deep ocean circulation

Deep Ocean CirculationDeep Ocean CirculationMotion in the Ocean, Part

2, “The Great Conveyor Belt”

Jack Barth(barth@coas.oregonstate.edu)

NASA web site:http://oceanmotion.org

Surface CirculationSurface Circulation

Surface and Deep Ocean Circulation help move heat from equator to

pole

Atmosphere & Ocean each responsible for Atmosphere & Ocean each responsible for about half of heat transferabout half of heat transfer

How does the Deep Ocean How does the Deep Ocean respond to Surface respond to Surface

Circulation?Circulation?

The main gyres move heat and salt Resulting DENSITY variations lead

to vertical flow (sinking) Formation of “water masses”,

characterized by Temperature + Salinity = Density

Density Variation in Sea WaterDensity Variation in Sea Water

Isopycnals = constant Isopycnals = constant densitydensity

Density Variation in Sea WaterDensity Variation in Sea Water

Nearly all the water in the oceans is Nearly all the water in the oceans is coldcold

North Atlantic Surface North Atlantic Surface CirculationCirculation

Density-Driven Water FlowDensity-Driven Water Flow

Called “Called “Thermohaline Thermohaline CirculationCirculation”, because ”, because temperaturetemperature and and salinitysalinity together determine together determine density of seawaterdensity of seawater““ThermoThermo” = ” = temperaturetemperature““halinehaline” = ” = saltsalt

Where does the Ocean’s Where does the Ocean’s Deepest Water Come Deepest Water Come

From?From?

The densest seawater is The densest seawater is coldcold and and saltysalty

This is formed at high latitudes in the This is formed at high latitudes in the North and South Atlantic:North and South Atlantic:

North Atlantic Deep Water (NADW)North Atlantic Deep Water (NADW)Antarctic Bottom Water (AABW)Antarctic Bottom Water (AABW)

Density of Sea WaterDensity of Sea Water

Density Rules!Density Rules!

Deep Water MassesDeep Water Masses

Deep/bottom water formation sites

Antarctic Bottom Water (AABW) in Weddell, Ross Seas and Adelie Coast

North Atlantic Deep Water

L. Talley (SIO)

Antarctic Bottom Water (AABW)

Weddell Sea major site of AABW formation AABW circles Antarctica and flow

northward as deepest layer in Atlantic, Pacific and Indian Ocean basins

AABW flow extensive– 45°N in Atlantic– 50°N in Pacific– 10,000 km at 0.03-0.06 km h-1; 250 y

North Atlantic Deep Water (NADW) Coastal Greenland (Labrador Sea) site of NADW

formation NADW comprises about 50% of the deep water to worlds

oceans NADW in the Labrador Sea sinks directly into the western

Atlantic– NADW forms in Norwegian Basins

Sinks and is dammed behind sills–Between Greenland and Iceland and Iceland and

the British Isles NADW periodically spills over sills into the North

Atlantic

Water MassesWater Massesand oceanand oceanmixingmixing determined by CTD determined by CTD (conductivity, (conductivity, temperature, depth) temperature, depth) measurementsmeasurements

Mediterranean WaterMediterranean Water

Mediterranean WaterMediterranean Water

Mediterranean WaterMediterranean Water

Deep Atlantic Deep Atlantic CirculationCirculation

This southward flow in one layer and northward flow below, with vertical motion at either end is called the “Atlantic Meridional Overturning Circulation (MOC)”

Ocean Circulation:The Great Conveyor Belt Surface water at high latitudes forms deep water Deep water sinks and flows at depth throughout

the major ocean basins Deep water upwells to replace the surface water

that sinks in polar regions Surface waters must flow to high latitudes to

replace water sinking in polar regions This Idealized circulation is called the “Great

(Thermohaline) Conveyer Belt”

Tracers in the OceanTracers in the Ocean

Track the motion (direction and velocity)

14C, cosmic rays in the upper atmos (half-life is 5700 years)3H, nuclear weapons testing (half-life is 12.5 years)CFCs, chlorinated fluoro-carbons –

banned in early 1970s

CFC Spreading in the Atlantic

•2000 m depth

•Deep Western Boundary Current

•Red is model result

The Great Conveyor BeltThe Great Conveyor Belt

Ocean Circulation and Climate

On long timescales, average ocean temperature affects climate

Most water is in deep ocean Average temperature of ocean is a function of

process of bottom-water formation transport of water around ocean basins

Deep water recycle times is ~1000 y– Thermohaline circulation moderates climate

over time periods of ~ 1000 y

Difference of winter surface temperatures from latitudinal average

The oceans are responsible for warmer temperatures on west

coasts of continents compared with east coasts

Photos courtesy of R. Seager (LDEO, U. Columbia)

The oceans are responsible for warmer temperatures on west

coasts of continents compared with east coasts

IPCC (2007), Fig. 10.15

Am

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Predicted changes in Atlantic MOC

slower

The idea of a “tipping point”Tipping points may produce changes that are much faster than the forcing; changes may be irreversible

Some impacts for Europe from shut-down of the Meridional Overturning Circulation

Reductions in runoff and water availability in southern Europe; major increase in snowmelt flooding in western Europe.

Increased sea-level rise on western European and Mediterranean coasts. Reductions in crop production with impacts on food prices. Changes in temperature affecting ecosystems in western Europe and the

Mediterranean (e.g., affecting biodiversity, forest products and food production).

Disruption to winter travel opportunities and increased icing of northern ports and seas.

Changes in regional patterns of increases versus decreases in cold- and heat-related deaths and ill-health.

Movement of populations to southern Europe Need to refurbish infrastructure towards Scandinavian standards.

http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch12s12-6-2.html

Carbon Cycle and Global Carbon Cycle and Global WarmingWarming

The temperature of bottom water formation determines how much CO2 is dissolved in deep ocean water

The rate of overturn of the oceans determines the “burial rate” of C from the atmosphere

Organic C accumulates in sediments, depending on the O2 content of deep ocean

Carbon Cycle and Global Carbon Cycle and Global WarmingWarming

Organic C in sediments is reduced to CH4 (methane gas)

Methane gas migrates upward and can be trapped as frozen “gas hydrates” near the ocean floor

Gas HydratesGas HydratesNewport, OR

Gas HydratesGas Hydrates

Gas Gas HydratesHydrates

Climate Change ConcernsClimate Change Concerns What happens when sea level falls?What happens when sea level falls?

(negative feed-back – polar ice forming)(negative feed-back – polar ice forming) What happens when deep water warms?What happens when deep water warms?

(positive feed-back – less CO(positive feed-back – less CO22 in water) in water)

Both effects liberate gas hydrates (CHBoth effects liberate gas hydrates (CH44), ), which combines with Owhich combines with O22 to form CO to form CO22, , ultimately reaching the atmosphereultimately reaching the atmosphere

Deep Ocean CirculationDeep Ocean Circulation Deep ocean water properties and circulation

play critical roles in earth’s climate system Modulates climate on long time scales (~100s-

1000s years) The ocean has an enormous capacity to absorb

and release greenhouse gases So, the rate, temperature and composition of

seawater circulating through the deep ocean is vitally important in assessing long term climate change

NASA web site: http://oceanmotion.org