Thermohaline Ocean Circulation

1
Thermohaline Ocean Circulation

• Stefan Rahmstorf

2
What is Thermohaline Circulation?

• Part of the ocean circulation which is driven by
  fluxes of heat and freshwater.
• Distinction of thermohaline vs wind-driven
• - Oceans density distribution ? pressure
  gradient (circulation)
• - Currents and mixing ? density distribution
• MOC is the north-south flow that includes
  wind-driven parts whereas THC is also associated
  with zonal overturning cells.

3

• Key features
• Deep water formation
• Spreading of deep waters NADW, AABW
• Upwelling of deep waters ACC region by Ekman
  pumping
• Near-surface currents to close the flow

4
What Drives the THC?

• High-latitude cooling ? high density ? sinking of
  water
• Ocean basin would be filled with dense water and
  the circulation would stop.
• It requires a mechanism that continually works to
  make the water in the box less dense, so that the
  source water remains denser than the water
  already in the box.
• Sandstroms theorem Heating at greater depth
  than cooling is required. This enables an ongoing
  replacement by colder water from above.

5
In the Real Ocean?

• Heating occurs at the surface due to the sun.
• The slow diffusion of heat by turbulent mixing
  caused by tides and the winds.

6
Nonlinear Behavior of the THC

• Tropics, subtropics warm, saltier ? low density
• High latitudes cold, fresher ? high density
• Thus, THC is thermally driven.
• Freshwater input
• ? density difference
• ? circulation
• ? salinity difference

7
The Circulations Effect on Climate

• Large heat transport of up to 1 PW in the North
  Atlantic.
• ? warm by 5K ? sea ice pushed back ? albedo
  feedback
• THC switched off
• 1) NH cools / SH warms ? shifts thermal equator
  and ITCZ southward
• 2) sea level around the northern Atlantic rise by
  up to 1m

8
THC in Quaternary Climate Changes

• Warm mode similar to present-day
• Cold mode NADW forming south of Iceland
• Switched-off mode occurred after major input of
  freshwater
• Heinrich events large input of freshwater by
  iceberg discharge
• ? Stopped NADW formation
• ? Proxy data shows cooling around
  mid-latitude Atlantic
• Younger Dryas meltwater floods
• DO events north-south shifts in convection
  location
• ? Dramatic warm events in Greenland and
  northern Atlantic Coinciding
  increase in salinity in the Irminger Sea pushes
  warm, salty Atlantic waters northward into the
  Nordic Seas
• Glacial conditions Southward shift of the main
  deep water formation (Nordic Seas ? South of
  Iceland) reduces ocean heat transport to high
  latitudes and expands sea ice.

9
The Future of the THC

• Global warming
• surface warming surface freshening
• ? Both reduce the density of high-latitude
  surface waters
• ? Inhibit deep water formation
• ? weakening of NADW formation
• ? serious impact on marine ecosystems, sea
  level and surface climate including a shift in
  ITCZ
• THC collapse low probability high impact
  risks associated with global warming

10
(No Transcript)
11
In the Real Ocean?

• Heating occurs at the surface due to the sun.
• The slow diffusion of heat by turbulent mixing
  caused by tides and the winds.
• Both THC and wind-driven circulation drive the
  observed MOC in the world oceans.

Westerlies over Southern Ocean ?Divergence of
surface currents ?Upwelling from deep waters
due to Drake Passage Effect ?High-latitude
North Atlantic with the highest surface
densities and stable stratification.