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Lecture 15: PROPERTIES OF SEA WATER

Salinity
Effect of salinity on physical properties
ENERGY TRANSMISSION/ABSORPTION IN WATER AND SEA WATER
Heat
Light
Sound

Powerpoint Lecture Slides


SEA WATER

Salinity - dissolved salt content
Average S = 35 g/kg (p.p.t, o/oo)
Range of S (99% of all sea water) = 30 -37 o/oo

Dissolved salts change inter-molecular interactions and thus physical properties of sea water.

Boiling point = 103°C

Freezing point lowered
Sea water begins to freeze at -2°C; salt is excluded from ice.
Remaining water is saltier, freezes at lower T.

Temperature of maximum density "disappears"
Density increases progressively to freezing point
Higher density promotes sinking of cold sea water

Density
... increases as S increases.
... increases as T decreases.

Importance to deep circulation in oceans:
Deep-water "masses" form at surface
... cooling (T decreases)
... freezing & evaporation (S increases)
Sink to a level (depth) governed by density, and spread out

LIGHT ENERGY: TRANSMISSION & ABSORPTION

Wave characteristics of energy - wavelength (wl), frequency
Electromagnetic spectrum
Solar energy

Radiation -- direct transmission
wl inversely related to surface T
Sun: 6,000°C -- visible
Earth: 18°C -- infrared

Absorbtion of solar radiation by the oceans
Radiation reaching the sea surface:
visible
infrared
(ultraviolet)
Absorbtion -- efficient, but wl-dependent

Depth % absorbed wavelengths absorbed
1 m 60 infrared (heats surface waters)
10 m 80 longer visible
150 m 99 [only short, green-blue-violet light remains]

Consequences:
Photosynthesis <150 meters
Blue color of sea water: wavelengths available for
reflection & scattering

Conduction -- heat transfer from warm object to cool object
Surface ocean - atmosphere conduction important
Conduction from surface ocean to deep -- less important
Water is a poor conductor

Convection -- heat transfer due to density-driven currents

cooling (surface)
|
| |
|
warming (base)

Convection in atmosphere -- important
Convection in oceans?
-- driven by cooling of surface water

Sound in the oceans

Propagated over very long distances -- not "attenuated"
Marine animals -- communication & detection
Scientists and the Navy -- depth & detection
Velocity varies with depth (depends on T, S, and pressure)
Max. V near surface:
refraction (bending) out of that zone
"shadow zone"
Min. V. at ~ 1,000 m -- SOFAR channel
refraction into that channel
global-scale transmission


Detailed notes begin here

Sea Water

The total dissolved salt content of a parcel of sea water is its salinity. The average salinity of sea water is 35 grams of dissolved salt per kilogram of sea water [35 g/kg, or 35 parts per thousand (p.p.t. or o/oo). 99% of all sea water has a salinity in the range 30 to 37 g/kg.

The presence of dissolved salts (as charged ions) effects the physical properties of sea water by altering interactions between H2O molecules:
* Boiling point is elevated. Average sea water would boil at 103 deg C. (not very important because no place in the surface ocean is that hot).
* Freezing point is depressed
- Sea water begins to freeze at -2 deg C.
- Salt is excluded from ice.
- Unfrozen water is saltier and freezes at an even lower temperature. (There will always be some saline brine left at very low T)
* Temperature of maximum density is depressed
- Seawater does not have temperature of maximum density above the freezing point
- As sea water cools, it becomes progressively denser until it freezes.
- Progressive increase in density promotes sinking and vertical circulation.
* Density:
- warm, surface waters1.021 g/cm^3
- cold, deep waters1.028 g/cm^3

As we shall discuss later, density of sea water increases as temperature decreases and as salinity increases. This is important to deep circulation in the oceans. Deep-water "masses" form at the surface of oceans by cooling and increase in salinity (freezing, evaporation). They sink to a level (depth) governed by density.

TRANSMISSION AND ABSORPTION OF ENERGY IN WATER AND SEA WATER

All forms of energy have wave-like characteristics, from very short (high-frequency) and very energetic gamma-rays and X-rays to very long (low-frequency) radio waves. The electromagnetic spectrum classifies and describes different forms of energy in terms of frequency and wavelength.

Essentially all of the energy received by the Earth is energy from the Sun in the form of light and heat. Many important physical features of the oceans, such as temperature variations and circulation, are realted to how sea water transmits and absorbs energy. In addition, the distribution of photosynthesizing organisms in the oceans is controlled by the absorbtion of light in sea water.

Radiation . . is the direct transmission of energy from its source. The dominant wavelength of energy radiated from any object in the Universe is inversely related to the surface temperature of the object. In other words, the hotter the object, the shorter (and more energetic) the waves emitted. For example, the surface of the Sun is 6,000 deg. C and emits mostly visible light. The average temperature of the ocean and continent surface is l8 deg C, so the Earth radiates energy in the infrared portion of the spectrum.

Absorbtion of solar radiation by the oceans. Most of the solar radiation reaching the sea surface is in the visible part of the spectrum. A smaller fraction is infrared radiation. (Most of the ultraviolet radiation from the Sun is absorbed in the atmosphere.) Water molecules absorb solar radiation efficiently. However, the extent of absorbtion depends on wavelength:

 Depth  % absorbed  wavelengths absorbed
 1 m  60  infrared (heats surface waters)
 10 m  80  longer visible
 150 m  99  only short, green-blue-violet light remains

All of the infrared radiation is absorbed in the top meter of the oceans. This is the process that "heats" the ocean. In other words, the absorbtion of solar heat energy is only effective in the topmost layers of the oceans. With depth, the longer wavelengths of visible light (red, orange, yellow) are absorbed. At depths greater than about 100 meters, only short wavelengths of visible light are transmitted (green, blue, violet).

The selective absorbtion of light has important consequences for photosynthesis in the oceans -- it limits the depth that photosynthesis can occur to about 150 meters. Another interesting aspect is that plants (terrestrial and marine) prefer using blue-green light in photosynthesis. Could this be an evolutionary adaptation?

Wavelength-selective absorbtion of light also explains why the sea is blue. Those are the most common wavelengths available for reflection and scattering from water molecules, salt ions, and particles in sea water.

Conduction . . is another molecular process in which heat energy is tranfered from a warm object to a cold object as "sensible heat" (i.e., we can sense an object becoming warmer or colder by conduction). Surface ocean water and the atmosphere in contact with the ocean can exchange heat by conduction. For example, heat flows conductively from the warmer ocean surface to the cooler atmosphere during winter months. Within the oceans, heat from warm surface waters is conducted downward to cooler waters at depth. But the process is not very efficient because water is a poor conductor of heat.

Convection . . is a process of heat transfer due to the flow of currents in a fluid. Convection currents are driven by density differences.

Warm currents rise because they are less dense than the surrounding fluid. They lose heat as they rise (by conduction and radiation) and sink to begin the cycle again. Convection is an efficient mechanism for the transfer and redistribution of heat in the atmosphere because the atmosphere receives most of its heat at the Earth's surface. But the oceans are heated at their top. So, only cooling of surface waters (mostly at high latitudes) can initiate convection. (Heat flow from ocean crust is not important, except maybe over mid-ocean ridge systems.)

Sound in the oceans

Sound in the oceans is propogated over very long distances. It is not attentuated (absorbed or scattered) very much during transmission. Marine animals utilize sound for communication and detection (sort of a natural SONAR). Marine scientist use reflected sound to measure ocean depths (Echo sounding, discussed previously).

The velocity of sound in sea water is not constant but varies with temperature, salinity, and pressure. The variation of sound velocity with depth has some interesting technical consequences.

Sound velocity is a maximum in shallow waters. Therefore sound waves are refracted (bent) upward and downward in that zone. This refraction distorts the apparent position of a subsurface object (like a submarine) that reflect sound waves and creates a "shadow zone" where objects may not be detected. Correcting for refraction requires a detailed knowledge of the temperature and salinity of the water column. That is why the U.S. Navy pioneered precise methods for determing T and S.

At a depth of about 1,000 meters, sound velocity is at a minimum. This zone is called the SOFAR channel (sound fixing and ranging). Sound waves are refracted into that zone and can be propogated very long distances. Experiments conducted in 1991 show that it possible to transmit and receive sound in the SOFAR channel on a global scale. This has important potential applications for global underwater communications.


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