation

• Why the hydrogen bomb is good
• Thermohaline circulation
• Consequences of ocean circulations

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Ocean topography shows us that the major global
downwelling occurs in polar regions
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Ocean circulations

• Surface ocean circulations are driven by winds
• Deep ocean circulations are driven by differences
  in density

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Density

• Mass/Volume
• Two processes are mostly responsible for changes
  in density in the ocean
• Temperature
• Salinity
• Density changes are larger vertically than
  horizontally

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Salinity

• A measure of the amounts of salt in the ocean
• Measured by parts per thousand, or per mil
• Equal to grams per kilogram
• Most of the ocean salts are chloride and sodium,
  the ingredients of table salt
• Produced by rock weathering and transport by
  rivers
• Removed by evaporation, biological and volcanic
  processes, and sea spray

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Salinity variation

• The global variation in salinity is the result of
  a balance between the inputs and outputs of salts
  and fresh water
• So areas with high evaporation and low input of
  fresh water will have high salinity
• Great Salt Lake, Red Sea, Mediterranean
• Coastal boundaries of oceans have lower salinity
  because of river input

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Effects of salinity

• Increases in salinity make water more dense than
  fresh water

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Temperature

• Colder water is denser than warm water
• Densest water is 4 deg C

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Vertical profiles

• The top surface 100-200 meters are called the
  mixed layer
• Below this is a transition zone of about 1000m
• Below the transition zone is the deep zone

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Pycnocline Thermocline
Halocline Pycnodensity
Thermoheat Halosalt
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Pycnocline

• The overall profile of density change with depth
  in the ocean, called the pycnocline, can be
  formed by a thermocline, halocline, or
  combination of the two
• Temperature changes are usually more important

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Transition zone

• This zone, characterized by sharp increases in
  density, is very stable
• Not much mixing can occur across this gradient
• Water movement occurs along sloping line of
  constant density

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Deep zone

• 80-90 of ocean volume is contained in the deep
  zone below 1000m
• Characterized by small gradients of temperature
  and salinity

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An apparent conundrum

• How can we have a deep ocean circulation if the
  profile of the ocean is so inherently stable?

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Polar waters

• The surface of polar waters is relatively denser
  than other surface waters
• Especially in winter
• Cold temperatures
• Formation of sea ice increases the salinity
• So there is less of a density gradient in polar
  regions

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Polar mixing

• Polar surface waters can become so dense that
  downwelling occurs
• Isopycnal mixing

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Regions of deep water formation

• The dense water needed to drive the ocean
  circulation is produced in two polar areas
• Greenland, forming the North Atlantic Deep Water
  (NADW)
• Antarctica, forming the Antarctic Deep Water
  (AADW) in the Weddell Sea

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NADW
Antarctic Deep Water (Weddell Sea Ice)
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Global salinity

• Geographic variation of salinity also has a major
  impact on ocean circulation

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GLOBAL VARIATION IN SALINITY Atlantic
evaporation exceeds precipitation and river
inputs. Input of saline water from the
Mediterranean and Caribbean Seas Pacific
evaporation about equal to inputs Consequence
major Atlantic-Pacific gradient in salinity
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Thermohaline circulation

• The combination of the deep water formation and
  salinity differences (both from sea ice formation
  and oceanic differences), leads to the
  thermohaline circulation
• Thermoheat
• Halinesalt
• Acts to redistribute energy and salinity

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Major source

• The Atlantic is the major pump for the
  thermohaline circulation
• Atlantic goes farther towards the poles and thus
  gets colder (more dense)
• Has higher salinity (more dense)

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The thermohaline circulation (global ocean
conveyor belt)
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Actions of the conveyor belt

• Bring cold water from the poles toward the
  equator
• Bring warm water from the equator toward the
  poles
• Acts to limit salinity differences

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Ocean differences

• Atlantic, with denser water, has an average deep
  water age of about 275 years
• Pacific, with less dense water, has an average
  age of about 510 years

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This figure, showing the distribution of tritium,
is direct proof of the theoretical concept of the
thermohaline circulation deep water formation
only occurs in polar latitudes
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Coriolis effect?

• Yes, but strongly limited by the deep ocean
  topography

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Overall effect of ocean circulations

• We have a known imbalance in the global energy
  budget
• Both the atmosphere and the ocean regulate this
  distribution
• About 50 each

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Local effects of ocean circulations

• If prevailing winds are from a warm ocean to
  land, this greatly moderates the climate
• Pacific Northwest
• Europe
• If prevailing winds are from a cold ocean, cold
  deserts result
• Southern Chile (Patagonia)

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Biological effects of deep water movement

• Photosynthesis in upper water
• Leads to phytoplankton
• Leads to zooplankton, etc.
• Organisms in these areas die, sink, and decompose
• Releasing their nutrients to the deep water
• Thus the upwelling of cold, deep water brings
  nutrients back to the surface