Upload
truongthien
View
221
Download
0
Embed Size (px)
Citation preview
1
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:…
2
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…
3
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)
4
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)
5
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?
6
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)
7
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)