Geography and Oceanography - Chapter 14 potx

Surface and Deep Ocean Circulation help move heat from equator to poleAtmosphere & Ocean each responsible for about half of heat transfer... How does the Deep Ocean respond to Surface Ci

Deep Ocean Circulation

Motion in the Ocean, Part 2,

“The Great Conveyor Belt”

Jack Barth

(barth@coas.oregonstate.edu)

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

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 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

■ 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

CFC Spreading in the Atlantic

•2000 m depth

•Deep Western Boundary

Current

•Red is model result

The 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

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 Warming

determines how much CO2 is dissolved in deep ocean water

the “burial rate” of C from the atmosphere

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 (~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