The Salinity-Temperature-Depth Relationship in the World Oceans

1. The Salinity-Temperature-Depth Relationship in the World Oceans Part Two – The Three-Layered Ocean and the Great Conveyor Belt November 2009

2. Oceans in 3-D • As we’ve seen from analyzing world maps and depth profiles, many variables in the world’s oceans vary both horizontallyand 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

3. 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

4. The Three-Layered Ocean 0 • Surface 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 layer • bottom of surface layer to approx. 1,500m • main thermocline can be found here • Deep & bottom layers • below 1,500m • uniformly cold (less than 4ºC) Deep & bottom layers Surface layer 200 Intermediate layer 1500 Depth (m) 5000

5. 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

6. 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

7. Global 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 belt. • Importance: • 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

9. Great Ocean Conveyor – A Critical Look IPCC – Intergovernmental Panel on Climate Change

10. The Great Ocean Conveyor • 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. • Indian Ocean • North Pacific Ocean • Both due to warming by equatorial temperatures and the parting of ocean water by equatorial tradewinds. Main Currents in the North Atlantic 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)

11. 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)

12. Thermohaline Circulation in 3D! • http://www.nodc.noaa.gov/cgi-bin/OC5/WOA05F/woa05f.pl?parameter=t

13. Role of THC in Climate Change • Brings warmth from the tropics to higher latitudes • Particularly in the North Atlantic • Has “shut off” in recent geologic past • (12,800 years ago - the Younger Dryas) • Evidence • Oxygen isotope and carbon dioxide isotopes in ice cores • Foraminifera and glacial deposits in sediment cores • Low latitude glaciology • LDEO - Abrupt Climate Change - Younger Dryas Explanation SATELLITES SEE GULF STREAM WARM WATERSThis 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

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

15. References Castro, 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