El NioLa Nia and Thermohaline Circulation

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El Niño/La Niña and Thermohaline Circulation
Lecture 14
OEAS-306
March 17, 2009

• Outline
• Review from Last Lecture
• ENSO--El Niño/La Niña
• Deep Water Masses
• Thermohaline Circulation

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Upwelling Velocity
N.E. Trade Winds
S.E. Trade Winds
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Normal Conditions

• Under normal conditions, high pressure
  dominates over the eastern Pacific, with low
  pressure over the western Pacific.
• This reinforces strong westward surface winds.
• Winds drive surface currents to the west leading
  to downwelling in western Pacific and upwelling
  in Eastern Pacific.

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Surface Water Temperature
Equatorial Upwelling
South American Upwelling
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Equatorial Water Temperature Under Normal
Conditions
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The Southern Oscillation
Every 3 to 8 years, the pressure field over the
Equatorial Pacific changes.
This is quantified by the Southern Oscillation
Index (SOI)
Strong Pressure Gradient
La Niña
El Niño
Weak Pressure Gradient
SOI is the normalized sea-level pressure
difference between Tahiti and Darwin, Australia.
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• Under El Niño conditions, the pressure
  gradients weaken or reverse, with low pressure
  shifting east over central Pacific.
• This reduces/reverses the westward surface winds.
• As a result, warm water moves eastward, shutting
  down the upwelling of South America.

El Niño Conditions
Warm water arrived off South America around
Christmas and was called El Niño, or Christ
child by Peruvian fisherman.
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Surface Water Temperature
Normal Conditions
El Niño Conditions
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El Niño conditions shut down South American and
Equatorial Upwelling
Normal Conditions
El Niño Conditions
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La Niña (the girl) is the opposite of El
Niño. Surface atmospheric pressure gradients are
intensified, increasing the trade winds, which
increases South American and Equatorial Upwelling.
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Shutting down the South American Upwelling has
significant biological consequences.
Strong El Niño event of 1972 combined with
over-fishing contributed to the collapse of the
Peruvian anchovy fishery.
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El Niño Southern Oscillation (ENSO) has
significant impact on Global Climate
Changes due to El Niño Conditions
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Remember, pressure gradients that drive
circulation can be either Barotropic or
Baroclinic.
Barotropic Pressure Gradient
Baroclinic Pressure Gradient
?z
z 0
?x
?x
More dense
Less dense
Uniform density

• Caused by horizontal variations in sea surface
  height.
• Is uniform with depth

• Caused by horizontal variations in density.
• Varies with depth

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Surface currents are driven primarily by the
wind. Wind sets up a pressure gradient
(barotropic) in response to Earths rotation to
maintain the geostrophic balance.
In contrast to surface currents, bottom currents
are driven primarily by density differences
(convection and baroclinic pressure gradients)
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Water Masses and Deep Ocean Circulation

• Water mass A region of water identifiable by a
  particular combination of physical and chemical
  properties (usually temperature and salinity).
• Water masses can be classified based on depth
• 1) Surface water to a depth of 200 meters
• 2) Central Water to the bottom of the main
  thermocline
• 3) Intermediate Water to about 1,500 meters
• 4) Deep Water below intermediate water but not
  in contact with bottom.
• 5) Bottom Water in contact with seafloor.
• Examples
• North Atlantic Deep Water (NADW) Cold (T 2-4
  C) and relatively salty (S 34.9 35.0 psu).
• Antarctic Bottom Water (AABW) Very Cold (T
  -0.4-1 C) and relatively fresh (S 34.6 34.8
  psu).
• Mediterranean Intermediate Water (MIW) Warm (T
  5-10 C) and very salty (S 35.5 35.9 psu).
• Ocean circulation is measured in Sverdrups (Sv)
  -- 1 Sv 106 m3/s

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Temperature and Salinity are conservative
tracersproperties that can only be altered at
the ocean boundaries or by mixing with water of
different properties.
T-S diagrams can be used to identify different
water masses.
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Simple Conceptual Model for Thermohaline
Circulation
(right) A model of thermocline circulation caused
by heating in lower latitudes and cooling in
higher latitudes. The thermocline at middle and
low latitudes is held up by the slow upward
movement of cold water.
(left) The water layers and deep circulation of
the Atlantic Ocean. Arrows indicate the direction
of water movement. Convergence zones are areas
where water masses approach one another.
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Deep Ocean Circulation is driven by Convection.
1) Convection is driven by the formation of dense
water at the surface (remember external effects
on density can only occur at boundaries). 2) Dense
water can be formed by either 1) reducing
temperature or 2) increasing salinity. 3) Dense
water sinks, driving the deep ocean
circulation. 4) This is called thermohaline
circulation because it is driven by changes in
temperature (thermo) and/or salinity (haline).
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Examples of Water Masses in the Atlantic Ocean
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• Dense Water Formation occurs Primarily in 3
  Regions due to 3 different mechanisms
• Norwegian and Greenland Seas (Surface
  Cooling)North Atlantic Deep Water.
• Antarctic Shelves (Freezing) Antarctic Bottom
  Water
• Mediterranean Sea and Red Sea (Evaporation)
  Mediterranean Intermediate Water and Red Sea
  Intermediate Water.
• Very little dense water is formed in the North
  Pacific OceanIt is too fresh, so surface cooling
  does not reduce density enough to drive
  convection.

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Formation of North Atlantic Deep Water (NADW)

• North Atlantic Current (end of the Gulf Stream),
  brings warm salty surface water into Norwegian
  and Greenland Seas.
• Here it undergoes rapid cooling.
• When it reaches a temperature of 2-3 C it sinks
  and spills back into the Atlantic Basin through
  Denmark Strait and Faroe Channel.

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Formation of Antarctic Bottom Water (AABW)

• Seasonally, the Antarctic (Southern Hemisphere)
  experiences much greater sea ice production than
  the Arctic (Northern Hemisphere).
• In the Antarctic winter, sea ice is forms around
  the continent of Antarctica.
• Strong winds blow ice off shore leaving behind
  open water (this region is called a polynya).
• Winds continually cool water producing more ice,
  which also gets blown offshore.
• During freezing process, salt is left behind
  (brine formation).
• This creates very cold (-0.4-1 C) and saline
  water that then sinks off the shelf to deep water.

• Continual offshore transport of ice by wind,
  makes polynyas sea ice factories
• Lots of heat is removed during freezing process
  (latent heat of freezing).
• Generally Antarctic waters are fresher than
  Arctic waters, so AABW must be colder than NADW
  to sink.

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Formation of Mediterranean Intermediate Water
(MIW)

• Excess evaporation makes Mediterranean saltier
  than Atlantic Ocean.
• This dense water spills through the Straight of
  Gilbraltar sinking into Atlantic.
• It mixes with Atlantic water becoming slightly
  less salty, and finally reaching a stable depth
  at 1000-2000 meters.

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Thermohaline Circulation Transports a Tremendous
amount of Heat
Average volume transport is 25 Sv or about
5,000 times more than flow over Niagara Falls
Global Heat Budget from Lecture 12

• Geologic record suggests strength of global
  thermohaline circulation has varied considerably
  with periods when it was shut down.
• Global warming could impact formation of dense
  deep water.
• Future reductions in thermohaline circulation
  could have significant impacts on Earths climate.

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• Summary
• Under normal conditions, the pressure gradient
  across the Pacific reinforces the trade winds,
  leading to westward surface currents. These
  currents result in upwelling along the west coast
  of South America.
• The El Niño South Oscillations (ENSO) refers to
  the periodic (3 to 8 year) weakening and
  strengthening of the equatorial atmospheric
  pressure gradient.
• Under El Niño conditions, the pressure gradient
  is weakened or reversed, allowing warm surface
  water to spread eastward across the equatorial
  Pacific, shutting down the South American
  upwelling.
• Under La Niña conditions, the atmospheric
  pressure gradient is intensified leading to
  stronger winds, and greater upwelling.
• The variability of ENSO has significant
  biological and climatological implications around
  the world.
• Water masses around the world reflect the surface
  conditions where they were formed.
• Water masses are usually classified by there
  physical and chemical properties and their depth.
• The deep circulation of the Worlds oceans is
  driven by Convection.
• Surface cooling, ice formation and evaporation
  all create dense surface water, which sinks
  driving the circulation.
• This circulation redistributes heat and plays a
  crucial role in the Earths climate.