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Ocean Circulation

Today’s topics• Density variations in the oceans• Layering of ocean water• Thermohaline Circulation

Let’s stop and think for a minute:

• Do you expect ocean water to be the sameeverywhere?

• What properties may change with latitude?• What properties may change with depth?• Could these variations drive circulation of

ocean water?

Answers:…

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You tell me:

• Do you expect ocean water to be the sameeverywhere?

• What properties may change with latitude?• What properties may change with depth?• Could these variations drive circulation of

ocean water?

Layered Ocean• Oceans layered like a cake

– Density layers are result ofTemperature, Salinity, andpressure variations

• These variations resultfrom Energy transfer– Between oceans and

atmosphere– Internal transfer by

advection• These variations drive

deep circulationAnd away we go…

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Density variations in Seawater:• Salinity effect:

– Dissolved salt = addition of mass– So, density seawater > freshwater– Salty water sinks

• Pressure effect:– increase Pressure ‘compresses’ water

resulting in increased Density– So, Density increases with Depth

• Temperature effect:– Increase T -> expansion of water

(decrease density)– Decrease T -> contraction of water

(increase density)– Cold water sinks

Combined effect of Temperatureand Salinity on Density

• Isopycnal = curves of constantDensity

• Isopycnal lines = Various T &S combinations resulting inthe same density

• Thus, water at the same depth(i.e. same density) can havedifferent T and S.– Mix these different waters and

density will increase and theysink (caballing)

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Temperature, Salinity and resulting density variations with depth:

• Near surface is well mixed, thus constant T and S• Halocline = rapidly changing Salinity• Thermocline = rapidly changing Temperature• Pycnocline = rapidly changing density

But isn’t this a little simplistic?What about variations in surfaceheating with latitude?

Density variations with latitude:

• Equatorial region: Enhanced precipitation decreses salinity of seawater(lower density).

• Mid-Latitudes: Arid conditions cause evaporation and increased salinity(greater density)

• High Latitude ~60º: heat lost to atmosphere makes cooler, denser water• Polar Regions: formation of sea ice increases salinity (salty cold dense)

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Bathymetry effect on deep polar water

• Deep Arctic polar water isrestricted to arctic ocean basin.

• Not so for Deep Antarctic polarwater

And so…

• Note: relatively static and simplistic• Let’s make it circulate and more realistic…• How? Change surface T and S perhaps?

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Thermohaline Circulation

• Thermohaline circulation:– deep density driven circulation– Driven by density changes at SURFACE

caused by changes in T and S

Larger scale: Antarctic

• Antarctic bottom water = densest ocean water (-0.5ºC, 1.0279 g/cc)– Results from formation of sea ice in Wedell Sea– 20-50x106 m3 forms per second– Sinks and FLOWS slowly (~500 to 1000 years to equator)

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North Atlantic Deep Water

• Forms in N. Atlantic near Iceland & Greenland• Cold air from Canada removes heat form N Atlantic• Similar waters form in …

Similar to N. Atlantic Deep Water

• Kamchatka• Antarctic Circumpolar Current (Antarctic

Intermediate Water)

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Look at Mediterranean?

• Evaporation > Precipitation by 300,000 km3

• Dense, salty, warm water over Gibraltar Sill

Simpler view of thermohalinecirculation pattern:

• Note that caballing occurs at some convergence zones