1. Deep Circulation in the Ocean
Chapter 13
Deep Circulation in the Ocean
Physical oceanography
Instructor: Dr. Cheng-Chien Liu
Department of Earth Sciences
National Cheng Kung University
Last updated: 4 January 2004

2. Introduction Surface circulation (upper km)
Abyss circulation (1 km ~ 4, 5 km)
Thermohaline circulation
Meridional Overturning Circulation
The vertical movements of ocean water masses caused by density differences that are due to variations in temperature and salinity
Carries cold water from high latitudes in winter to lower latitudes throughout the world

3. Importance of the Thermohaline Circulation
Three important consequences
Stratification
layers according to density; applies to fluids. Stable stratification occurs when density decreases continuously (but not necessarily uniformly) with distance from the Earth's center
Volume
Weak but comparable to the volumes of surface transports
Earth's heat budget and climate
The deep circulation varies from decades to centuries to millennia
Modulate climate over such time intervals?!!
The ocean may be the primary cause of variability over times ranging from years to decades, and it may have helped modulate ice-age climate

4. Importance of the Thermohaline Circulation (cont.)
The Oceans as a Reservoir of CO2
CO2  greenhouse gas
Amount of carbon
Ocean: 40,000 GtC
Land: 2,200 GtC
Atmosphere: 750 GtC
GTC: gigaton of carbon = 1012 kilograms of carbon
New carbon since Industrial Revolution: 150 GTC
Carbon cycled through the marine ecosystem in five years > 150 GTC

5. Importance of the Thermohaline Circulation (cont.)
The Oceans as a Reservoir of CO2 (cont.)
Major exchangeable reservoir
Not those lock-up reservoir
Rocks, shell, coral, …
The cold deep water
Image the cold cola
New CO2
Burning of fossil fuels and trees
Where do they go?

6. Importance of the Thermohaline Circulation (cont.)
The Oceans as a Reservoir of CO2 (cont.)
Forecast of the future climate  How much CO2 is stored in the ocean and for how long
The amount  T of the deep water
The storage time  the rate at which deep water is replenished
The deposition  whether the dead plants and animals that drop to the sea floor are oxidized.

7. Oceanic Transport of Heat
Global Conveyor Belt
Wally Broecker
Fig 13.1
crude estimate
40 Sv of 180C water northward
14 Sv of 20C water return southward
Lose 0.9 petawatts (1 petawatt = 1015 watt) in the north Atlantic north of 24°N
Another estimation: 1.2 ± 0.2 petawatts

8. Oceanic Transport of Heat (cont.)
The production of bottom water
Remarkably sensitive to small changes in S
Influence of S
More saline surface waters form denser water in winter than less saline water
The saltiest water is in the Atlantic and under the ice on the continental shelves around Antarctica.
Remarkably sensitive to small changes in mixing

9. Oceanic Transport of Heat (cont.)
Role of the Ocean in Ice-Age Climate Fluctuations
Ice core
Two from Greenland ice sheet and three through the Antarctic sheet
A continuous record of atmospheric conditions (400,000 years)
Annual layers  counted to get age
Deeper in the core, where annual layers are hard to see, depth  age
Occasional world-wide dustings of volcanic ash provide common markers in cores
Oxygen-isotope ratios in the ice give temperatures over parts of the northern hemisphere
Bubbles in the ice give atmospheric CO2 and methane concentration
Pollen, chemical composition, and particles give information about volcanic eruptions, wind speed, and direction
Thickness of annual layers gives snow accumulation rates
Isotopes of some elements give solar and cosmic ray activity

10. Oceanic Transport of Heat (cont.)
Role of the Ocean in Ice-Age Climate Fluctuations (cont.)
Deep-sea sediments core
Ocean Drilling Program
Sea-surface temperature
Salinity above the core
Production of north Atlantic deep water
Ice volume in glaciers
Production of icebergs

11. Oceanic Transport of Heat (cont.)
Role in Ice-Age Climate Fluctuations (cont.)
Abrupt T variability over the past 100,000 years
The oxygen-isotope record in the ice cores
Many times during the last ice age, temperatures near Greenland warmed rapidly over periods of years, followed by gradual cooling over longer periods (Dansgaard et al., 1993)
Dansgaard/Oeschger event
11,500 years ago, temperatures over Greenland warmed by 80C in 40 years in three steps, each spanning 5 years (Alley, 2000)
Other studies have shown that much of the northern hemisphere warmed and cooled in phase with temperatures calculated from the ice core
The climate of the past 8,000 years was constant
Our perception of climate change
Based on highly unusual circumstances
All of recorded history has been during a period of warm and stable climate

12. Oceanic Transport of Heat (cont.)
Role in Ice-Age Climate Fluctuations (cont.)
Heinrich events (iceberg production time ticeberg)
Hartmut Heinrich and colleagues (Bond et al., 1992)
Studying the sediments in the north Atlantic
Periods: coarse material was deposited on the bottom in mid-ocean
Mechanism: only icebergs can carry such material out to sea
Indication: times when large numbers of icebergs were released into the north Atlantic
T variability + ticeberg  the meridional overturning circulation
Icebergs melted  the surge of fresh water   the stability of the water column  shut off the production of North Atlantic Deep Water  the transport of warm water in the north Atlantic   producing very cold northern hemisphere climate (Figure 13.2)  pushed the polar front further south
The location of the front, and the time it was at different positions can be determined from analysis of bottom sediments

13. Oceanic Transport of Heat (cont.)
Role in Ice-Age Climate Fluctuations (cont.)
Antarctic warming
When the meridional overturning circulation shuts down, heat normally carried from the south Atlantic to the north Atlantic becomes available to warm the southern hemisphere
Hysteresis (Figure 13.3)
Four states (two of which are stable)
1: The present circulation  stable
2: Deep water is produced mostly near Antarctica, and upwelling occurs in the far north Pacific (as it does today) and in the far north Atlantic
3: The circulation is shut off  stable
4: The return to normal salinity does not cause the circulation to turn on. Surface waters must become saltier than average for the first state to return (Rahmstorf, 1995)

14. Oceanic Transport of Heat (cont.)
Role in Ice-Age Climate Fluctuations (cont.)
Dansgaard/Oeschger-Heinrich tandem events
Heinrich events seem to precede the largest Dansgaard/Oeschger events
Heinrich event  shuts off the Atlantic thermohaline circulation  very cold North Atlantic  1000 years later  Dansgaard/Oeschger event with rapid warming
Have global influence
Related to warming events seen in Antarctic ice cores
Temperatures changes in the two hemispheres are out of phase
When Greenland warms, Antarctica cools
Little Ice Age
A weakened version of this process with a period of about 1000 years
May be modulating present-day climate in the north Atlantic
May have been responsible for the Little Ice Age from 1100 to 1800

15. Oceanic Transport of Heat (cont.)
Summary
Not yet understand well
The variability in S, Tair and deep-water formation
What causes the ice sheets to surge?
Surges  T  water vapor from the tropics (a greenhouse gas)
Surges  T  an internal instability of a large ice sheet
How the oceanic circulation responds to changes in the deep circulation or surface moisture fluxes?
Recent work by Wang, Stone and Marotzke (1999), who used a numerical model to simulate the climate system, shows that the meridional overturning circulation is modulated by moisture fluxes in the southern hemisphere

16. Oceanic Transport of Heat (cont.)
Summary (cont.)
100,000-year cycle
Every 100,000 years for the past million years, ice sheets have advanced over the continents
Correlation: Earth's orbital eccentricity, deep-sea temperature, and atmospheric carbon-dioxide concentration
Ice-sheet volume lagged behind CO2 changes in the atmosphere
CO2 changes  ice sheets change, not the other way around
The oceans play a key role in the development of the ice ages

17. Theory for the Thermohaline Circulation
Stommel, Arons and Faller theory
Three papers from 1958 to 1960
Three fundamental ideas
Supplied by deep convection
Cold, deep water is supplied by deep convection at a few high-latitude locations in the Atlantic, notably in the Irminger and Greenland Seas in the north and the Weddell Sea in the south
Mixing
Brings the cold, deep water back to the surface.
Geostrophic flow
The abyssal circulation is strictly geostrophic in the interior of the ocean, and therefore potential vorticity is conserved.

18. Theory for the Thermohaline Circulation (cont.)
Velocity
Sverdrup equation
Integration
Direction: everywhere toward the poles
Fig 13.4
The abyssal flow in the interior of the ocean

19. Theory for the Thermohaline Circulation (cont.)
Western boundary current Tw
Deep western boundary (Stommel and Aron)
Simplified ocean (equator + 2 meridians + flat bottom)
Source at the pole S0
Tw = -2S0 at j = 900
Tw gradually diminishes to zero at j = 00
If S0 exceeds the volume of water upwelled in the basin  the western boundary current carries water across the Equator
Fig 13.4: the western boundary current sketched in the north Atlantic
No source
Tw gradually diminishes to zero at j = 300
Fig 13.4: the western boundary current sketched in the north Pacific
Fig 13.5: ridges control the flow of deep circulation
Time scale of deep circulation: hundreds to thousands years

20. Theory for the Thermohaline Circulation (cont.)
Some comments
Convection  Mixing
Convection
Reduce the potential energy of the water column
Self powered
Mixing in a stratified fluid
Increases the potential energy
Driven by an external process.
The meridional over-turning circulation is very sensitive to Az
Numerical models of the deep circulation show that the meridional over-turning circulation is very sensitive to the assumed value of vertical eddy diffusivity in the thermocline (Gargett and Holloway, 1992).
Tw is very sensitive to Az
Numerical calculations by Marotzke and Scott (1999) indicate that the transport is not limited by the rate of deep convection, but it is sensitive to the assumed value of vertical eddy diffusivity, especially near side boundaries.

21. Theory for the Thermohaline Circulation (cont.)
Some comments (cont.)
Mixing takes place at
Seamounts, mid-ocean ridges, and along strong currents such as the Gulf Stream
Numerical models  large errors
The deep circulation calculated from numerical models probably has large errors
Heat transport may be not that sensitive to S
Because the meridional overturning circulation is pulled by mixing and not pushed by deep convection, the transport of heat into the north Atlantic may not be as sensitive to surface salinity as described above

22. Observations of the Deep Circulation
Difficulties in observing the abyssal circulation
Direct measurement is not available until recently
The measurements do not produce a stable mean value for the deep currents
Mean = 1 mm/s but variability = 100 mm/s
Solution
Indirect observation
Inferred from measured distribution of temperature, salinity, oxygen, silicate, tritium, fluorocarbons and other tracers
These measurements are much more stable than direct current measurements
Observations made decades apart can be used to trace the circulation.

23. Observations of the Deep Circulation (cont.)
Water mass
Originate from meteorology
Cold  front  warm
Atmosphere: large contrast in both r and T  strong wind
Sea : small contrast in both r and T  weak currents
Definition
Common formation history
Physical entities with a measurable volume
Delineation: T-S plot
The properties formed at the surface on in the ML
Conserved properties
Only change little by mixing as the water mass sinks
(T, S)  unique water mass
Fig 13.6
Left: d-T, d-S plots
Right: S-T plot

24. Observations of the Deep Circulation (cont.)
Fig 13.7: mixing of water masses
Mixing two water types leads to a straight line on a T-S diagram
Fig 13.8: densification
Because the lines of constant density on a T-S plot are curved, mixing increases the density of the water. This is called densification

25. Observations of the Deep Circulation (cont.)
Fig 13.9:
A T-S plot calculated from hydrographic data collected in the south Atlantic
The mixing among three water masses shows the characteristic rounded apexes
Table 13.1:
Three important water masses listed in order of decreasing depth
Antarctic Bottom Water AAB
North Atlantic Deep Water NADW
Antarctic Intermediate Water AIW

26. Observations of the Deep Circulation (cont.)
Core method
Core
A layer of water with extreme value (in the mathematical sense) of salinity or other property as a function of depth
An extreme value is a local maximum or minimum of the quantity as a function of depth
Assumptions
The flow is along the core
Water in the core mixes with the water masses above and below the core and it gradually loses its identity
The flow tends to be along surfaces of constant potential density
Fig (South Atlantic Ocean)  works very well
AIW
NADW
ABW

27. Observations of the Deep Circulation (cont.)
Problems with the core method
The flow is probably not along the core
Lateral boundary (seamounts, mid-ocean ridges)  weak vertical mixing
Flow in a plan  the core (along the western boundary )  horizontal mixing
Fig 13.11:
T-S plots of water in the various ocean basins

28. Observations of the Deep Circulation (cont.)
Other tracers S
Conserved
Influences s much less than T O2
Partly conserved
Its concentration is reduced by the respiration by marine plants and animals and by oxidation of organic carbon
Silicates
Used by some marine organisms
They are conserved at depths below the sunlit zone
Phosphates
Used by all organisms
Provide additional information

29. Observations of the Deep Circulation (cont.)
Other tracers (cont.)
3He
Conserved
There are few sources, mostly at deep-sea volcanic areas and hot springs
3H (tritium)
Produced by atomic bomb tests in the atmosphere in the 1950s
Enters the ocean through the mixed layer
Useful for tracing the formation of deep water
Fluorocarbons (Freon used in air conditioning)
Injected into atmosphere
Can be measured with very great sensitivity
They are being used for tracing the sources of deep water
Sulphur hexafluoride SF6
Can be injected into sea water
Can be measured with great sensitivity for many months

30. Antarctic Circumpolar Current
Significance of the Antarctic Circumpolar Current (ACC)
Transport waters among three oceans
Contribute the deep circulation in all basins
Fig 13.12: Density plot in the Drake Passage
Three fronts
the Subantarctic front
the Polar front
the Southern ACC front
Distribution (Fig 13.13)
Density slopes at all depths  the current extend to the bottom

31. Antarctic Circumpolar Current (cont.)
Transpose of ACC
Slow (10 cm/s – 50 cm/s) but deep and wide  transpose more water than the western boundary water
125  11 Sv
Range: 95 – 158 Sv
Fig 13.14: Variability of transpose in the ACC
Max  late winter and early spring

32. Antarctic Circumpolar Current (cont.)
Influence of topography steering
Circumpolar Deep Water
A mixture of deep water from all oceans
The upper branch of the current contains oxygen-poor water from all oceans
The lower (deeper) branch contains a core of high-salinity water from the Atlantic
A giant mix-master
The coldest and saltiest water  on the continental shelf around Antarctica in winter
Too dense to cross the Drake Passage
Seeps into other basins
It is not the circumpolar water

33. Important concepts
The deep circulation of the ocean is very important because it determines the vertical stratification of the oceans and because it modulates climate.
The cold, deep water in the ocean absorbs CO2 from the atmosphere, therefore temporarily reducing atmospheric CO2.
Eventually, however, most of the CO2 must be released back to the ocean. (Some is used by plants, some is used to make sea shells).
The production of deep bottom waters in the north Atlantic causes a transport of one petawatt of heat into the northern hemisphere which warms Europe.
Variability of deep water formation in the north Atlantic has been tied to large fluctuations of northern hemisphere temperature during the last ice ages

34. Important concepts (cont.)
The theory for the deep circulation was worked out by Stommel and Arons in a series of papers published from 1958 to They showed that vertical velocities are needed nearly everywhere in the ocean to maintain the thermocline, and the vertical velocity drives the deep circulation.
The deep circulation is driven by vertical mixing, which is largest above mid-ocean ridges, near seamounts, and in strong boundary currents.
The deep circulation is too weak to measure directly. It is inferred from observations of water masses de ned by their temperature and salinity and from observation of tracers.
The Antarctic Circumpolar Current mixes deep water from all oceans and redistributes it back to each ocean. The current is deep and slow with a transport of 125 Sv