Geology Reference
In-Depth Information
Surface water temperatures are extremely variable, obviously influ-
enced by location and season. The minimum temperature found in polar
latitudes approaches the freezing point of nearly
2 1C. Equatorial
oceanic waters can reach 30 1C. Temperature variations with depth are
far from consistent. In a region where mixing prevails, as observed
especially in the surface waters, a layer forms with a relatively uniform
temperature. The zone immediately beneath normally exhibits a sharp
change in temperature, known as the thermocline. The thermocline in
the ocean extends down to about 1000 m within equatorial and tem-
perate latitudes. It acts as an important boundary in the ocean, sepa-
rating the surface and deep layers and limiting mixing between these two
reservoirs.
Below the thermocline, the temperature changes only little with depth.
The temperature is a non-conservative property of seawater because
adiabatic compression causes a slight increase in the in situ temperature
measured at depth. For instance in the Mindanao Trench in the Pacific
Ocean, the temperature at 8500 and 10,000 m is 2.23 and 2.48 1C,
respectively. The term potential temperature is defined to be the tem-
perature that the water parcel would have if raised adiabatically to the
ocean surface. For the examples above, the potential temperatures are
1.22 and 1.16 1C, respectively. Potential temperature of seawater is a
conservative index.
Salinity in the surface waters in the open ocean range between 33 and
37 (Figure 3), the main control being the balance between evaporation
and precipitation. The highest salinities occur in regional seas, where the
evaporation rate is extremely high, namely the Mediterranean Sea (38-
39) and the Red Sea (40-41). Within the World Ocean, the salinity is
greatest in latitudes of about 201 where the evaporation exceeds precip-
itation. Lower salinities occur poleward as evaporation diminishes and
near the equator where precipitation is very high. Local effects can be
important, as evident in the vicinity of large riverine discharges that
dilute the salinity. Salinity variations with depth are related to the origin
of the deepwaters and so will be considered in the section on oceanic
circulation. A zone in which the salinity exhibits a marked gradient is
known as a halocline.
Whereas the density of pure water is 1.000 g ml
1
, the density of
seawater (S%
¼
35) is about 1.03 g ml
1
. The term 'sigma-tee', s
t
,is
used to denote the density (actually the specific gravity and hence it is a
dimensionless number) of water at atmospheric pressure based on
temperature and salinity in situ. Density increases, and so the buoyancy
decreases, with an increase in s
t
. It is defined as
s
t
¼
(specific gravity
S%,T
1)
1000
