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Ocean and Climate II Salinity Thermohaline circulation The great conveyor belt http://www.gerhardriessbeck.de/ Temperature, Salinity, Pressure Three layer model: 1. mixed layer, T,S,P≈constant 2. Cline layer, decrease of T, increase of S and P, 3. abyss layer, moderate change of T, S, P Hydrographic profile along Atlantic Ocean 25oW longitude Temperature profile every 2.5oC. High line density indicates the thermocline, abyssal range shows only slow temperature decline. Salinity profile in psu, every 0.5 psu (>35psu) to 1 psu (<35psu). The strongest gradient in salinity appears to exist in the thermocline layer as indicated before. Temperature gradient T z Tp Tm Tp e z z* empirical values : mean surface temperature : Tm 19.60C polar temperature at abyss depth : Tm 1.20C 25.00 T z 1.20C 18.80 C e z z* Temperature oC z* 500m 20.00 15.00 10.00 5.00 0.00 10.00 100.00 Depth m 1000.00 Latitudinal change of temperature and salinity The latitudinal temperature and salinity pattern of the oceans indicates a sharp temperature drop towards higher latitude with slight decrease of salinity (melting ice). Empirical approximations for T , S T , S ref 1 d T ref dT T 1 104 K 1 Thermal expansion coefficient (Temperature dependence of density) S 1 ref d dS S 7.6 104 psu1 Salinity dependence of density 0 ref T T T0 S S S0 o≈26.7kg/m3 for T0 =15oC and S0 =36psu are reference numbers, extracted from the T-S contour plot. Conditions change and differ for different depth layers. K c 2 sound Example Calculate the density of ocean water for equatorial water temperatures of T=28oC and a salinity of S=35psu and for polar temperatures of T=4oC at the same salinity of 35psu with a reference density ref = 1000 kg/m3! 0 ref T T T0 S S S0 T0 150 C 288K T 1 104 K 1 S0 36 psu S 7.6 104 psu1 26.5 kg kg 4 1 4 1 S 36 psu 1000 10 K 288 K T 7 . 6 10 psu 3 3 m m T 280 C 301K 26.5 kg kg kg 2 . 06 24 . 44 m3 m3 m3 T 40C 277 K 26.5 S 35 psu warm 1024 kg m3 The density increases with decreasing temperature at constant salinity! S 35 psu kg kg kg 0 . 34 26 . 84 m3 m3 m3 cold 1027 kg m3 Ocean water density Buoyancy and Density Archimedes principle: A body wholly or partially submerged in a fluid is buoyed up by a force equal to the weight of the displaced fluid! For floating object : Fi 0 i Fbuoyancy Fpressure Fgravity WH 2O WH 2O mH 2O g H 2O g V Wobject object g V object H O 2 object H O 2 Object will sink Object will float with a fraction x of its volume submerged: x Vsubmerged Vobject object H O 2 Example Ice has a density of ice=916.7kg/m3 compared to water which has a higher density H2O=1000kg/m3, calculate the buoyancy force FB and the fraction x of the submerged volume of a typical iceberg with a total volume of V=900,000 m3. object 916.7 x 0.917 H O 1000 2 FB H 2O g Vsubmerged H 2O g x V kg m FB 1000 3 9.81 2 0.917 900,000m 3 m s m FB 8.1 109 kg 2 8.1 109 N 8.1MN s With sea water density of sw≈1025kg/m3 the fraction x would be smaller, since the submerged volume would be reduced, but the buoyancy force would remain the same. Thermohaline circulation Warm water ocean currents such as the Gulf current originate in tropic ocean regions flowing north. The warm surface water cools to the atmosphere and has a high evaporation rate. Cooling increases the density as shown before, evaporation increases the salinity and therefore further increases the density of the ocean water. The high density salt water sinks to lower layers driving the returning deep water flow, forming the so-called ocean conveyor belt. The layer structure of ocean changes in polar regions due to the down-welling of high density water with high salinity. The decrease of temperature and salinity cause down welling of salty surface water from ocean currents; the mixed layer reaches towards much larger depths and couples directly with the cold abyss layer, which is at the same low temperature level. The sinking water triggers a deep cold water flow directed towards the equatorial zones of low latitude. Pronounced deep mixing zones appear at latitudes of 70oN but only 50oS. The high latitude reach of 70oN is due to the warm water flow of Gulf stream. Consider a water volume element at the ocean surface of 50m depth and 2500m2 surface moving north as part of the gulf current. Emerging from the Gulf of Mexico the water has a density of 1=1024kg/m3, reaching the Arctic Ocean, evaporation has increased the density to 2=1027kg/m3. Calculate the buoyancy of this body of salty water compared to an averaged ocean water density if H2O=1026kg/m3 and determine the location from the density map at which it is completely submerged. Water flows at surface x1 object H 2O 1024 0.998 1026 FB H 2O g Vsubmerged H 2O g x V kg m FB1 1026 3 9.81 2 0.998 125,000m 3 1,255,552,724 N m s object 1027 Density is too high, water sinks down x2 1.001 H 2O 1026 FB2 1,259,390,633 N Density Conditions near ocean surface 1024 kg/m3 1026 kg/m3 1027 kg/m3 Latitude kg x 1; 1026 3 m Corresponds to 45oN latitude. Real conditions are determined by the interplay between temperature and salinity conditions! Observations near the arctic coast Salinity for AO1 58o N Salinity for A24 A24 AO1 Thermohaline circulation near Greenland coast, visualized by the high salinity turn-over current at 3000 m. Thermohaline circulation near Greenland coast, visualized by the high salinity turn-over current at 2000 m. Temperature around Antarctica Salinity conditions around Antarctica Decrease in salinity at surface (50 m depth) due to mixing with melting water; increase in salinity at 800 m depth due to thermohaline circulation. Pacific Ocean salinity conditions Double salinity layer due to thermohaline currents shown in distribution at different depths and in depth profile. Thermohaline circulation is one of the drivers of the ocean conveyor belt! The great Ocean Conveyor Belt http://pmm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt Threat to Europe? The THOR project