Deep Ocean CirculationDeep Ocean Circulation

Motion in the Ocean, Part 2, Motion in the Ocean, Part 2, oror

Who wants to ride the Great Who wants to ride the Great Conveyor Belt?Conveyor Belt?

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

Circulation?Circulation?

The main gyres move heat and saltThe main gyres move heat and salt Resulting DENSITY variations lead Resulting DENSITY variations lead

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

characterized by Temperature, characterized by Temperature, SalinitySalinity

Density Variation in Sea WaterDensity Variation in Sea Water

Isopycnals = constant Isopycnals = constant densitydensity

North Atlantic CirculationNorth Atlantic Circulation

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” = temperature” = temperature““halinehaline” = salt” = salt

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

Water Water MassesMassesand oceanand oceanmixingmixing

Mediterranean WaterMediterranean Water

Mediterranean WaterMediterranean Water

Mediterranean WaterMediterranean Water

Deep Atlantic Deep Atlantic CirculationCirculation

Tracers in the OceanTracers in the Ocean

Track the motion (direction and Track the motion (direction and velocity)velocity)

1414C, cosmic rays in the upper atmosC, cosmic rays in the upper atmos (half-life is 5700 years) (half-life is 5700 years)33H, nuclear weapons testingH, nuclear weapons testing (half-life is 12.5 years) (half-life is 12.5 years)CFCs, chlorinated fluoro-carbonsCFCs, chlorinated fluoro-carbons

The Great Conveyor BeltThe Great Conveyor Belt

Closing Isthmus of Closing Isthmus of Panama and onset of the Panama and onset of the

Ice Ages?Ice Ages?

Consequences of Global Consequences of Global FlowFlow

Ocean turnover is about 1500 years (time Ocean turnover is about 1500 years (time for a round trip on the conveyor belt) for a round trip on the conveyor belt)

Deep water (made in the polar Atlantic) Deep water (made in the polar Atlantic) contains abundant Ocontains abundant O22 and CO and CO22

The high OThe high O22 content promotes oxidation of content promotes oxidation of bottom sedimentsbottom sediments

The COThe CO22 content controls CCD (Carbonate content controls CCD (Carbonate Compensation Depth)Compensation Depth)

Carbon Cycle and Global Carbon Cycle and Global WarmingWarming

The temperature of bottom water formation The temperature of bottom water formation determines how much COdetermines how much CO22 is dissolved in is dissolved in deep ocean water deep ocean water

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

Organic C accumulates in sediments, Organic C accumulates in sediments, depending on the Odepending on the O22 content of deep ocean content of deep ocean

Carbon Cycle and Global Carbon Cycle and Global WarmingWarming

Organic C in sediments is reduced Organic C in sediments is reduced to CHto CH44 (methane gas) (methane gas)

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

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

(negative feed-back)(negative feed-back) What happens when deep water warms?What happens when deep water warms?

(positive feed-back)(positive feed-back) 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

The ocean has an enormous capacity The ocean has an enormous capacity to absorb and release greenhouse to absorb and release greenhouse gasesgases

So, the rate, temperature and So, the rate, temperature and composition of seawater circulating composition of seawater circulating through the deep ocean is vitally through the deep ocean is vitally important in assessing long term important in assessing long term climate changeclimate change