Deep Ocean Circulation Motion in the Ocean, Part 2, “The Great Conveyor Belt” Jack Barth NASA web site:

Surface Circulation

Surface and Deep Ocean Circulation help move heat from equator to pole Atmosphere & Ocean each responsible for about half of heat transfer

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

Density Variation in Sea Water Isopycnals = constant density

Density Variation in Sea Water Nearly all the water in the oceans is cold

North Atlantic Surface 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

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) n n Weddell Sea major site of AABW formation n n AABW circles Antarctica and flow northward as deepest layer in Atlantic, Pacific and Indian Ocean basins n n AABW flow extensive – –45°N in Atlantic – –50°N in Pacific – –10,000 km at km h -1 ; 250 y

North Atlantic Deep Water (NADW) n n Coastal Greenland (Labrador Sea) site of NADW formation n n NADW comprises about 50% of the deep water to worlds oceans n n NADW in the Labrador Sea sinks directly into the western Atlantic – –NADW forms in Norwegian Basins n n Sinks and is dammed behind sills – –Between Greenland and Iceland and Iceland and the British Isles n n NADW periodically spills over sills into the North Atlantic

Water Masses and ocean mixing determined by CTD (conductivity, temperature, depth) measurements

Mediterranean Water

Deep Atlantic Circulation 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 n n Surface water at high latitudes forms deep water n n Deep water sinks and flows at depth throughout the major ocean basins n n Deep water upwells to replace the surface water that sinks in polar regions n n Surface waters must flow to high latitudes to replace water sinking in polar regions n n This Idealized circulation is called the “Great (Thermohaline) Conveyer Belt”

Tracers in the Ocean F F Track the motion (direction and velocity) F 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 – banned in early 1970s

CFC Spreading in the Atlantic 2000 m depth Deep Western Boundary Current Red is model result

The Great Conveyor Belt

Ocean Circulation and Climate n n On long timescales, average ocean temperature affects climate n n Most water is in deep ocean n n Average temperature of ocean is a function of n n process of bottom-water formation n n transport of water around ocean basins n n 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 Amount of water per second in Atlantic MOC 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 n n Reductions in runoff and water availability in southern Europe; major increase in snowmelt flooding in western Europe. n n Increased sea-level rise on western European and Mediterranean coasts. n n Reductions in crop production with impacts on food prices. n n Changes in temperature affecting ecosystems in western Europe and the Mediterranean (e.g., affecting biodiversity, forest products and food production). n n Disruption to winter travel opportunities and increased icing of northern ports and seas. n n Changes in regional patterns of increases versus decreases in cold- and heat-related deaths and ill-health. n n Movement of populations to southern Europe n n Need to refurbish infrastructure towards Scandinavian standards.

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

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

Gas Hydrates Newport, OR

Gas Hydrates

Climate Change Concerns F What happens when sea level falls? (negative feed-back – polar ice forming) F What happens when deep water warms? (positive feed-back – less CO 2 in water) 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 F Deep ocean water properties and circulation play critical roles in earth’s climate system F F Modulates climate on long time scales (~100s- 1000s years) F F The ocean has an enormous capacity to absorb and release greenhouse gases F F 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: