Summary from last class… Importance of large-scale ocean circulation –climate, biogeochemistry,...
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Transcript of Summary from last class… Importance of large-scale ocean circulation –climate, biogeochemistry,...
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”
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
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
A Water Molecule
•Water is a non-polar molecule
•Weak hydrogen bonding
Consequences of Water’s Hydrogen
Bonding•Water forms a lattice or
aggregation of many
molecules (polywater)
Consequences of Water’s Hydrogen
Bonding•High specific heat (break the lattice!!)
•High latent heat for phase changes
•Great solvent
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
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
More on Hydrogen Bonding
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
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
Temperature
Equatorial Pacific - WOCE150W
Temperature
60S - WOCE150W
Pacific Temperature
eWOCE gallery – www.ewoce.org
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
Salinity
Salinity is measured
electrically now
Units are “practical
salinity units” (psu)
Often bottles
are used
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.)
Typical T & S
Profiles
Features
Mixed layer
Thermocline
Halocline
Pacific Salinities
eWOCE gallery – www.ewoce.org
Atlantic Salinity
Ocean Distribution of & S
Mean ocean ~ 4 C & S ~ 34.8 psu
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
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
Potential Temperatu
re
World OceanAtlas 1994
Global SST
Annual Change in SST
Annual Change in SST
January July
Net Air-Sea Heat FluxesRed = Heat In Blue = Heat Out
Interannual SST Changes
March 1998relative toaverage March
World OceanAtlas 1994
Global Salinity
Global Salinity
Global Salinity & Air-Sea Fluxes
Global Salinity
1 Sverdrup = 106 m3 s-1
Why is the Atlantic so salty?
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
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
T-S Diagram (full range)
Max density
Freezing
Values of t
T-S Diagram (typical range)
Density Calculator
http://www.phys.ocean.dal.ca/~kelley/seawater/WaterProperties.html
http://gyre.umeoce.maine.edu/physicalocean/Tomczak/Utilities/index.html
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
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