The Salinity-Temperature-Depth Relationship in the World OceansThe Salinity-Temperature-Depth

Relationship in the World Oceans

Part Two – The Three-Layered Ocean and the Great Conveyor Belt

November 2009

Part Two – The Three-Layered Ocean and the Great Conveyor Belt

November 2009

• As we’ve seen from analyzing world maps and depth profiles, many variables in the world’s oceans vary both horizontally and vertically.

• This 3rd vertical dimension is controlled by the density of the water.

QUESTION: What two variables have the greatest effect on the density of sea water?

ANSWER: Temperature and salinity

Stratification

• Less dense water (warmer/less salty/or both) floats at the surface

• Denser water (colder/saltier/or both) tends to sink

• Creates stratification– essentially causes layers

to form in the ocean.

– imparts vertical stability to the ocean

The Three-Layered Ocean• Surface layerSurface layer

– surface to approx. 200m– also called the mixed layer, because it

is mixed by wind, waves, currents, & storms

– may be the only layer found in shallow coastal waters over the continental shelf

– seasonal thermoclines may form here

• Intermediate layerIntermediate layer– bottom of surface layer to approx.

1,500m– main thermocline can be found here

• Deep & bottom layersDeep & bottom layers– below 1,500m– uniformly cold (less than 4ºC)

Deep& bottom layers

Dep

th (

m)

Intermediate layer

0

1500

200

5000

Surface layer

Stability

• Stability – likelihood that a water column in the ocean will remain stratified.– High stability

• large density difference between deep and shallow water• keeps water column stratified

– Low stability • slight density difference between deep and shallow water• relatively easy to mix the two layers

Overturn

– Instability (unstable) • surface layer becomes more dense than deep layer

• surface water sinks resulting in downwelling

• process known as overturn– What might a depth profile look like when downwelling occurs?

– What climate conditions might cause downwelling?

• once water sinks, its temperature and salinity do not change since the processes that change them are surface phenomena

– becomes a “signature” for the water mass

– oceanographers can follow these water masses as they circulate throughout the oceans

Global Thermohaline CirculationGlobal Thermohaline Circulation• Gets its name from fact that it is driven by density

differences in water masses which are in turn caused by temperature and salinity differences.

• It acts as a giant conveyor beltgiant conveyor belt..

• ImportanceImportance:– Extends throughout ocean depths– Regulates the Earth’s climate– Transports heat and nutrients throughout globe– Chemically mixes the world’s oceans– Brings oxygen-rich surface water to the deep sea

Great Ocean Conveyor – A Critical LookGreat Ocean Conveyor – A Critical Look

IPCC – Intergovernmental Panel on Climate Change

The Great Ocean ConveyorThe Great Ocean Conveyor

• Water sinks…Water sinks…– Norwegian Sea (North Atlantic

Deep Water – NADW)• Forms due to heat loss and high

salinity

– Labrador Sea • Deep convection

– Weddell Sea (Antarctic Bottom Water – ABW) – densest

• Forms mostly because of low temperature

• ……and returns to surface.and returns to surface.– Indian Ocean

– North Pacific Ocean• Both due to warming by equatorial

temperatures and the parting of ocean water by equatorial tradewinds.

An oceanic roundabout. As warm ocean currents in the subpolar gyre gradually cool, they warm Europe and may trigger seesaws in climate (McCartney et al., 1996, Oceanus, 39, 19-23)

Main Currents in the North AtlanticMain Currents in the North Atlantic

The thermohaline circulation "conveyor belt". Purple arrows indicate cold, deep ocean currents. Red arrows show shallow, warm water circulation patterns. Credit: Image courtesy CLIVAR (after W. Broecker, modified by E. Maier-Reimer)

Thermohaline Circulation in 3D!

• http://www.nodc.noaa.gov/cgi-bin/OC5/WOA05F/woa05f.pl?parameter=t

Role of THC in Climate ChangeRole of THC in Climate Change

• Brings warmth from the tropics to Brings warmth from the tropics to higher latitudes higher latitudes – Particularly in the North AtlanticParticularly in the North Atlantic

• Has “shut off” in recent geologic Has “shut off” in recent geologic past past – (12,800 years ago - the Younger (12,800 years ago - the Younger

Dryas)Dryas)– EvidenceEvidence

• Oxygen isotope and carbon dioxide Oxygen isotope and carbon dioxide isotopes in ice coresisotopes in ice cores

• Foraminifera and glacial deposits in Foraminifera and glacial deposits in sediment coressediment cores

• Low latitude glaciologyLow latitude glaciology• LDEO - Abrupt Climate Change - Younger LDEO - Abrupt Climate Change - Younger

Dryas ExplanationDryas Explanation

SATELLITES SEE GULF STREAM WARM WATERS

This is a NASA satellite image of the warm waters of the Gulf Stream running up the U.S. eastern seaboard. The Gulf Stream shows up as a winding rope of orange and yellow (indicating warm waters) against the cooler green and blue waters. Credit: MODIS Ocean Group NASA/GSFC SST product by U. Miami

Additional Links

• http://www.youtube.com/watch?v=uSBzRWs2nNU&fmt=18

ReferencesCastro, Peter, & Michael E. Huber. Marine Biology. 5th. New York: The McGraw-Hill Companies, Inc.,

2005.

Hoffman, Jennifer. Science 101: Ocean Science. Irvington, NY: Harper Collins Publishers, Inc., 2007.

Kunzig, Robert. "In deep water." Discover Dec 1996: 86-96.

Naik, Naomi. "2: What scientific evidence do we have that abrupt climate change has happened before?." Abrupt Climate Change. 2003. Lamont Doherty Earth Observatory of Columbia University. 3 Mar 2008 <http://www.ldeo.columbia.edu/res/pi/arch/examples.shtml>.

Russell, Randy. "Transfer and Storage of Heat in the Oceans." Windows to the Universe. 6 June 2007. The Regents of the University of Michigan. 3 Mar. 2008 <http://www.windows.ucar.edu/tour/link=/earth/Water/ocean_heat_storage_transfer.html