Presentation on theme: "Topic 14 Density Driven Currents"— Presentation transcript:
1 Topic 14 Density Driven Currents GEOL 2503Introduction to Oceanography
2 THERMOHALINE CIRCULATION VERTICAL circulation driven by DENSITYDensity of sea water mainly controlled by temperature and salinityTemperature is more importantWarm water has low density, and oceans are heated from the top. So how does water sink?
3 IMPORTANCE OF VERTICAL CIRCULATION DOWNWELLINGCarries oxygen to the deep oceanUPWELLINGCarries carbon dioxide and nutrients to the surface
4 Driven by surface processes Recall atmospheric circulationThese zones control evaporation and precipitation patternsLatitude controls surface temperature
10 Ocean Surface Water Density Summary Temperature and Salinity are the major controls on density of ocean surface waterAs salinity increases, density increasesAs salinity decreases, density decreasesAs temperature increases, density decreasesAs temperature decreases, density increases
11 Controls on Salinity Increased by: Decreased by: evaporation sea ice formationDecreased by:precipitationsea ice meltinginflow of river water
12 Many combinations of salinities and temperatures produce the same density
13 Surface Processes Less-dense water stays at surface Warmer, less salineDenser water tends to sinkColder, more salineThe result is a density-layered ocean
14 Changes with Depth Well-mixed surface layer to about 100 m Increasing density to about 1,000 mConstant density below 1,000 mRegion between 100-1,000 m is called a pycnocline‘Pycno-’ refers to density
15 Same for Temperature, Salinity Thermocline—zone with a rapid change in temperature with depthHalocline—zone with a rapid change in salinity with depthPycnocline—zone with a rapid change in density with depthAll occur between about 100-1,000 m
19 There is no pycnocline (or halocline or thermocline) in high latitudes because of rapidly sinking water
20 Density-Driven Circulation Vertical circulation in the oceans is driven by densityDensity is mostly controlled by surface changes in temperature and salinityBecause of this, vertical circulation is called thermohaline circulation
21 Upwelling and Downwelling Vertical motions of water up or down:Sinking water—downwellingRising water—upwellingContinuity of flow is basic concept
22 Another way to drive upwelling and downwelling Push surface water together or apartHas nothing to do with densityProcesses called surface convergences and divergences
23 Divergence zones—surface water pushed apart by winds and surface currents Convergence zones—surface water pushed together by winds and surface currents
24 Ocean surface convergence and divergence driven by global wind patterns
28 Antarctic Convergence Also known as the Polar FrontIt’s a line encircling Antarctica where cold, northward-flowing Antarctic waters sink beneath the relatively warmer waters of the sub-Antarctic.It is a dynamic boundary, meaning it's precise location may shift, but is generally located between 48º S and 61º S latitude
31 Water MassesWater with similar temperature and density characteristicsAcquire characteristics at the surfaceThermohaline circulation moves masses throughout the ocean depthsNamed by:Where fromWhere found
37 Southern Ocean Water Masses and Circulation Schematic depth-latitude diagram showing the major circulation and water masses of the Southern Ocean.The following water masses are highlighted:(1) Antarctic Bottom Water flowing along the abyssal ocean,(2) Circumpolar Deep Water upwelling into the Antarctic Divergence Zone,(3) Antarctic Intermediate Water in the temperature range 4-6°C, and(4) Subantarctic Mode Water in the upper ocean north of the Subantarctic Front (SAF).
38 T-S Curves Temperature and Salinity (T-S) plotted on graphs Remember, water masses have characteristic patterns of temperature and salinitySo T-S curves delineate water masses
45 A. thermoclineB. mixed layerC. layer with the highest densityD. curve for low latitudesE. curve for high latitudes
46 A. thermocline and pycnocline B. warmest waterC. seasonally warm waterD. Antarctic Bottom WaterE. Antarctic Intermediate WaterF. North Atlantic Deep WaterG. upwelling
47 Polynya (polynia) An area of open water surrounded by sea ice Now used as geographical term for areas of sea in Arctic or Antarctic regions which remain unfrozen for much of the year.The term derives from the Russian word for a hole in the iceAdopted in the 19th century by polar explorers to describe navigable portions of the sea.