Transcript Document
Lecture 8: The Oceans (2) EarthsClimate_Web_Chapter.pdf, p. 24-27 Deep-Ocean Circulation The thermohaline circulation = the density-driven circulation (thermo- for heat, and -haline for salt). Other names: the global ocean conveyor belt, or meridional overturning circulation (MOC). Wind driving and density driving of the oceanic circulation are closely coupled and cannot be easily separated. Global Conveyor Belt 1. Warm and salty shallow water from Tropics is carried northward through Atlantic 2. Water cools in North Atlantic and sinks east of Greenland as cold and salty dense water 3. North Atlantic Deep Water flows toward the Equator and into the South Atlantic, where it meets yet colder and denser Antarctic Bottom Water flowing toward the Equator 2 1 3 Global Conveyor Belt (Cont’d) 4. Combined flow eastward below Africa and into the Pacific Ocean 5. Deflected by Asia 6 5 7 6. Journey through Pacific causes warming and slow rise to surface 4 7. Warm and less dense Pacific surface waters move south and west to converge toward Equator as shallow surface water to continue warming Global Conveyor Belt (Cont’d) 8. 1,000 years for a complete cycle 9. Conveyor Belt aided by imbalance between loss of water by evaporation in Atlantic and gain of water by precipitation and runoff 10. Bering Strait prevents free exchange between Arctic and Pacific Oceans inhibits Arctic-Pacific Conveyor Belt Impacts on global climate 1. Supplying heat to the polar regions, and regulating sea ice there 2. Determining the concentration of carbon dioxide in the atmosphere 3. Warming Western Europe by about 2°C relative to the similarly located west coast of Canada 4. 12,000 years ago, meltwater from ice sheets disrupted deep water formation and subsidence in the North Atlantic and caused a cooling climate period in Europe known as the Younger Dryas. Global warming and shutdown of THC Hypothesis: global warming melting Greenland ice sheet, increasing precipitation and river discharge interrupt THC cooling in Europe Summary: • What role does deep-ocean circulation play in climate change? – Transport heat from tropics to higher latitude and from surface to deep ocean. • Where does surface water sink to deep ocean and where does deep ocean water surfaces globally? – Surface water mainly sinks to deep ocean in sub-polar North Atlantic and Arctic ocean, deep ocean water surfaces in eastern ocean boundary, Antarctic coast and the equatorial upwelling regions. • What change might occur to the thermohaline circulation in a warmer climate? How would such a change influence the global climate? – Melting of ice and warming of surface temperature would reduce density of the sea water in the sub-polar North Atlantic and Arctic oceans, consequently weakens the thermohaline circulation. Should this occur, it might cause abrupt climate change in the northern hemisphere middle and high latitudes.