The Great Ocean Conveyor_F10_Lect11

The Great Ocean Conveyor_F10_Lect11 - The Great Ocean...

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1 The Great Ocean Conveyor: Thermohaline Circulation Fall 2010, Lecture 11
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2 Seawater Density The density of seawater depends primarily on two factors o Temperature Cold water is denser than warm water o Salt content (salinity) The more salt that is dissolved in seawater, the denser it is
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3 Temperature Variation The temperature of the world's ocean is variable over the surface of the ocean o Temperature ranges from less than 0°C (32°F) near the poles to nearly 30°C (84°F) in the tropics Seawater is heated from the surface downward by sunlight o At depth, most of the ocean is cold o 75% percent of the water in the ocean falls within the temperature range of −1 to +6°C (30 to 43°F)
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4 Salinity The amount of salts dissolved in water is called salinity Salinity is measured in g per 1000 ml and a special symbol is used: ‰ by weight. ‰ is read “parts per thousand”, or more commonly as parts per mille Open ocean water has an average salinity of about 35 0/00 (equivalent to 3.5%) o 75 percent of the water in the ocean falls within the salinity range of 34 to 35‰ Scientists use metric measurements and express quantities of dissolved substances as grams per liter, or 1000 milliliters Thus, we use parts per thousand to express salinity
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5 Salinity Variation Red Sea = 40‰ Mediterranean Sea = 38‰ Average Seawater = 34.7‰ Black Sea = 18‰ Baltic Sea = 8‰
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6 Evolution of a Salty Ocean The salinity of the ocean has evolved over time Early in earth’s history, the oceans were fresh water As the result of water circulation within the hydrolgic cycle, rainwater falling on land dissolves minute amounts of salt, and carries it slowly to the ocean When ocean water evaporates, it leaves salt behind, gradually increasing the salinity of the ocean Until the solubility limit of some substance dissolved in the ocean is reached, this process continues to operate
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7 Brackish Water As water enters the sea, it mixes with salt water from the ocean Much of this mixing occurs in estuaries, such as Chesapeake and San Francisco Bays Water in estuaries is brackish, meaning it has a salinity between fresh water and open ocean seawater
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8 Baltic Sea The inflowing salt water stays deep, beneath a halocline The halocline is a density induced barrier between the surface and deep waters Little mixing occurs between so the surface and deep waters, so the salinity of the surface waters is 8‰ Baltic Sea is a brackish inland sea Surface discharge is 940 km 3 /yr Sub-surface flow is 475 km 3 /yr Streams contribute 660 km 3 /yr
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9 Black Sea The Black Sea has a net positive outflow of about 300 km 3 /yr There is a return flow of denser, saline water from the Aegean Sea, a part of the Mediterranean Sea This accounts for the observed 18‰ salinity
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10 Mediterranean Sea Evaporation greatly exceeds precipitation and river runoff in the Mediterranean
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The Great Ocean Conveyor_F10_Lect11 - The Great Ocean...

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