Thermohaline circulation What have we learned

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Thermohaline circulationWhat have we learned

• What does the THC look like?
• What drives the THC?
• What is the evidence for changes in the THC in
  past climates?
• Are two stable modes of the THC possible?
• What is the role of ice?
• What happens to the THC in a cold climate? In a
  warm climate?
• How does the THC influence climate?

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What is the THC?

• Deep water formation in the NA
• Transformation of NADW to AABW and AAIW
• Return flow and upwelling locations are unclear
• The Gulf Stream and THC are not the same thing,
  only 20 of Gulf Stream is THC driven

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What drives the THC? Is it pushed or pulled?

• Turbulent mixing maintains the abyssal
  stratification in equilibrium
• Changing the density in the formation regions can
  change the THC temporarily
• The southern ocean plays an important role in
  controlling the THC as well.

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What is the evidence for changes in the THC in
past climates?

• Heinrich events (tentative evidence for changes
  in the North Atlantic Basin)
• D-O events (Greenland centric)
• Changes in THC from glacial to inter-glacial
  times
• There are changes in ocean state not related to H
  and D-O events

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Are two stable modes of THC possible?

• Box models show T (strong) dominated and S (weak)
  dominated circulations
• Ocean only models show two steady states possible
  with different initial conditions
• Coupled models show transient THC changes that go
  away after several decades

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What is the role of sea-ice?

• Sea-ice edge is controlled by surface fluxes and
  ocean heat flux convergence
• Sea-ice cover has strong impact on climate
• Ice-albedo feedbacks

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What happens to the THC in a cold climate? Warm
climate?

• During glacial times, the THC was weaker and
  shallower in the North Atlantic.
• During warm climate, the THC can shut down
  temporarily, and weaken owing to increase
  ice-melt and precipitation.

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How does the THC influence climate?

• The ocean heat transport is small poleward of
  about 40N
• The THC contributes a large fraction of the total
  heat transports in the NA, but small in the North
  Pacific where the shallow overturning circulation
  dominates
• The ocean heat transport is asymmetric about the
  equator owing to the THC (especially in NA)
• When THC is shut down, there is local cooling
  around the NE Atlantic, and global effects
  mediated by the tropical Atlantic climate.

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Outstanding questions(1)

• How does the climate system adjust to changes in
  the THC?
• What is the relative role of mixing and deep
  water formation in the THC strength?
• Will the atmosphere take up the slack in heat
  transport if the THC shuts down?
• What changes in fresh water in the NA are
  necessary to shut down the THC? What is the role
  of the Greenland ice cap?
• What is the probability of abrupt cooling during
  global warming?

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Outstanding questions (2)

• Was the THC the driver or responder to abrupt
  climate change events in the paleo record?
• What is the explanation for abrupt warming, and
  slow cooling during D-O events?
• Is the current THC in equilibrium or transient?
• Does mixing drive or throttle the THC?
• Is the current climate near an unstable
  equilibrium?
• Where does the return flow-upwelling occur?

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Abrupt climate change

• An abrupt change takes place faster than the
  responsible forcing.
• Abrupt climate change evidence Y-D (probably
  due to the discharge of Lake Agassiz)
• D-O events seen in NA and other places in NH, but
  tenuous at best. SH antiphasing evidence is not
  believable.
• Some Holocene abrupt climate change events 4200
  BP

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NOAA Fact Sheet

• The YD and a large number of abrupt changes
  during the last ice age call D/O evenst have been
  corroborated in multiple ice cores from
  Greenland, Antarctica and tropical mountains,
  marine sediments from the NA, the tropical A,
  eastern Pacific, and Indian Oceans and from
  various records on land. Other, smaller abrupt
  changes have been linked ot societal disruptions.
  Evidence for some of these events are more
  regional in nature, and points to far less
  dramatic changes. However, these events did
  occur so rapidly and unexpectedly that human or
  natural systems had difficultly adapting to them.
  

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Uncertainties in paleo record

• Spatial footprint of record. Connections between
  SH and NH.
• Interpretation of proxy variables
• Causes of abrupt climate change
• The scenario must characterize the staes among
  which the climate system has jumped
• Mechanisms for the jump
• Teleconnection mechanisms to the rest of the
  planet
• Flywheel capable of holding in new state

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Precursors to Abrupt climate change

• THC changes
• Tropical climate changes

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Causes of abrupt climate change

• A sudden change in external forcing (I.e.
  freshwater discharge from a glacial lake)
• Slow changes in external forcing to move climate
  system across a threshold
• Internal, chaotic changes in the climate system

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Role of THC in Abrupt climate change under global
warming

• THC slows down in models under global warming
  scenarios
• Ocean heat transport can impact climate of
  western Europe, but possible just to ameliorate
  global warming changes
• OHT controlled by THC and wind-driven shallow
  overturning circulation

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Final comments

• Paleoclimate record shows abrupt climate change
  occurs. Is there a reason to assume that current
  climate would be more docile?
• Global warming is a real and serious threat, with
  potentially disastrous impacts on societies, with
  or without abrupt climate change