summary from last class… importance of large-scale ocean circulation –climate, biogeochemistry,...
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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”
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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 Lecture 3 of Tomzcak’s web coursegyre.umeoce.maine.edu/physicalocean/Tomczak/IntroOc/lecture03.html
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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
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A Water Molecule
•Water is a non-polar molecule
•Weak hydrogen bonding
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Consequences of Water’s Hydrogen
Bonding•Water forms a lattice or
aggregation of many
molecules (polywater)
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Consequences of Water’s Hydrogen
Bonding•High specific heat (break the lattice!!)
•High latent heat for phase changes
•Great solvent
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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
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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 = f(S,T,p) = [kg m-3]
(S,T,p=0) 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
= +1 kg m-3 for T = -5 C
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Temperature
Equatorial Pacific - WOCE150W
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Temperature
60S - WOCE150W
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Pacific Temperature
eWOCE gallery – www.ewoce.org
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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
Units are “practical
salinity units” (psu)
Often bottles
are used
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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.)
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Typical T & S
Profiles
Features
Mixed layer
Thermocline
Halocline
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Pacific Salinities
eWOCE gallery – www.ewoce.org
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Atlantic Salinity
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Ocean Distribution of & S
Mean ocean ~ 4 C & S ~ 34.8 psu
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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
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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 (T=0 & S=35) will
heat ~0.3C if moved to 3000 m depth
Defines potential temperature or
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Potential Temperatu
re
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World OceanAtlas 1994
Global SST
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Annual Change in SST
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Annual Change in SST
January July
Net Air-Sea Heat FluxesRed = Heat In Blue = Heat Out
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Interannual SST Changes
March 1998relative toaverage March
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World OceanAtlas 1994
Global Salinity
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Global Salinity
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Global Salinity & Air-Sea Fluxes
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Global Salinity
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1 Sverdrup = 106 m3 s-1
Why is the Atlantic so salty?
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Seawater Density
Equation of state = f(S,T,p) = [kg m-
3]
(S,T,p=0) range from 1020 to 1040 kg m-3
Shorthand sigma-t: t = (S,T,0) - 1000
t(S,T) ranges from 20 to 40
–Similarly, sigma-theta: = (S,,0) -
1000
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Seawater Density
Density of seawater is controlled by
Temperature, salinity & pressure
= f(S,T,p)
Rule of Thumb => = +1 kg m-3
IF S = +1 psu OR T = -5C OR p = +100 db
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T-S Diagram (full range)
Max density
Freezing
Values of t
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T-S Diagram (typical range)
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Density Calculator
http://www.phys.ocean.dal.ca/~kelley/seawater/WaterProperties.html
http://gyre.umeoce.maine.edu/physicalocean/Tomczak/Utilities/index.html
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Review
Fundamental seawater properties
–Salinity, temperature & pressure
Density is the important variable
–in situ density (S,T,p)
–Sigma-t (S,T,0) – 1000
–Sigma- (S,,0) – 1000
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Review
Rules of thumb -> = +1 kg m-3
T = -5C, S = 1 psu or p = 100
db
Global surface T & S driven largely
by air-sea exchanges
Dense water sinks… now we're
talking dynamics