1) Density is calculated from temperature, salinity, and pressure. Pressure matters because temperature changes with pressure. The full equation to calculate density has many terms, and is called the equation of state for seawater. It can be approximated as: ΔDensity = α*ΔT+β*ΔS α and β are coefficients of thermal expansion and saline contraction. α is roughly 0.2 kg m -3 °C -1 β is roughly 1 kg m -3 psu -1 These values are where the 15°C and 3 psu come from. In the ocean, α and β also depend on salinity, temperature, and pressure. 2) Temperature changes of 15°C are more common than salinity changes 3 psu, so temperature tends to have a bigger effect on density than salinity does. As a general rule, cold water sinks and salt is just carried along with it. More on the next slide. 3) Cold salty water will sink, and warm fresh water will float. But cold fresh water may have the same density as warm salty water. This is part of what makes the ocean interesting. Ocean Layering: Density, Salinity, Temperature, and Circulation Sylvia Cole, Scripps Institution of Oceanography, San Diego CA Maureen Quessenberry, University City High School, San Diego CA
Transcript
1) Density is calculated fromtemperature, salinity, andpressure. Pressure mattersbecause temperature changeswith pressure. The full equationto calculate density has manyterms, and is called the equationof state for seawater. It can beapproximated as: ΔDensity = α*ΔT+β*ΔSα and β are coefficients ofthermal expansion and salinecontraction. α is roughly 0.2 kg m-3 °C-1
β is roughly 1 kg m-3 psu-1
These values are where the15°C and 3 psu come from. Inthe ocean, α and β also dependon salinity, temperature, andpressure.
2) Temperature changes of 15°C are more commonthan salinity changes 3 psu, so temperature tends tohave a bigger effect on density than salinity does. As ageneral rule, cold water sinks and salt is just carriedalong with it. More on the next slide.
3) Cold salty water will sink, and warm fresh water will float. Butcold fresh water may have the same density as warm salty water.This is part of what makes the ocean interesting.
Ocean Layering: Density, Salinity, Temperature, and CirculationSylvia Cole, Scripps Institution of Oceanography, San Diego CAMaureen Quessenberry, University City High School, San Diego CA
1) Density is mostlydetermined bytemperature. Thedensest water is foundnear the poles, and thelightest water is foundnear the equator.
2) The coldest wateris found near thepoles and thewarmest water nearthe equator.
3) The saltiest wateris found near thecenter of the gyres,but this is NOTwhere the densestwater is found.
4) The main point is that temperature will primarily determine wherewater sinks. There are exceptions to this, and one will bementioned later (the salty Mediterranean Sea).
The figures are average temperature, salinity, and density at thesurface from the World Ocean Atlas 2005. They are an average ofall observations in the last several decades. Seehttp://www.nodc.noaa.gov/OC5/WOA05/pr_woa05.html for moreinformation.
This wateris veryfresh, andnot verydense.
2) The ocean is a big densitycolumn with more layers, andsmoother changes betweenlayers. The lightest water ison top, and the heaviest onthe bottom.
3) The density profile issmooth because water at anydepth mixes a little with waterabove and below. In mostplaces, most of the water isvery dense, with a thin layerof light water near thesurface. The profile shownhere is from the South PacificOcean (20°S, 170°W). It wastaken as part of a sectionfrom near Antarctica toHawaii for the World OceanCirculation Experiment. See‘02t.slides.globallayers’(http://earthref.org/cgi-bin/erda.cgi?n=1014) formore information.
1) This is the approximate density profile for the densitycolumn demonstrated at the beginning of class. Some of thelayers have a big change in density, and some of them onlychange a little. It only takes a small change in density forone liquid to float on top of another. The numeric values foreach liquid are given in ‘01t.activity.densitycolumn’(http://earthref.org/cgi-bin/erda.cgi?n=1001).
1) The figure is a schematic of the globalconveyor belt circulation after the popularone by W. S. Broecker*. The conveyor beltcirculation is also known as the meridionaloverturning circulation, or the thermohalinecirculation.
2) Scientists are still working on manydetails of this circulation. It is not even clearwhat causes the circulation. It may bepartially driven by dense water sinking, orby the energy available for water to rise.Lifting cold water from the bottom to thesurface takes energy, and that has to comefrom somewhere. It primarily comes fromenergy put into the ocean by the tides andwinds.
*Broecker, W S, 1987. The biggest chill.Natural History Magazine, Vol. 97, pp. 74-82.
3) For a ninth grader, the important points are that thereis a circulation that is global and connects all of theoceans, and the circulation involves the verticalmovement of water. This circulation is very different fromwhat happens at the surface. The global conveyor belt isNOT driven by the wind, or organized into gyres.Students should understand that these global currentsnear the bottom of the ocean are caused by differencesin ocean density. Density causes water to sink in someplaces, which means it will rise in other places and therewill be horizontal currents connecting where water sinksand rises.
4) You may want to draw this on the board:
Causes vertical currents. Coldwater moves down and warmwater moves up.
Causes horizontal currents.Cold water moves right andwarm water moves left.
1) The figure is adapted from Fig 9.20 ofGarrison*. This shows where water sinksin the ocean, and roughly how deep itsinks.
Red dots: All of the water that sits on thebottom of the ocean sank near theselocations. As water sinks it entrainsnearby water and is modified as it travelsaround the globe. Most of the ocean haswater that originated near Antarctica.
Green dots: These water masses sink tothe middle of the ocean and then spreadhorizontally. The one by theMediterranean Sea sinks because it isvery salty. The Mediterranean Sea isvery salty because there is moreevaporation than precipitation there.
In some places, near Antarctica andRussia, ice is involved in making waterdenser. When sea ice freezes, it leavesmost of the salt behind. This helps toincrease the density of nearby andalready cold water.
What about the Arctic? Water does sinkthere, but the bottom topography(underwater hills and mountains)prevents it from spreading around therest of the ocean. It is trapped in theArctic.
2) The main points are that 1) only dense water sinks, 2) densewater is found in only a few places, usually where water is coldnear the polar regions, and 3) sinking happens rapidly comparedto rising. Sinking is not a continuous process. If you go to one ofthese locations, you may or may not observe sinking water. Ithappens in fits and spurts, and involves interactions with thewater, ice, and the atmosphere around it. In some places, waterwill only sink for a short time in winter.
*Garrison, Oceanography: An Introduction to Marine Science. USA:Thomson Learning, 2002.
1) There is no real diagram forwhere water rises, water riseseverywhere very slowly.
The average speed of waterrising is roughly 1cm per day(from Garrison, citation onprevious slide). This meansthat it would take 1095 yearsto rise 4000 m. This is anaverage, it may take a fewhundred years, or a fewthousand years.
2) What causes water to rise? Water “rises” or becomes warmer andless dense when the ocean is mixed in the vertical (see the diagramto the right). Vertical mixing happens at smaller scales, parcels 10’sof meters high are mixed together. The mixing occurs when wavesbreak. Just like there are surface waves that break on the surfaceand mix air into the water and water into the air, there are wavesbelow the surface that mix slightly denser water with slightly lighterwater when they break. These are called internal waves and arecaused by winds blowing on the surface and tidal currents that runinto continental shelves, ridges, or other underwater obstructions.
Unmixed
Water is denser
MixedWater is lighter
DensityD
epth
This is just a reminder of what the big picture is. The timescale for spreadinghorizontally around this circuit is about the same as rising, about 1000 years.
Rising water in this diagram is a simplification, it happens everywhere in the ocean.Sinking water really does occur in just a few places.
The figure mainly shows the circulations involved with water that sinks to the bottom.Water that sinks to the middle of the ocean can have similar patterns, with flowbetween the different ocean basins.
1) Just like surfacecurrents, deep currentstransport many things.They can transport heat,salt, nutrients, sediment,gasses, pollution,whatever is in the water.
One of the most importantthings the oceantransports is heat,because this affects ourclimate on land.
Our climate is alsoaffected by how fast theocean mixes in thevertical. If the mixing wereincreased or decreased,our climate would change.Weaker mixing means agreater temperaturedifference between thesurface and the bottom,so the surface wouldprobably be warmer.
2) The third point has to do with nutrients, which is what biology needsto grow. The rising part of this circulation is important for bringingnutrients up from the bottom. If water did not rise, not as much lifecould grow in the ocean. The global conveyor belt circulation is notthe only way for water to rise in the ocean, but it is still important.
