Transcript Lecture 1. Course Introduction

Deep Ocean Circulation
Motion in the Ocean, Part 2,
“The Great Conveyor Belt”
Jack Barth
([email protected])
NASA web site:
http://oceanmotion.org
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?
 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 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
 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?
 The
densest seawater is cold and salty
 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
North Atlantic Deep Water
Antarctic Bottom Water (AABW) in Weddell, Ross
Seas and Adelie Coast
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
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– 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)
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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 Masses
and ocean
mixing
determined by CTD
(conductivity,
temperature, depth)
measurements
Mediterranean Water
Mediterranean Water
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
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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 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 Belt
Ocean Circulation and Climate
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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
The oceans are responsible for warmer
temperatures on west coasts of
continents compared with east coasts
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)
Amount of water per second in
Atlantic MOC
Predicted changes in Atlantic MOC
slower
IPCC (2007), Fig. 10.15
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
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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 coldand 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 Warming
 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 Warming
 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 Hydrates
Newport, OR
Gas Hydrates
Gas Hydrates
Climate Change Concerns
 What
happens when sea level falls?
(negative feed-back – polar ice forming)
 What happens when deep water warms?
(positive feed-back – less CO2 in water)
 Both effects liberate gas hydrates (CH4),
which combines with O2 to form CO2,
ultimately reaching the atmosphere
Deep Ocean Circulation
Deep ocean water properties and circulation
play critical roles in earth’s climate system
 Modulates climate on long time scales (~100s1000s 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
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NASA web site:
http://oceanmotion.org