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DensityDensity--Driven Driven DownwellingDownwellingand and ThermohalineThermohalineCirculationCirculation

The Great Ocean Conveyor The Great Ocean Conveyor

Deep Deep circulationcirculation

�� Deep waters form in the Deep waters form in the high North Atlantic and high North Atlantic and around Antarctica.around Antarctica.

�� Intermediate waters form Intermediate waters form in the Mediterranean Sea in the Mediterranean Sea (MIW) and the Southern (MIW) and the Southern Hemisphere polar front Hemisphere polar front region (AAIW).region (AAIW).

�� Deep water masses, like Deep water masses, like nearnear--surface currents, are surface currents, are influenced by the influenced by the CoriolisCorioliseffecteffect

–– As deep waters flow As deep waters flow equatorwardequatorward, they hug the , they hug the lower continental slopes and lower continental slopes and rises on the western sides of rises on the western sides of the ocean basins.the ocean basins.

–– They are also channeled by They are also channeled by topographic features on the topographic features on the seafloor. seafloor.

60°N

30°N

0° (Equator)

30°S

60°S

Gulf of Mexico

Caribbean Sea

North Atlantic

South Atlantic

Mediterranean Sea

Weddell Sea (Antarctica)

Greenland-Norwegian Sea

North Atla

ntic Deep Water

(NADW)Med. Intermed.

Water (MIW)

Antacrcitc Intermediate Water

(AAIW)

An

tarc

tic B

ott

om

Wat

er

(AA

BW

)AAIW

Circumpolar Water (CPW)

60°N

30°N

0° (Equator)

30°S

60°S

Subtropical Gyre

Subtropical Gyre

Gulf of Mexico

North Atlantic

South Atlantic

Mediterranean Sea

North Atlantic Current

Can

ary

Cur

rent

Gulf Stre

am

N. Equatorial Current

Eq. Countercurrent

S. Equatorial Current

Ben

gue l

aC

urr

ent

Bra

zil C

urre

nt

West Wind Drift

East Wind Drift

Caribbean Sea

60°N

30°N

0° (Equator)

30°S

60°S

Caribbean Sea

North Atlantic

South Atlantic

Mediterranean Sea

Weddell Sea (Antarctica)

Greenland-Norwegian Sea

North Atla

ntic Deep Water

(NADW)Med. Intermed.

Water (MIW)

Antacrcitc Intermediate Water

(AAIW)

An

tarc

tic B

ott

om

Wat

er

(AA

BW

)AAIW

Circumpolar Water (CPW)

Surface vs. deep Surface vs. deep circulationcirculation

�� NearNear--surface waters move surface waters move independently of deep and independently of deep and intermediate waters because of the intermediate waters because of the pycnoclinepycnocline that separates them.that separates them.

�� DownwellingDownwelling & upwelling& upwelling link the link the surface & deep ocean.surface & deep ocean.

The Global Conveyor Net import of near-surface waterinto the Atlantic

Net export of deep waterout of the Atlantic

The The global conveyorglobal conveyor describes the complete circuit of global ocean describes the complete circuit of global ocean circulation involving the circulation involving the horizontal & vertical flowhorizontal & vertical flowof nearof near--surface surface and deep waters.and deep waters.

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deep & intermediate watersacquire their physical & chemical

characteristics at the surface

Why is there so muchvariation in dissolved O2

across the ocean surface?

Cold water holds moredissolved oxygenthan warm water

deep waters lose O2 over time due toanimal respiration and the decomposition

of organic matter; the older the water mass,the longer it has been away from the surface,

the lower its O2 content and the higherits CO2 content

the oldest deep watersare in the North Pacific

Better views of the conveyorBetter views of the conveyor

�� Can you translate the Can you translate the terms in this diagram terms in this diagram (even without (even without knowing German)?knowing German)?

�� Hint: think “deep”Hint: think “deep”

�� Note the Gulf Stream in its Note the Gulf Stream in its proper position.proper position.

�� Note upwelling of NADW Note upwelling of NADW in the in the Weddell Weddell Sea and Sea and subsequent formation of subsequent formation of AABW.AABW.

�� Note formation of bottom Note formation of bottom water in the Ross Sea also.water in the Ross Sea also.

BODENWASSER

TIEFENWASSER

OBERFLACHENWASSER

NADW formation and NADW formation and clihmateclihmate

?

A few decades at most – YIKES!

heat released to atmosphere

glacial period (“ice age”) interglacial (warm) period

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Oceanic gateways and climateOceanic gateways and climate�� longlong--term changes (millionsterm changes (millions--ofof--years time scale) years time scale)

which rewhich re--organized global ocean circulationorganized global ocean circulation

�� 3 oceanic gateways (remember Ma = “Mega3 oceanic gateways (remember Ma = “Mega--annumsannums” ” = millions of years ago)= millions of years ago)

1)1) Drake Passage (opened ~34 Ma)Drake Passage (opened ~34 Ma)

2)2) Central American seaway (closed ~5 Ma) Central American seaway (closed ~5 Ma)

3)3) Indonesian seaway (moved north ~3Indonesian seaway (moved north ~3--4 Ma)4 Ma)

1

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Drake Passage openingDrake Passage opening�� prior to ~40 Ma …prior to ~40 Ma …

–– isthmus of land existed which connected southern tip of South isthmus of land existed which connected southern tip of South America to Antarctic PeninsulaAmerica to Antarctic Peninsula

–– Brazil Current acted like Gulf Stream todayBrazil Current acted like Gulf Stream today»» swift, strong, warm, swift, strong, warm, polewardpoleward--flowingflowing»» kept Antarctica warmkept Antarctica warm

�� after 34 Ma (Eoceneafter 34 Ma (Eocene--Oligocene boundary) …Oligocene boundary) …–– Drake Passage opened, land bridge disappearedDrake Passage opened, land bridge disappeared–– Antarctic Circumpolar Antarctic Circumpolar

Current began to flowCurrent began to flow»» isolated Antarctic continentisolated Antarctic continent»» allowed glacial ice sheets allowed glacial ice sheets

to form on Antarcticato form on Antarctica

�� or was it something else?or was it something else?–– maybe changes in maybe changes in

atmospheric COatmospheric CO22 did itdid it–– DeConto DeConto and Pollard, 2003 and Pollard, 2003

modeling paper in modeling paper in NatureNature

Central American Seaway closureCentral American Seaway closure�� prior to ~5 Ma …prior to ~5 Ma …

–– open passage existed between North and South Americaopen passage existed between North and South America–– allowed exchange of tropical water between ancient Pacific and Aallowed exchange of tropical water between ancient Pacific and Atlantictlantic

»» salinity in both was similarsalinity in both was similar»» no (or very weak) Gulf Stream existedno (or very weak) Gulf Stream existed

�� after ~2.7 Ma …after ~2.7 Ma …–– Isthmus of Panama fully formed, closed Central Am. SeawayIsthmus of Panama fully formed, closed Central Am. Seaway–– salinity increased in Atlantic,salinity increased in Atlantic,

decreased in Pacificdecreased in Pacific–– Gulf Stream intensifiedGulf Stream intensified

»» warm, saltwarm, saltierier waters nowaters northrth»» ocean conveyor startedocean conveyor started»» onset of Northern Hem.onset of Northern Hem.

ice sheets & glacial cyclesice sheets & glacial cycles�� wait a second …wait a second …

–– Gulf Stream Gulf Stream warms warms N. Hem.N. Hem.–– so why ice sheets???so why ice sheets???

Gulf Stream and Northern Gulf Stream and Northern Hemisphere Glacial CyclesHemisphere Glacial Cycles

�� intensified Gulf Stream carried not only more heat, but also intensified Gulf Stream carried not only more heat, but also more more moisturemoisture to the Northern Hemisphereto the Northern Hemisphere

�� rain, snow fell on Eurasiarain, snow fell on Eurasia�� great rivers flowed into great rivers flowed into

Arctic Ocean, freshening Arctic Ocean, freshening water and forming sea icewater and forming sea ice–– fresh water shortfresh water short--circuited circuited

ocean conveyorocean conveyor–– ice reflected sunlightice reflected sunlight–– both both coolcool the N. Hem., the N. Hem.,

resulting in an ice ageresulting in an ice age–– when conveyor when conveyor onon again, again,

get a warm (and fuzzy) get a warm (and fuzzy) interglacial periodinterglacial period See http://www.whoi.edu/oceanus/viewArticle.do?id=2508

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Indonesian Seaway switchIndonesian Seaway switch�� prior to ~4 Ma …prior to ~4 Ma …

–– New Guinea farther southNew Guinea farther south–– allowed warm South Pacific water to flow through Indonesian Seawallowed warm South Pacific water to flow through Indonesian Seawayay–– persistent El persistent El NiNiññoo--like state like state

»» moved warm water from tropics to high latitudesmoved warm water from tropics to high latitudes»» kept N. Hemisphere icekept N. Hemisphere ice--freefree

todaytoday

~4 Ma~4 Ma

~3 Ma~3 Ma

�� after ~3 Ma …after ~3 Ma …–– northward displacement of New Guineanorthward displacement of New Guinea–– switched the source of flow switched the source of flow

through Indonesiathrough Indonesia–– relatively cold North Pacific relatively cold North Pacific

waterswaters–– persistent Lapersistent LaNiNiññaa--like statelike state

»» reduced transfer of heat reduced transfer of heat from tropics to highfrom tropics to highlat’slat’s

»» onset of Northern Hem.onset of Northern Hem.ice sheets & glacial cyclesice sheets & glacial cycles

�� Cane and Molnar, 2001 paper in Cane and Molnar, 2001 paper in NatureNature

So the question is …So the question is …

What happens next ???What happens next ???

OR…OR…