Deep Ocean Circulation Motion in the Ocean, Part 2, “The Great Conveyor Belt” Jack Barth...
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Transcript of Deep Ocean Circulation Motion in the Ocean, Part 2, “The Great Conveyor Belt” Jack Barth...
Deep Ocean CirculationDeep Ocean CirculationMotion in the Ocean, Part
2, “The Great Conveyor Belt”
Jack Barth([email protected])
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
ou
nt
of
wate
r p
er
seco
nd
in
A
tlan
tic
MO
C
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