Deep Circulation in the Ocean Physical oceanography Instructor: Dr. Cheng-Chien LiuCheng-Chien Liu...

Deep Circulation in the OceanDeep Circulation in the Ocean

Physical oceanographyInstructor: Dr. Cheng-Chien Liu

Department of Earth Sciences

National Cheng Kung University

Last updated: 4 January 2004

Chapter 13Chapter 13

IntroductionIntroduction

Surface circulation (upper km)Surface circulation (upper km) Abyss circulation (1 km ~ 4, 5 km)Abyss circulation (1 km ~ 4, 5 km) Thermohaline circulationThermohaline 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

Importance of the Thermohaline Importance of the Thermohaline CirculationCirculation

Three important consequencesThree 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

• VolumeWeak but comparable to the volumes of surface transports

• Earth's heat budget and climateThe deep circulation varies from decades to centuries to millenniaModulate 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

Importance of the Thermohaline Importance of the Thermohaline Circulation (cont.)Circulation (cont.)

The Oceans as a Reservoir of COThe Oceans as a Reservoir of CO22

• CO2 greenhouse gas• Amount of carbon

Ocean: 40,000 GtCLand: 2,200 GtCAtmosphere: 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



(cont.)(cont.)• Major exchangeable reservoir

Not those lock-up reservoir Rocks, shell, coral, …

The cold deep water Image the cold cola

• New CO2Burning of fossil fuels and treesWhere do they go?



(cont.)(cont.)• Forecast of the future climate

How much CO2 is stored in the ocean and for how longThe amount T of the deep waterThe storage time the rate at which deep water is

replenishedThe deposition whether the dead plants and animals that

drop to the sea floor are oxidized.

Oceanic Transport of HeatOceanic Transport of Heat

Global Conveyor BeltGlobal Conveyor Belt• Wally Broecker

• Fig 13.1

• crude estimate40 Sv of 180C water northward14 Sv of 20C water return southward Lose 0.9 petawatts (1 petawatt = 1015 watt) in the north

Atlantic north of 24°NAnother estimation: 1.2 ± 0.2 petawatts

Oceanic Transport of Heat (cont.)Oceanic Transport of Heat (cont.)

The production of bottom water 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


Role of the Ocean in Ice-Age Climate Role of the Ocean in Ice-Age Climate FluctuationsFluctuations• Ice core

Two from Greenland ice sheet and three through the Antarctic sheetA continuous record of atmospheric conditions (400,000 years)Annual layers counted to get ageDeeper in the core, where annual layers are hard to see, depth ageOccasional world-wide dustings of volcanic ash provide common

markers in coresOxygen-isotope ratios in the ice give temperatures over parts of the

northern hemisphereBubbles in the ice give atmospheric CO2 and methane concentrationPollen, chemical composition, and particles give information about

volcanic eruptions, wind speed, and directionThickness of annual layers gives snow accumulation ratesIsotopes of some elements give solar and cosmic ray activity


Role of the Ocean in Ice-Age Climate Role of the Ocean in Ice-Age Climate Fluctuations (cont.)Fluctuations (cont.)• Deep-sea sediments core

Ocean Drilling ProgramSea-surface temperatureSalinity above the coreProduction of north Atlantic deep waterIce volume in glaciersProduction of icebergs


Role in Ice-Age Climate Fluctuations (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 1-100 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 constantOur perception of climate changeBased on highly unusual circumstances

All of recorded history has been during a period of warm and stable climate


Role in Ice-Age Climate Fluctuations (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 AtlanticPeriods: coarse material was deposited on the bottom in mid-oceanMechanism: only icebergs can carry such material out to seaIndication: times when large numbers of icebergs were released into the north

Atlantic

• T variability + ticeberg the meridional overturning circulationIcebergs 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


Role in Ice-Age Climate Fluctuations (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)


Role in Ice-Age Climate Fluctuations (cont.)Role in Ice-Age Climate Fluctuations (cont.)• Dansgaard/Oeschger-Heinrich tandem events

Heinrich events seem to precede the largest Dansgaard/Oeschger eventsHeinrich event shuts off the Atlantic thermohaline circulation very

cold North Atlantic 1000 years later Dansgaard/Oeschger event with rapid warming

Have global influenceRelated to warming events seen in Antarctic ice coresTemperatures changes in the two hemispheres are out of phase

When Greenland warms, Antarctica cools

• Little Ice AgeA 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


SummarySummary• 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


Summary (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

Theory for the Thermohaline Theory for the Thermohaline CirculationCirculation

Stommel, Arons and Faller theoryStommel, Arons and Faller theory• Three papers from 1958 to 1960

• Three fundamental ideasSupplied 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.

Theory for the Thermohaline Theory for the Thermohaline Circulation (cont.)Circulation (cont.)

VelocityVelocity• Sverdrup equation

• Integration

• Velocity

• Direction: everywhere toward the poles

• Fig 13.4The abyssal flow in the interior of the ocean


Western boundary current Western boundary current TTww• Deep western boundary (Stommel and Aron)

• Simplified ocean (equator + 2 meridians + flat bottom)Source at the pole S0

Tw = -2S0 at = 900

Tw gradually diminishes to zero at = 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 = 300

Fig 13.4: the western boundary current sketched in the north Pacific

Fig 13.5: ridges control the flow of deep circulationTime scale of deep circulation: hundreds to thousands years


Some commentsSome 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 AzNumerical 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 AzNumerical 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.


Some comments (cont.)Some comments (cont.)• Mixing takes place at

Seamounts, mid-ocean ridges, and along strong currents such as the Gulf Stream

• Numerical models large errorsThe deep circulation calculated from numerical models

probably has large errors

• Heat transport may be not that sensitive to SBecause 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

Observations of the Deep CirculationObservations of the Deep Circulation

Difficulties in observing the abyssal Difficulties in observing the abyssal circulationcirculation• Direct measurement is not available until recently• The measurements do not produce a stable mean

value for the deep currentsMean = 1 mm/s but variability = 100 mm/s

SolutionSolution• 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.

Observations of the Deep Circulation Observations of the Deep Circulation (cont.)(cont.)

Water massWater mass• Originate from meteorology

Cold front warmAtmosphere: large contrast in both and T strong windSea : small contrast in both and T weak currents

• DefinitionCommon formation historyPhysical entities with a measurable volume

• Delineation: T-S plotThe properties formed at the surface on in the MLConserved propertiesOnly change little by mixing as the water mass sinks(T, S) unique water massFig 13.6

Left: d-T, d-S plots Right: S-T plot


Fig 13.7: mixing of water massesFig 13.7: mixing of water masses• Mixing two water types leads to a straight line

on a T-S diagram

Fig 13.8: densificationFig 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


Fig 13.9: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:Table 13.1:• Three important water masses listed in order of

decreasing depthAntarctic Bottom Water AABNorth Atlantic Deep Water NADWAntarctic Intermediate Water AIW


Core methodCore 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

• AssumptionsThe flow is along the coreWater in the core mixes with the water masses above and below the

core and it gradually loses its identityThe flow tends to be along surfaces of constant potential density

• Fig 13.10 (South Atlantic Ocean) works very wellAIWNADWABW


Problems with the core methodProblems 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:Fig 13.11:• T-S plots of water in the various ocean basins


Other tracersOther tracers• S

ConservedInfluences much less than T

• O2Partly conservedIts concentration is reduced by the respiration by marine plants and

animals and by oxidation of organic carbon

• SilicatesUsed by some marine organismsThey are conserved at depths below the sunlit zone

• PhosphatesUsed by all organismsProvide additional information


Other tracers (cont.)Other tracers (cont.)• 3He

ConservedThere are few sources, mostly at deep-sea volcanic areas and hot springs

• 3H (tritium)Produced by atomic bomb tests in the atmosphere in the 1950sEnters the ocean through the mixed layerUseful for tracing the formation of deep water

• Fluorocarbons (Freon used in air conditioning)Injected into atmosphereCan be measured with very great sensitivityThey are being used for tracing the sources of deep water

• Sulphur hexafluoride SF6Can be injected into sea waterCan be measured with great sensitivity for many months

Antarctic Circumpolar CurrentAntarctic Circumpolar Current

Significance of the Antarctic Circumpolar Significance of the Antarctic Circumpolar Current (ACC)Current (ACC)• Transport waters among three oceans• Contribute the deep circulation in all basins

Fig 13.12: Density plot in the Drake PassageFig 13.12: Density plot in the Drake Passage• Three fronts

the Subantarctic frontthe Polar frontthe Southern ACC front

• Distribution (Fig 13.13)• Density slopes at all depths the current extend to

the bottom

Antarctic Circumpolar Current (cont.)Antarctic Circumpolar Current (cont.)

Transpose of ACCTranspose 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 ACCMax late winter and early spring

Antarctic Circumpolar Current (cont.)Antarctic Circumpolar Current (cont.)

Influence of topography steeringInfluence of topography steering Circumpolar Deep WaterCircumpolar Deep Water

• A mixture of deep water from all oceansThe upper branch of the current contains oxygen-poor water from all oceansThe lower (deeper) branch contains a core of high-salinity water from the

Atlantic

• A giant mix-master

The coldest and saltiest water The coldest and saltiest water on the on the continental shelf around Antarctica in wintercontinental shelf around Antarctica in winter• Too dense to cross the Drake Passage• Seeps into other basins• It is not the circumpolar water

Important conceptsImportant concepts

The deep circulation of the ocean is very important because it The deep circulation of the ocean is very important because it determines the vertical stratification of the oceans and because it determines the vertical stratification of the oceans and because it modulates climate. modulates climate.

The cold, deep water in the ocean absorbs COThe cold, deep water in the ocean absorbs CO22 from the from the atmosphere, therefore temporarily reducing atmospheric COatmosphere, therefore temporarily reducing atmospheric CO22. .

Eventually, however, most of the COEventually, however, most of the CO22 must be released back to must be released back to the ocean. (Some is used by plants, some is used to make sea the ocean. (Some is used by plants, some is used to make sea shells). shells).

The production of deep bottom waters in the north Atlantic The production of deep bottom waters in the north Atlantic causes a transport of one petawatt of heat into the northern causes a transport of one petawatt of heat into the northern hemisphere which warms Europe. hemisphere which warms Europe.

Variability of deep water formation in the north Atlantic has been Variability of deep water formation in the north Atlantic has been tied to large fluctuations of northern hemisphere temperature tied to large fluctuations of northern hemisphere temperature during the last ice agesduring the last ice ages

Important concepts (cont.)Important concepts (cont.)

The theory for the deep circulation was worked out by Stommel The theory for the deep circulation was worked out by Stommel and Arons in a series of papers published from 1958 to 1960. They and Arons in a series of papers published from 1958 to 1960. They showed that vertical velocities are needed nearly everywhere in showed that vertical velocities are needed nearly everywhere in the ocean to maintain the thermocline, and the vertical velocity the ocean to maintain the thermocline, and the vertical velocity drives the deep circulation. drives the deep circulation.

The deep circulation is driven by vertical mixing, which is largest The deep circulation is driven by vertical mixing, which is largest above mid-ocean ridges, near seamounts, and in strong boundary above mid-ocean ridges, near seamounts, and in strong boundary currents. currents.

The deep circulation is too weak to measure directly. It is inferred The deep circulation is too weak to measure directly. It is inferred from observations of water masses de ned by their temperature from observations of water masses de ned by their temperature and salinity and from observation of tracers. and salinity and from observation of tracers.

The Antarctic Circumpolar Current mixes deep water from all The Antarctic Circumpolar Current mixes deep water from all oceans and redistributes it back to each ocean. The current is oceans and redistributes it back to each ocean. The current is deep and slow with a transport of 125 Sv deep and slow with a transport of 125 Sv

Deep Circulation in the Ocean Physical oceanography Instructor: Dr. Cheng-Chien LiuCheng-Chien Liu...

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