Download Properties of sea-water and their distribution in the oceans

Properties of sea-water and their
distribution in the oceans
• What are the physical properties of seawater? (temperature,
salinity, density, etc.)
• How we observed these properties?
• How one property relate to another?
• What is the typical distribution of these properties in the world
oceans?
• How much they vary in space? (horizontally & vertically)
• How much they vary in time? (daily, seasonally, annually)
• How are they affected by different forcing? (wind, sun, etc.)
Special characteristics of water molecules:
•One oxygen and two hydrogen atoms (H2O)- strong interaction: very stable
•Polar molecule- dipole (positive & negative sides)
- withstand electric field (allow conductivity measurements)
•High solvent ability- able to dissolve more substance than any other fluid
•High surface tension- capillary waves, friction between wind and water
•Anomaly- max density of pure water at 4°C, above freezing (0°C)… but
different for salt water…
H: atomic No.=1
O: atomic No.=8
High Surface Tension:
Hydrogen bonding creates “skin”
allows bugs to “ walk on water”
allows surface “Capillary Waves”
Solvent ability: seawaters are salty
NaCl
Effects of Salinity on the Properties of Seawater:
•Lowers freezing point
•Lowers temperature of
maximum density
•Lowers evaporation rate
Liquid water
NOTE:
•water with S<24.7
max density > freezing point
•water with S>24.7
max density < freezing point

lakes turn over as they
freeze, while oceans remain
stratified as they freeze
 Without this property, no
marine biology as we know
it (or life on earth!)
Seawater freezes
before reaching
max density
-1.33
Ice
24.7
(from Pinet, 1998)
Temperature
• One of the first ocean parameters observed and studied
(easy to measure).
• Affects many other properties in the ocean (e.g., density,
speed of sound, etc.) as well as weather and climate.
• Many oceanic features and circulation patterns can be
inferred from temperature measurements.
• Scientific units: Celsius or Kelvin (absolute temperature)
°C = °K – 273.15
(not Fahrenheit!, °F = °C*5/9 + 32)
1000 km
depth
1 km
1 km
Latitude
Antarctica
Alaska
North-south temperature across the Pacific Ocean:
 Isotherms: lines of constant temperature
 How oceanographers obtain such sections?
 Horizontal-vertical oceanic scales
Density distribution is
affected by heating and
cooling at the surface
and water mass
transports:
Cold-dense waters are
formed in high latitudes
and sink to deeper
layers in lower latitudes
Isopycnals:
lines of constant
density
Typical temperature profiles
Mixed Layer
Seasonal Thermocline
Permanent Thermocline
Thermocline: region of rapidly
changing temperature with depth
Surface Mixed-Layer: region of
nearly constant temperature
(mixed by winds)
The seasonal thermocline
Balance between:
solar heating, surface cooling and wind mixing
more solar heating
less wind mixing
less solar heating
more wind mixing
July
large seasonal
changes in
mid-latitudes
small seasonal
changes in
tropics and in
high-latitudes
January
Annual mean surface temperature distribution:
• Almost Zonal- small change with longitude, large change with latitude, warmer near
equator, colder in high latitudes
•Non-zonal pattern is affected by ocean circulation: Lat>40N warmer in east side of
basins; 40S<Lat<40N warmer in west side of basins
In situ and potential temperature
• in situ temperature: the observed temperature,
T(x,y,z,t)
• potential temperature: the temperature of a parcel of
water after it has been raised adiabatically from some
depth to the surface, θ=func(S,T,P).
– adiabatic: without heat exchange with environment
– water sinks  pressure increases  water compresses 
energy increases  water warms
(example: T(5000m)=1°C; θ=0.57°C)
– removing compressibility (pressure) effects helps to trace
water masses and mixing processes
Note: compressibility of the atmosphere is much larger than
water (up to 10°C/1km, thus its cold at mountain tops!)
Note: the relation between water properties (temperature, salinity, pressure &
density) are based on empirical formulas – best fit of lots of seawater samples.
Note: in deep trenches potential temperature is isothermal and water is
neutrally stable, even though in situ temperature increases with depth.
Salinity
• Units: ‰, ppt (parts per thousand), pss (practical salinity
scale) or psu (practical salinity units- controversial units…)
Almost all the elements
found on earth are
present in seawater:
• the amounts may
change from place to
place (river flows,
biological activities, etc.)
• However conservative
constituents are those
that maintain the same
proportions in most of
the oceans
Salinity
• Simple definition: “total amount of dissolved material in grams
in one kilogram of sea water”
– Almost impossible to measure (what about gasses?,
material lost when drying?, etc.)
• Complex definition:
Salinity
- how the definition of salinity evolved over the years:
• Definition based on Chlorinity:
S = 1.80655 x Cl
Why? 1. Salinity proportional to amount of Chlorine in sea water
2. Chlorine is accurately measured by chemical analysis
Accuracy: ±0.02‰
• Salinity based on Electrical Conductivity: C=func(T,S)
Why? Most practical and quick way to measure
However, need to measure temperature very accurately
(±0.01°C error in T  ±0.01‰ error in S)
Accuracy: ±0.001‰ to ±0.003‰
Salinity
“salinity based on conductivity is not really a new definition of
salinity, but a relation between Chlorinity and Conductivity
relative to standard seawater” (Millero, 1996)
• Practical Salinity Scale (PSS-1978): Salinity defined using only
conductivity (not Chlorinity)
Define standard
Potassium Chloride
(KCl) sample
Why is it important to
measure salinity very
accurately?
The variations of salinity are
much smaller than variations
in temperature (34.6-34.8ppt)
(in the deep ocean- coastal
oceans are different story…)
Histogram of water
masses colder than 4°C
Typical distribution of salinity in the oceans:
- mostly zonal (but not as much as temperature)
- minimum north of equator
- maximum at 25°N & 25°S
- North Atlantic saltiest ocean, North Pacific least saline
Mean salinity at 1000 m shows why N. Atlantic saltier than N. Pacific:
Mediterranean outflow
Latitudinal distribution of salinity
Salinity in mid & low
latitudes is controlled by
evaporation & precipitation
In high latitudes fresh
water runoff may be
important
Vertical profiles of salinity in the open ocean
halocline
thermocline
Light in the sea
• Oceans are much more effective absorber of
light than the atmosphere (50% can
penetrate the entire atmosphere)
• Most of the sun’s short wave radiation that
reaches the surface of the ocean is absorbed
by the top few meters  heating the ocean
 sun controls variation of ocean
temperatures
• How much is absorbed depends on the
clarity of the water, wave-length, and even
biology
• Solar energy penetration decays
exponentially:
E(z)=E0e-kz
k=vertical attenuation coefficient
Turbid coastal water
Clearest ocean water
The penetration of
radiation into the
ocean depends on:
• clarity of water
(including biological
activity)
• wavelength
• depth
• scattering
• surface reflection
Clear ocean water
Turbid coastal water
The absorption and
reflection of the sun’s
light spectrum in the
ocean cause clear ocean
to look blue and turbid
coastal water to look
green-yellow or brown
Ocean color is important for remote sensing of
chlorophyll, which indicates biological activity, but
also traces oceanic features
Temperature
Chlorophyll
California coast
Red Tide:
• Excessive bloom of specific
phytoplankton (algae) that have a
reddish color.
• The toxin released by the algae
causes severe disturbance for
biological activity and fisheries
Next Class:
• Density and the Equation of State (EOS)
• Hydrostatic Stability
• Sound Waves in the Ocean