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Summary from last class…

Summary from last class…. Importance of large-scale ocean circulation climate, biogeochemistry, marine resources Characteristic “Types” of Ocean Circulation Patterns of circulation can be generalized for different regions Suggests that dynamical processes are the same

aaron-keith
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Summary from last class…

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  1. Summary from last class… • Importance of large-scale ocean circulation • climate, biogeochemistry, marine resources • Characteristic “Types” of Ocean Circulation • Patterns of circulation can be generalized for different regions • Suggests that dynamical processes are the same • Wind-driven, Buoyancy-driven & Coastal • Scales - “spit on a basketball”

  2. Sea Water Properties • Water mass characteristics • Salinity, temperature, nutrients, oxygen • Key property is seawater density • Changes in vertical - inhibit mixing • Changes in horizontal - drive currents • See Lecture 3 of Tomzcak’s web course gyre.umeoce.maine.edu/physicalocean/Tomczak/IntroOc/lecture03.html

  3. What is Seawater? • Seawater is 96.5% pure water • About 3.5% is other materials dissolved salts, gases & organic substances as well as particles • Physical properties are mainly determined by pure water

  4. A Water Molecule • Water is a non-polar molecule • Weak hydrogen bonding

  5. Consequences of Water’s Hydrogen Bonding • Water forms a lattice or aggregation of many molecules (polywater)

  6. Consequences of Water’s Hydrogen Bonding • High specific heat (break the lattice!!) • High latent heat for phase changes • Great solvent

  7. Consequences of Water’s Hydrogen Bonding • Ice crystals take up more space than liquid water • Ice Floats!! (rare for liquids) • Maximum density is water at 4C • Critical for freshwater systems

  8. More about ice... • Seasonal turnover in lakes • As lakes cool they reach temperature of maximum density (4C) & overturn • Later ice forms at the surface, sheltering the interior from winter conditions • This allows fish over winter under the ice

  9. More on Hydrogen Bonding

  10. Back to Oceans... Density of seawater is controlled by • temperature • salinity (dissolved salt content) • pressure (related to depth) Equation of state r = f(S,T,p) = [kg m-3] r(S,T,p=0) range from 1020 to 1030 kg m-3

  11. Temperature • Temperature generally decreases with depth in the ocean • Except where ice is formed, temperature changes primarily regulate density • Rule of thumb Dr = +1 kg m-3 for DT = -5 C

  12. Temperature Equatorial Pacific - WOCE150W

  13. Temperature 60S - WOCE150W

  14. Pacific Temperature eWOCE gallery – www.ewoce.org

  15. Salinity Ocean waters are “salty” Salinity ~ [mass “salts”]/[mass seawater] The “salts” (Cl-, SO4-2, Na+, K+, etc.) are in approximate constant proportion • Law of salinity (residence time is huge) • Measure one ion [Cl-] - estimate salinity

  16. Salinity Salinity is measured electrically now Units are “practical salinity units” (psu) Often bottles are used

  17. Salinity Salinity varies from 32 to 37 psu Atlantic is saltier than the Pacific… • Why?? Good water mass tracer Lower/higher values are unusual (riverine, huge evaporation, etc.)

  18. Typical T & SProfiles Features Mixed layer Thermocline Halocline

  19. Pacific Salinities eWOCE gallery – www.ewoce.org

  20. Atlantic Salinity

  21. Ocean Distribution of q & S Mean ocean q ~ 4 C & S ~ 34.8 psu

  22. Pressure Pressure is due to the weight of sea water lying above a depth (hydrostatic) Pressure varies from 0 to >5000 db p = 0 is atmospheric pressure Note: 1 db pressure ~ 1 m depth

  23. Potential Temperature Hydrostatic pressure will heat a water parcel as descends within the ocean Adiabatic lapse rate is ~0.0001 C/m A surface parcel (T=0 & S=35) will heat ~0.3C if moved to 3000 m depth Defines potential temperature or q

  24. Potential Temperature

  25. Global SST World Ocean Atlas 1994

  26. Annual Change in SST

  27. Annual Change in SST January July Net Air-Sea Heat Fluxes Red = Heat In Blue = Heat Out

  28. Interannual SST Changes March 1998 relative to average March

  29. Global Salinity World Ocean Atlas 1994

  30. Global Salinity

  31. Global Salinity & Air-Sea Fluxes

  32. Global Salinity

  33. Why is the Atlantic so salty? 1 Sverdrup = 106 m3 s-1

  34. Seawater Density Equation of state r = f(S,T,p) = [kg m-3] r(S,T,p=0) range from 1020 to 1040 kg m-3 Shorthand sigma-t: st = r(S,T,0) - 1000 st(S,T) ranges from 20 to 40 • Similarly, sigma-theta: sq = r(S,q,0) - 1000

  35. Seawater Density Density of seawater is controlled by Temperature, salinity & pressure • r = f(S,T,p) Rule of Thumb => Dr = +1 kg m-3 IF DS = +1 psu OR DT = -5C OR Dp = +100 db

  36. T-S Diagram (full range) Values of st Max density Freezing

  37. T-S Diagram (typical range)

  38. Density Calculator http://www.phys.ocean.dal.ca/~kelley/seawater/WaterProperties.html http://gyre.umeoce.maine.edu/physicalocean/Tomczak/Utilities/index.html

  39. Review Fundamental seawater properties • Salinity, temperature & pressure Density is the important variable • in situ density r(S,T,p) • Sigma-t r(S,T,0) – 1000 • Sigma-qr(S,q,0) – 1000

  40. Review Rules of thumb -> Dr = +1 kg m-3 DT = -5C, DS = 1 psu or Dp = 100 db Global surface T & S driven largely by air-sea exchanges Dense water sinks… now we're talking dynamics

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