Physics of
Seawater
Water is …

a chemical compound (H2O) made
up of two atoms of hydrogen and
one atom of oxygen;

in liquid state between the temperatures of 0º C and 100º C;

perhaps the only substance that is
present in vast quantities in solid,
liquid and gaseous states.
The water molecule





is light
is stable as liquid over a
wide temperature-range
has high heat capacity and
latent heat
freezes over, not under, and
is an excellent solvent
Water stays liquid over a wide
range of temperatures
-
-
+ +
++
-
When water freezes
to ice, the angle of
hydrogen bonding
expands from 105° to
109°.
+
105°
+
As the space taken by
27 water molecules is
now used by 24
molecules, the density
of ice is less than the
density of water, i.e.,
-
-
+ +
++
-
water freezes over.
+
109°
+
Density (g/cm3)
Temperature (°C)
Salinity lowers water’s
 freezing point
and
 point of
Density (g/cm3)
1.03
maximum
density
1.02
and raises
the boiling
point.
1.01
1.00
0.99
Temperature (°C)
Seawater
therefore
freezes at –
2°C and boils
at 103°C.
Electromagnetic Spectrum of Sunlight
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?
22
 Where do we encounter such situations?
Temperature (°C)
23
20°
24
25
Y
26
10°
X
27
28
29
0°
31‰
33‰
35‰
Salinity
37‰
SOFAR (Sound Fixing
And Ranging) channel
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).
• This example shows how much heat is needed to
6. 25 cal of heat will change 1 g water vapor
at 100°C to 1 g water vapor at 150°C
cal 50°C1g (Specific Heat)
= ½g°C
−
−
−
Temperature measures the thermal
state of matter
−
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)
Temperature (°C)
change the temperature of 1 g ice at -50°C to 1 g
Water vapor at 150°C.
150
5. 540 cal of heat will change 1 g water
• The following changes
at 100°C to 1 g water vapor at 100°C
cal
occur in this process
=
540
g 1g (Latent Heat)
D
− ice from -50°C to 0°C (this
100
4. 100 cal of heat will change 1 g
water at 0°C to 1 g water at 100°C
cal 100°C1g (Specific Heat)
= 1g°C
50
B
0
E
This is the
temperature
range for
liquid water
C
2. 25 cal of heat will change it
cal -50 A
to 1 g of ice at 0°C = ½ g°C
0
 50°C1g (Specific Heat)
Let us use the following constants:
cal
Specific Heat = 1 g°C for water and
cal
½ g°C for ice/vapor
Latent Heat = 80 cal/g to melt ice
540 cal/g to boil water
F
1. Start with 1 g
of ice at -50°C
3. 80 cal of heat will change itcal
from 1 g ice at 0°C
to 1 g water at 0°C, i.e., 80 g 1g (Latent Heat)
200
400
Heat (calories)
600
800
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)
Therefore,
Total heat needed = (25+80+100+540+25) or 770 calories
Why do we have seasons?
 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.
•NASA’s Earth Seasons
Vernal equinox
Summer solistice
Autumnal equinox
Winter solistice
Northern Southern
hemihemisphere
sphere
March 21 March 21
June 22
Dec 22
Sept 22
Sept 22
Dec 22
June 22
 Do tropics have
 Northern hemisphere thus has its longest day (or
seasons?
summer solistice) around June 22, and the shortest
 Would seasons
day (or winter solistice) around Dec 22, whereas the
exist if the Earth’s
opposite occurs in the southern hemisphere.
spin axis was not
 Seasons typically characterize the temperate latitudes
inclined at all?
(23½°– 66½° N and S), therefore, whereas tropics
receive Sunlight all year round.
Source: http://vortex.plymouth.edu/sun/sun3d.html
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
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
0.1 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
0.2 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
0.5 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
1 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
2 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
3 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
4 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth:
5 Km
http://ingrid.ldgo.columbia.edu/SOURCES/.LEVITUS94/.ANNUAL/html+viewer?plotcoast=draw+land
Depth
Temperature
Therefore, thermocline
(i.e., the inflection point
in temperature-depth
graph) is ...
Tropical
all year
round, in
summer at
temperate
latitudes.
Polar latitudes all
year round, in winter at
temperate latitudes

permanent
in the tropics;

seasonal at
temperate latitudes,
i.e., present in
summer, missing in
winter; and

absent in the polar
waters.
42.0
38.0
37.5
37.0
36.5
36.0
35.5
34.5
35.0
34.0
33.5
33.0
32.5
30.0
24.0
18.0
Global variations in sea surface salinity
Salii
ty i

oceausmag.whoi.edu/v392/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)
Dry
50
Salinity
0
Wet
Evaporation - Precipitation (cm)
Surface salinity of the world ocean is high
where evaporation exceeds precipitation,
and low where the opposite holds.
E-P
- 50
40°N
20°N
0°
20°S
40°S
Equator
Halocline
30°N/S
Depth
Well defined and permanent haloclines therefore
exist at the equator and
Salinity (‰)
at the 30°N and 30°S
latitudes:
• At the equator
because high precipitation 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 (‰)
Sea Surface
Temperatures
http://www.scivis.nps.navy.mil/~braccio/images/T_big.gif
Sea Surface Salinity
http://www.scivis.nps.navy.mil/~braccio/images/S_big.gif
Ocean Temperatures
at 160m Depth
http://www.scivis.nps.navy.mil/~braccio/images/E_big.gif
Ocean Salinity
at 160m Depth
http://www.scivis.nps.navy.mil/~braccio/images/A_big.gif
T-S plot for mapping the pycnoline
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
15.0
14.2
12.1
10.0
9.0
8.0
13.2
12.7
6.4
4.9
4.0
3.5
3.0
2.6
1.5
37.3
36.0
35.3
35.0
33.5
33.0
37.0
36.7
35.2
34.8
34.5
34.5
34.4
34.3
34.1
Temperature (°C)
Tabulated 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 temperature, salinity and density depth-profiles separately to map this. Notice how easily
this insight could be drawn from the T-S plot itself!
Depth Temp Salinity
20
(m)
(°C)
(‰)
100
200
300
700
400
500
10
600
0
32
800
900
1000
1100
1200
1400
1300
1500
34
36
Salinity (‰)
38