Deep Ocean Circulation Motion in the Ocean, Part 2, or Who wants to ride the Great Conveyor Belt?

Surface Circulation

How does the Deep Ocean respond to Surface Circulation? F The main gyres move heat and salt F Resulting DENSITY variations lead to vertical flow (sinking) F Formation of “water masses”, characterized by Temperature, Salinity

Density Variation in Sea Water Isopycnals = constant density

North Atlantic Circulation

Density-Driven Water Flow F Called “Thermohaline Circulation”, because temperature and salinity together determine density of seawater “Thermo” = temperature “haline” = salt

Where does the Ocean’s Deepest Water Come From? F The densest seawater is cold and salty F This is formed at high latitudes in the North and South Atlantic: North Atlantic Deep Water (NADW) Antarctic Bottom Water (AABW)

Density of Sea Water

Density Rules!

Deep Water Masses

Water Masses and ocean mixing

Mediterranean Water

Deep Atlantic Circulation

Tracers in the Ocean F Track the motion (direction and velocity) F 14 C, cosmic rays in the upper atmos (half-life is 5700 years) 3 H, nuclear weapons testing (half-life is 12.5 years) CFCs, chlorinated fluoro-carbons

The Great Conveyor Belt

Closing Isthmus of Panama and onset of the Ice Ages?

Consequences of Global Flow F Ocean turnover is about 1500 years (time for a round trip on the conveyor belt) F Deep water (made in the polar Atlantic) contains abundant O 2 and CO 2 F The high O 2 content promotes oxidation of bottom sediments F The CO 2 content controls CCD (Carbonate Compensation Depth)

Carbon Cycle and Global Warming F The temperature of bottom water formation determines how much CO 2 is dissolved in deep ocean water F The rate of overturn of the oceans determines the “burial rate” of C from the atmosphere F Organic C accumulates in sediments, depending on the O 2 content of deep ocean

Carbon Cycle and Global Warming F Organic C in sediments is reduced to CH 4 (methane gas) F Methane gas migrates upward and can be trapped as frozen “gas hydrates” near the ocean floor

Gas Hydrates

Climate Change Concerns F What happens when sea level falls? (negative feed-back) F What happens when deep water warms? (positive feed-back) F Both effects liberate gas hydrates (CH 4 ), which combines with O 2 to form CO 2, ultimately reaching the atmosphere

Deep Ocean Circulation F The ocean has an enormous capacity to absorb and release greenhouse gases F So, the rate, temperature and composition of seawater circulating through the deep ocean is vitally important in assessing long term climate change