physics of seawater

Physics of Physics of SeawaterSeawater

Water is …Water is … a chemical compound (H2O) made

up of two atoms of hydrogen and one atom of oxygen;

in liquid state between the tempe-ratures of 0º C and 100º C;

perhaps the only substance that is present in vast quantities in solid, liquid and gaseous states.

The water moleculeThe water molecule is lightis light is stable as liquid over a wide is stable as liquid over a wide

temperature-rangetemperature-range has high heat capacity and latent has high heat capacity and latent

heatheat freezes over, not under, andfreezes over, not under, and is an excellent solventis an excellent solvent

Water stays liquid over a wide range of temperatures

+ -

+ + + -

- -

- -

-

-

+ -

+ -

105°

+ -

+ + + -

- -

- -

-

-

+ -

+ -

109°

When water freezes When water freezes to ice, the angle of to ice, the angle of hydrogen bonding hydrogen bonding

expands from 105° to expands from 105° to 109°. 109°.

As the space taken by As the space taken by 27 water molecules is 27 water molecules is now used by 24 now used by 24 molecules, the density molecules, the density of ice is less than the of ice is less than the density of water, i.e., density of water, i.e., water freezes overwater freezes over..

Temperature (°C)

Den

sity

(g/c

m3 )

1.03

1.02

1.01

1.00

0.99

Den

sity

(g/c

m3 )

Temperature (°C)

Salinity lowers water’sSalinity lowers water’s freezing pointfreezing pointandand

point of point of maximum maximum densitydensity

and raisesand raisesthe boilingthe boilingpoint.point.

Seawater Seawater therefore therefore freezes at freezes at ––2°C2°C and boils and boils at 103at 103°C.°C.

Electromagnetic Spectrum of Sunlight

22

24

23

25

26

27 28 29

37‰33‰31‰0°

20°

10°

Tem

pera

ture

(°C

)

35‰Salinity

X

Y

The numbers for the density isoclinals here are the density factor [(= 1000 x (density – 1)] values, with density measured in gm/cm3.

Suppose we mix two water samples, X and Y, having different temperatures and salinities but the same density. What will be the temperature, salinity and density

of the resulting mixture? Where do we encounter such situations?

SOFAR (Sound Fixing And Ranging) channel

Heat (calories)0 200 400 600 800

This is the temperature

range for liquid water

0

150

50

100

-50

Heat is the energy needed to change the temperature of a body or material (e.g., 1 calorie is the heat needed to change the temperature of 1 gram of water by 1°C)

Tem

pera

ture

(°C

)Te

mpe

ratu

re m

easu

res

the

ther

mal

st

ate

of m

atte

r

Heat versus Temperature• Heat, the energy needed to change the temperature of a body, can be specific (i.e.,

temperature change at constant phase or state) or latent (i.e., state or phase change at constant temperature).

A

1. Start with 1 g of ice at -50°C

B

2. 25 cal of heat will change it to 1 g of ice at 0°C = ½ 50°C1g (Specific Heat)

calg°C

C

3. 80 cal of heat will change it from 1 g ice at 0°C to 1 g water at 0°C, i.e., 80 1g (Latent Heat)cal

g

D

4. 100 cal of heat will change 1 g water at 0°C to 1 g water at 100°C

= 1 100°C1g (Specific Heat)

calg°C

E

5. 540 cal of heat will change 1 g water at 100°C to 1 g water vapor at 100°C = 540 1g (Latent Heat)cal

g

F

6. 25 cal of heat will change 1 g water vapor at 100°C to 1 g water vapor at 150°C

= ½ 50°C1g (Specific Heat)calg°C

• This example shows how much heat is needed to change the temperature of 1 g ice at -50°C to 1 gWater vapor at 150°C.

• The following changes occur in this process− ice from -50°C to 0°C (this

involves specific heat)− ice to water at 0°C (this involves

latent heat)− water from 0°C to 100°C (this

involves specific heat)− water to water vapor at 100°C (this

involves latent heat)− water vapor from 100°C to 150°C

(this involves specific heat)

Let us use the following constants:Specific Heat = 1 for water and

½ for ice/vaporLatent Heat = 80 cal/g to melt ice

540 cal/g to boil water

calg°C

calg°C Therefore,

Total heat needed = (25+80+100+540+25) or 770 calories

The 23½° tilt of Earth’s spin axis means that the two poles do not get the same amount of solar heat at the same time.

North pole is tilted toward the sun from about March 22 to about Sept 22, when south pole tilts away from the Sun.

Northern hemisphere thus has its longest day (or summer solistice) around June 22, and the shortest day (or winter solistice) around Dec 22, whereas the opposite occurs in the southern hemisphere.

Seasons typically characterize the temperate latitudes (23½°– 66½° N and S), therefore, whereas tropics receive Sunlight all year round. Source:

http://vortex.plymouth.edu/sun/sun3d.html

Do tropics have seasons?

Would seasons exist if the Earth’s spin axis was not inclined at all?

Northern hemi-

sphereMarch 21

June 22Sept 22Dec 22

Vernal equinoxSummer solistice

Autumnal equinoxWinter solistice

Southern hemi-

sphereMarch 21

Dec 22Sept 22June 22

•NASA’s Earth Seasons

http://vortex.plymouth.edu/sun/sun3d.htmlm

http://kids.msfc.nasa.gov/Earth/Seasons/Seasons.htm

http://geography.uoregon.edu/envchange/clim_animations/gifs/tmp2m_web.gif

Seasonal temperature variations can be explained in terms of the latitudinal and seasonal variations in the surface energy balance.

http://geography.uoregon.edu/envchange/clim_animations/gifs/tmp2m_web.gif

Depth:0 Km

http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land

Depth:0.05 Km


Depth:0.1 Km


Depth:0.2 Km


Depth:0.5 Km


Depth:1 Km


Depth:2 Km


Depth:3 Km


Depth:4 Km


Depth:5 Km


permanent in the tropics;

seasonal at temperate latitudes, i.e., present in summer, missing in winter; and

absent in the polar waters.

Therefore, thermocline (i.e., the inflection point

in temperature-depth graph) is ...

Dep

thTemperature

Tropical all year round, in summer at temperate latitudes.

Polar latitudes all year round, in winter at temperate latitudes

32.5

33.0

30.0

24.0

18.0

33.5

34.0

34.5

35.0

35.5

36.0

36.5

37.0

37.5

38.0

42.0

Global variations in sea surface salinity

‰

:// . . / 39 2/ .http oceanusmag whoi edu v n schmitt html

http://oceanusmag.whoi.edu/v39n2/schmitt.html

Precipitation-Evaporation (P-E) represents the difference between precipitation and evaporation.

Data: NCEP/NCAR Reanalysis Project. 1959-97 Climatologies Animation: Department of Geography, University of Oregon, March 2000 (http://geography.uoregon.edu/envchange/clim_animations/gifs/pminuse_web.gif)

http://geography.uoregon.edu/envchange/clim_animations/gifs/pminuse_web.gif











0° 20°S 40°S20°N40°N

- 50

50

0

Evap

orat

ion

- Pre

cipi

tatio

n (c

m)

Dry

Wet E - P

Salinity

Surface salinity of the world ocean is high where evaporation exceeds precipitation,

and low where the opposite holds.

Salinity (‰)D

epth

Equa

tor 30°N

/S

Halo-cline

Well defined and permanent haloclines therefore exist at the equator and at the 30°N and 30°S latitudes:

• At the equator because high preci-pitation there makes the surface waters fresh/less salty.

• At the 30°N and 30°S latitudes because excess evaporation there makes the surface waters very salty.

January 1986 sea surface (0-50 m)

salinity (‰)

http://www.scivis.nps.navy.mil/~braccio/images/T_big.gif

Sea Surface Temperatures

http://www.scivis.nps.navy.mil/~braccio/images/S_big.gif

Sea Surface Salinity

http://www.scivis.nps.navy.mil/~braccio/images/E_big.gif

Ocean Temperatures at 160m Depth

http://www.scivis.nps.navy.mil/~braccio/images/A_big.gif

Ocean Salinity at 160m Depth

1.027

g/cm3

1.028 g/cm3

1.029

g/cm3

1.025g/cm3

1.026

g/cm3

36 383432Salinity (‰)

0

20

10

Tem

pera

ture

(°C

)

14001500

100200

300400

500

600

700

800

1000900

1100 12001300

Depth(m)

100200300400500600700800900

100011001200130014001500

Temp(°C)

15.014.212.110.0

9.08.0

13.212.7

6.44.94.03.53.02.61.5

Salinity(‰)

37.336.035.335.033.533.037.036.735.234.834.534.534.434.334.1

T-S plot for mapping the pycnolineTabulated below are the temperature and salinity data obtained at different depths at about 10°N in the central Atlantic. Note how the data clearly show the presence of very salty and warmer waters at 700-800 m depths. Pycnocline is clearly present here (due to the influx of the Mediterranean waters). Indeed, there was no need to plot the tem-perature, salinity and density depth-profiles separately to map this. Notice how easily

this insight could be drawn from the T-S plot itself!

physics of seawater

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