42
Martin V. ANGEL
of depth. The pressure ( p) at any depth (z) results
from the weight (gs ) of the overlying water per unit
area (where g is the gravitational constant and s
is the density of the sea water). In the open ocean
the local effects of variations in atmospheric pressure
are small, but at larger scales they can play a key
role in some of the large-scale physical processes,
such as the generation of El Ni˜ no Southern Ocean
Oscillation (ENSO) events (see below, p. 65). In
shallow inshore waters, fluctuations in atmospheric
pressure can generate storm surges, which can result
in catastrophic flooding when sea level rises quite
suddenly by several metres. During hurricanes in
coastal areas more people are drowned in the floods
caused by the storm surges than are killed by the high
winds. Water is only very slightly compressible, so
that there are only slight increases in in situ density
with increasing depth. If sea water at a temperature
of 0ºC, with a density of 1028.1 kg m
−3 at the surface,
is lowered to a depth of 100 m and its temperature
is kept constant, its density will increase to 1028.6.
Lowering it further to 1000 m will increase its density
to 1032.8, and at 4000 m its density will be 1046.4. If
a sample of seawater is collected at depth and brought
back to the surface in a fully insulated container,
it cools down because its volume expands slightly.
Ecologically and physiologically, the effects of pressure
tend to be relatively small compared to those of other
environmental factors such as temperature. However,
it has a strong influence on the dissolution of calcium
carbonate, and on those organisms that use gas-bladders
or gas-filled vacuoles to regulate their buoyancy.
There are marked geographical variations in the
density of sea water at the surface of the oceans. These
are generated by variations in solar heating, and by the
balance between water being lost from the surface by
evaporation (which cools the surface and increases the
salinity) and inputs of fresh water from rainfall (which
can either cool or warm the surface, but also reduces
surface salinity). Where additions of fresh water from
rainfall are higher than the losses through evaporation,
the density of the surface water in the upper windmixed layer decreases, so that it becomes more buoyant
and less likely to mix with the water below it – that is,
it stabilizes the upper water column. Conversely where
losses by evaporation exceed the inputs from rain,
surface salinity increases and temperatures decrease,
both factors increasing the density of the surface water.
Once the surface water becomes denser than the water
it overlies, it sinks into the interior of the ocean at
convergences, sliding down along surfaces of equal
density or isopycnals (e.g., McCartney, 1992), until it
reaches the depth at which its density equals that of the
surrounding water. Once there, because vertical mixing
is limited the water retains its characteristic properties
of temperature and salinity, and these bodies of water
are known as “water masses”. These water masses can
be tracked over extensive distances as they move within
the deep circulation patterns of the ocean (e.g., Dickson
et al., 1988), and specific communities of plankton and
nekton tend to be associated with them. Finer details
of the large-scale circulation are now being followed
using natural chemical markers, such as
3 He emitted
from hydrothermal vents, and anthropogenic chemical
tracers such as chlorofluorocarbons or CFCs (Smethie,
1993) and radioactive isotopes (Schlosser et al., 1995).
In a few regions large outflows of fresh riverine water
play a significant role in reducing the density of surface
seawater – notably in the Arctic, where the freshwater
outflows of the large Russian rivers stabilize the upper
water column, reducing the fertility of Arctic waters.
Another example is the outflow of the Amazon, which
has a strong influence on the circulation and ecology
of the southern Caribbean and the equatorial Atlantic.
Conversely, in the semi-enclosed Mediterranean (and
the Red Sea) there is a significant excess of evaporation
over the freshwater inputs from rain and riverine
outflows (the latter have now been drastically reduced
in the Eastern Mediterranean as a result of the
construction of the Aswam Dam on the Nile), so that
salinity is exceptionally high.
As fresh water is cooled its density increases to
a maximum at 4ºC, but then it decreases as the
temperature is further cooled towards freezing point. In
contrast, the density of sea water continues to increase
until it reaches its freezing point at about −1.9ºC (it
may be noted that salinity affects the freezing point; the
higher the salinity, the lower the freezing point). When
sea water freezes the ice formed is almost free of salt,
so that the water which remains unfrozen is not only
saltier but also denser. This is the process that underlies
the formation of Bottom Waters in the Weddell Sea
(Southern Ocean) and off Greenland in the North-east
Atlantic; this ensures not only that the deep ocean is
cold but also that it is well ventilated with oxygen (see
Chapter 2).
Dissolution of gases in sea water
Gases, such as oxygen, nitrogen and CFCs that do
Martin V. ANGEL
of depth. The pressure ( p) at any depth (z) results
from the weight (gs ) of the overlying water per unit
area (where g is the gravitational constant and s
is the density of the sea water). In the open ocean
the local effects of variations in atmospheric pressure
are small, but at larger scales they can play a key
role in some of the large-scale physical processes,
such as the generation of El Ni˜ no Southern Ocean
Oscillation (ENSO) events (see below, p. 65). In
shallow inshore waters, fluctuations in atmospheric
pressure can generate storm surges, which can result
in catastrophic flooding when sea level rises quite
suddenly by several metres. During hurricanes in
coastal areas more people are drowned in the floods
caused by the storm surges than are killed by the high
winds. Water is only very slightly compressible, so
that there are only slight increases in in situ density
with increasing depth. If sea water at a temperature
of 0ºC, with a density of 1028.1 kg m
−3 at the surface,
is lowered to a depth of 100 m and its temperature
is kept constant, its density will increase to 1028.6.
Lowering it further to 1000 m will increase its density
to 1032.8, and at 4000 m its density will be 1046.4. If
a sample of seawater is collected at depth and brought
back to the surface in a fully insulated container,
it cools down because its volume expands slightly.
Ecologically and physiologically, the effects of pressure
tend to be relatively small compared to those of other
environmental factors such as temperature. However,
it has a strong influence on the dissolution of calcium
carbonate, and on those organisms that use gas-bladders
or gas-filled vacuoles to regulate their buoyancy.
There are marked geographical variations in the
density of sea water at the surface of the oceans. These
are generated by variations in solar heating, and by the
balance between water being lost from the surface by
evaporation (which cools the surface and increases the
salinity) and inputs of fresh water from rainfall (which
can either cool or warm the surface, but also reduces
surface salinity). Where additions of fresh water from
rainfall are higher than the losses through evaporation,
the density of the surface water in the upper windmixed layer decreases, so that it becomes more buoyant
and less likely to mix with the water below it – that is,
it stabilizes the upper water column. Conversely where
losses by evaporation exceed the inputs from rain,
surface salinity increases and temperatures decrease,
both factors increasing the density of the surface water.
Once the surface water becomes denser than the water
it overlies, it sinks into the interior of the ocean at
convergences, sliding down along surfaces of equal
density or isopycnals (e.g., McCartney, 1992), until it
reaches the depth at which its density equals that of the
surrounding water. Once there, because vertical mixing
is limited the water retains its characteristic properties
of temperature and salinity, and these bodies of water
are known as “water masses”. These water masses can
be tracked over extensive distances as they move within
the deep circulation patterns of the ocean (e.g., Dickson
et al., 1988), and specific communities of plankton and
nekton tend to be associated with them. Finer details
of the large-scale circulation are now being followed
using natural chemical markers, such as
3 He emitted
from hydrothermal vents, and anthropogenic chemical
tracers such as chlorofluorocarbons or CFCs (Smethie,
1993) and radioactive isotopes (Schlosser et al., 1995).
In a few regions large outflows of fresh riverine water
play a significant role in reducing the density of surface
seawater – notably in the Arctic, where the freshwater
outflows of the large Russian rivers stabilize the upper
water column, reducing the fertility of Arctic waters.
Another example is the outflow of the Amazon, which
has a strong influence on the circulation and ecology
of the southern Caribbean and the equatorial Atlantic.
Conversely, in the semi-enclosed Mediterranean (and
the Red Sea) there is a significant excess of evaporation
over the freshwater inputs from rain and riverine
outflows (the latter have now been drastically reduced
in the Eastern Mediterranean as a result of the
construction of the Aswam Dam on the Nile), so that
salinity is exceptionally high.
As fresh water is cooled its density increases to
a maximum at 4ºC, but then it decreases as the
temperature is further cooled towards freezing point. In
contrast, the density of sea water continues to increase
until it reaches its freezing point at about −1.9ºC (it
may be noted that salinity affects the freezing point; the
higher the salinity, the lower the freezing point). When
sea water freezes the ice formed is almost free of salt,
so that the water which remains unfrozen is not only
saltier but also denser. This is the process that underlies
the formation of Bottom Waters in the Weddell Sea
(Southern Ocean) and off Greenland in the North-east
Atlantic; this ensures not only that the deep ocean is
cold but also that it is well ventilated with oxygen (see
Chapter 2).
Dissolution of gases in sea water
Gases, such as oxygen, nitrogen and CFCs that do