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Lesson 1: Physical Properties of Seawater
Lesson Outline
I. Hydrographic Variables
II. Measuring Temperature & Salinity
III. Global Distribution of Temperature & Salinity
IV. Heat Capacity
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Properties of Seawater
Hydrography is the scientific study of the properties of seawater.
This lesson focuses on the hydrographic variables of pressure,
temperature, salinity, and density of seawater.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Density of Seawater
The density ρ of seawater is defined as the mass per unit volume.
The density of pure water at 4º C is 1 g/cm3 = 1 kg/L = 1000 kg/m3
The density of seawater varies depending on the salinity, temperature,
and pressure.
At atmospheric pressure, the density of seawater usually varies
between 1020 and 1030 kg/m3.
Since the variability is so small, oceanographers typically subtract
1000 from the density to get a value between 20 and 30 known as
sigma-t (that is, σt = ρ – 1000)
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Pressure
At sea level, the pressure exerted on a
surface by the full height of the
atmosphere is defined as one atmosphere
(atm), equal to 14.7 lb/in2.
The pressure exerted on a submerged
surface by a stationary column of water is
called hydrostatic pressure.
A column of water just 10-meters high
exerts a hydrostatic pressure equivalent to
1 atm.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Salinity
The salinity of a sample of seawater is the concentration (by weight)
of dissolved solids in the sample.
By convention, the weight of the dissolved solids is measured in
grams and the weight of the water is measured in kilograms. Since
there are 1,000 grams in a kilogram, salinity is expressed in parts per
thousand (‰).
In the open ocean, away from the influence of fresh water runoff
from land, the salinity of the ocean varies from 32‰ to 37‰.
In estuarine waters, like Tampa Bay, the salinity ranges from the
20‰ to about 30‰.
As the salinity of water increases, the density also increases; that is,
the density of water varies directly with salinity.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Temperature
In general, the density of seawater varies inversely as the temperature.
That is, when the
temperature of seawater
increases the density
decreases.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
A Stratified Ocean
Ocean waters can be divided into three layers, depending on their
densities.
Less dense waters form a top layer
called the surface mixed zone.
The temperature and salinity of this
layer can change often because it is in
direct contact with the air.
The next layer is the pycnocline, or transition zone.
This transition zone is a barrier between the surface zone and a bottom
layer, allowing little water movement between the two zones.
The bottom layer is the deep zone, where the water remains cold and
dense.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Vertical Profiles of Temperature, Salinity, and Density
All three profiles exhibit a layer
of sharp vertical change, called a
thermocline (for temperature),
halocline (for salinity), and
pycnocline (for density).
The density plot shows that ocean
waters are stably stratified with
less dense water near the surface
and denser water at depth.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Measuring Temperature, Salinity, and Density
A conductivity-temperature-depth
meter, or CTD, is the most common
instrument in use today for measuring
the salinity, temperature, and pressure
of seawater.
Instruments for measuring other ocean
variables, such as dissolved gasses,
chlorophyll concentration, and
suspended organic and inorganic
particles, are often attached to the
CTD.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
CTD Sensors
A CTD measures conductivity by passing an electrical current through
the water (electrical currents flow more readily through water with a
higher salt content).
By comparing the result to conductivities of water with known
salinities, the conductivity is converted to a salinity, expressed in PSU
(practical salinity units).
The temperature of the seawater is measured directly with an electronic
thermometer.
The pressure is recorded by a pressure gauge and converted to a depth.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Deploying the CTD
The package of sensors on the CTD Conducting
cable
is lowered from the research ship
on a conducting cable.
The cable carries signals from the
ship to the CTD and data from the
instrument back to computers on
board the ship.
CTD
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Calculating Density: The Equation of State
The density (ρ) of seawater is calculated from temperature (T), salinity
(S), and pressure (P) using the equation of state for seawater, a
complicated mathematical formula usually programmed into the CTD
software.
where
and
Equation of State Calculator
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Linearized Equation of State
A simplified, linear version of the equation of state that can be used to
approximate the density of seawater in most areas of the ocean to
within ±0.5 kg/m3 is:
ρ = ρ0 + [a(T – T0) + b(S – S0) + kP]
where ρ0 = 1027 kg/m3; T0 = 10° C; S0 = 35‰
a = –0.15 kg/m3 per °C (coefficient of thermal expansion)
b = 0.78 kg/m3 per ‰ (coefficient of saline contraction)
k = 0.0045 kg/m3 per decibar (coefficient of compressibility)
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Example: Calculating Density
Use the equation of state to estimate the density of a surface seawater
sample with a temperature of 12º C and salinity of 36‰.
Substituting the coefficients into the equation of state gives
ρ = 1027 + [–0.15 (T – 10) + 0.78(S – 35) + .0045P]
Substituting T = 12, S = 36, and P = 0 db (at the surface) into the
equation gives
ρ = 1027 + [–0.15 (12 – 10) + 0.78(36 – 35) + .0045(0)]
= 1027 + [–0.15 (2) +.78]
= 1027 + .48
= 1027.48 kg/m3
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Temperature-Salinity Diagrams
Another method for quickly estimating the density of seawater is to use
a temperature-salinity diagram (TS-diagram).
TS Diagram
(Contours in σt)
Example: Estimate the
30
density of seawater with a
22
25 23
temperature of 12º C and a
salinity of 36‰. Express
Temperature (ºC)
24
20
25 the result to the nearest
15 26
whole unit.
27
10 P The density at the point P
28
5 is approximately:
29
0
σt = 27.5 or ρ = 1027.5
33.5 34.0 34.5 35.0 35.5 36.0 36.5 37.0
kg/m3.
Salinity (‰)
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Global Distribution of Surface Salinity
Ocean currents,
evaporation and
precipitation, river
outflow, and geological
features such as
coastlines and deep sea
mountain ranges all
combine to distribute the
salinity in world’s
oceans.
World Ocean Atlas (Levitus)
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Global Distribution of Sea Surface Temperatures
The same factors also
combine to distribute the
ocean temperatures.
World Ocean Atlas (Levitus)
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Vertical Distribution of Temperature and Salinity
The vertical
distributions of
temperature, salinity,
and density are also
important to global
ocean circulation and
climate.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Local Hydrography Measurements
Hydrographic data is collected at
a sequence of stations along a
transect, then combined to
produce a cross-sectional picture
of the water properties.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Physical States of Water
Water exists in three physical states: solid, liquid, and gas.
When heated, water changes from its liquid state to its gaseous state
known as water vapor.
A calorie is the heat energy required to
Water Vapor (100º C)
raise the temperature of one gram of water
585 cal by one degree centigrade.
Water (100º C)
100 cal
It requires 100 calories to raise one gram of
pure water from 0º C to 100º C.
Water (0º C)
80 cal To change one gram of water at 100º C into
Ice (0º C) water vapor requires an additional input of
585 calories.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Heat Capacity
The ability of a substance to resist a temperature change is called its
heat capacity.
Water has a very high heat capacity; only ammonia and Freon have a
higher heat capacity than water.
Unit III: Physical Oceanography
Lesson 1: Physical Properties of Seawater
Heat Storage and Transfer
The First Law of Thermodynamics states that energy cannot be
destroyed.
When a gram of water is evaporates, the water vapor “stores” 585
calories of heat energy.
If the water vapor travels toward the pole and at some point precipitates
back to a liquid as rain, then the 585 calories of heat energy are
released.
The evaporation of water at one latitude
and its precipitation at another transports
the heat energy stored in the water vapor.
Unit III: Physical Oceanography
