OSU OC 103 - e-OC103_Lesson17 (5 pages)

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e-OC103_Lesson17



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e-OC103_Lesson17

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Pages:
5
School:
Oregon State University
Course:
Oc 103 - Exploring The Deep: Geography Of The World's Oceans

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OC103 Lesson 17 Deep Ocean Circulation In the last lesson we covered the wind driven currents of the surface ocean While the surface circulation is the more obvious to us it turns out it affects only 10 of ocean water volume The remaining 90 of ocean water is too deep to be affected by wind This lesson covers where that deep water comes from and how it circulates through the deep ocean in slow currents driven by density differences between water masses with different temperatures and salinities This slow movement of deep water helps regulate our climate and delivers nutrients to organisms living in the deep Density Driven Water Flow Deep water circulation is driven by density differences as in heavier water sinks and lighter water rises and is called Thermohaline Circulation because temperature and salinity T S together are what determine the density of seawater Processes that cause variations in temperature and salinity occur mainly at the surface warming and cooling by the atmosphere evaporation freezing So once a parcel of water becomes dense and sinks away from the surface it tends to keep its T S characteristics for a long time 1000 years or longer until it eventually circulates back up to the surface Density Variation in Sea Water Most ocean water is within a narrow range of T S The figure at right shows the natural range of T S of ocean waters 75 of ocean water has the very narrow range of T S characteristics covered by the small dark blue square near the bottom of the figure at 0 5 C and 34 35 while 99 of ocean water has T S characteristics that fit within the light blue area But even these small variations in T S are enough to create the density differences that drive deepwater circulation Water Masses The figure below is a north south cross section through the Atlantic Ocean showing the locations and directions of water flowing beneath the surface The different packages or tongues of water with distinctive combinations of temperature and salinity are called water masses Each water mass gained its unique characteristics when it formed at the surface in a particular area For example the water mass that forms in the Mediterranean Sea is warm and salty due to the warm dry climate causing a lot of evaporation there and when it flows out into the Atlantic it can still be distinguished from the surrounding water by its distinctive combination of temperature and salinity It is the small water mass labeled MIW in the figure below Water masses are usually named for where they formed and or where they flow in the ocean For example the Mediterranean water mass is called Mediterranean Intermediate Water MIW because it formed in the Med and flows through the ocean at an intermediate depth about 2 km deep below the less salty less dense surface waters and above the denser colder deep waters We are not interested in the gory details



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