Sea Water Properties

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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 Chapters 1 2 of Tomzcaks readings

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What is Seawater?

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

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A Water Molecule

• Water is a polar molecule!!
• Weak hydrogen bonding

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Consequences of Waters Hydrogen Bonding

• Water forms a lattice or aggregation of many
  molecules (polywater)

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Consequences of Waters Hydrogen Bonding

• High specific heat (break the lattice!!)
• High latent heat for phase changes
• Great solvent

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Consequences of Waters 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

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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

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More on Hydrogen Bonding
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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,p0) range from 1020 to 1030 kg m-3

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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

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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

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Salinity

• Salinity is measured electrically now
• Salinity is in practical units (psu)
• Often bottles are used

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Salinity

• Salinity varies from 32 to 37 psu
• Good water mass tracer
• Lower/higher values are unusual (riverine, huge
  evaporation, etc.)
• Rule of thumb
• Dr 1 kg m-3 for DS 1 psu

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Typical T SProfiles

• Features
• Mixed layer
• Thermocline
• Halocline

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CalCoFI
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Temperature
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Salinity
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CalCoFI Cruise 9804
Line 80
Line 90
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Line 90
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Off Pt Arena Off San Clemente Off Punta
Baja
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Pressure

• Pressure is due to the weight of sea water lying
  above a depth (hydrostatic)
• Pressure varies from 0 to gt5000 db
• p 0 is atmospheric pressure
• Rules of thumb
• 1 db pressure 1 m depth
• Dr 1 kg m-3 for Dp 100 db

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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 (T0 S35) will heat 0.3C if
  moved to 3000 m depth
• Defines potential temperature or q

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Potential Temperature
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Potential Temperature
0.05 C at 500 m
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Ocean Distribution of q S

• Mean ocean q 4 C S 34.8 psu

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Seawater Density

• Equation of state r f(S,T,p) kg m-3
• r(S,T,p0) 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

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T-S Diagram (full range)
Values of st
Max density
Freezing
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T-S Diagram (typical range)
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Density Profile
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Temperature Inversion
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Density Profile
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CalCoFI Cruise 9804
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Coastal Upwelling Changes Density
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Density for Line 90
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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-q r(S,q,0) 1000

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Review

• Rules of thumb -gt 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