enough to reduce reflection and there is no snow
cover, solar radiation is able to penetrate into and
through sea ice. In doing so it heats both the interior of the ice and the water column below (Holland
et al., 1997). Leads are also important in allowing
radiation to penetrate into the ocean thus heating
the water column. The sea ice is then melted from
below.
1.2.3 Ocean storage of heat and
fresh water
The ocean’s central role in the climate system
comes from its ability to store and transport heat,
fresh water (and carbon) over a wide range of time
and space scales. A simple indication of the ocean’s
importance is that 3 m of seawater has about
the same heat capacity as the whole atmospheric
column above it (Gill, 1982). On the diurnal time
scale, the ocean stores heat during the day and
releases it to the atmosphere at night. Under calm
conditions, solar heating can warm the upper few
metres by about a degree, provided the water column is not too clear. Heat loss during the following night will cool the surface, making the water
denser. The resulting convection will develop a
shallow diurnal mixed layer a few metres to tens of
metres thick.
Beyond the diurnal time scale, kinetic energy
from wind and waves together with current shear
at the base of the surface layer sustains an upper
mixed layer a few tens of metres in thickness. The
daily net heating (or cooling) of the ocean is averaged over such a mixed layer. In summer, net heating adds buoyancy to the surface waters, increasing
the stratification. This, coupled with generally
weaker winds in summer, makes for shallower
mixed layers. The stability of the water column
can be further increased by the addition of fresh
water to the surface ocean through rainfall, river
discharge, spring runoff from snow melt on land
and through the melting of sea ice. Because these
salinity-stabilized surface mixed layers are very
shallow, they are able to warm up quicker in
spring than they would if they were stabilized by
thermal effects alone.
The depth of the winter mixed layer represents
the water depth that participates directly in the
seasonal cycle of storage and release of heat, fresh
water and gases. The depth of this layer can reach
a few hundred metres in parts of the subtropical
and higher-latitude ocean (Fig. 5.4.2, Hanawa and
Talley, Chapter 5.4), greatly slowing the daily,
weekly and monthly change in temperature. These
deep layers are formed on the equatorward (or
warm) side of major currents such as the eastward
extension of western boundary currents and the
Antarctic Circumpolar Current, particularly where
frequent outbreaks of cold continental air masses
result in large heat loss to the atmosphere. These
sites are the locations for the formation of Mode
waters (Fig. 5.4.3, Hanawa and Talley, Chapter
5.4), which are subsequently advected into the ocean
interior. Anomalies in the heat content of these
Mode waters may persist for several years despite
being exposed to the atmosphere each winter when
the shallow seasonal thermocline is removed (Sutton
and Allen, 1997). Such ocean anomalies may be
important in determining the predictability of interannual climate anomalies (Rodwell et al., 1999).
Heat loss in a few special regions (e.g. the
Labrador Sea, the Golfe de Lyon in the western
Mediterranean Sea and in the high-latitude
Southern Ocean) can be intense, reaching several
hundred watts per square metre over areas a few
hundred kilometres across during particularly
intense winters. The winter mixed layers so formed
extend to over a thousand metres and sometimes to
the seafloor (Lazier et al., Chapter 5.5). Once
water enters this deep layer, it remains there for
centuries before returning to the surface and
contact with the atmosphere. The volume of this
layer is some three to four times the volume of the
thermocline. It is through these regions that the
deep ocean is ventilated and renewed.
1.2.4 Ocean circulation
The ocean circulation is driven by air–sea fluxes of
momentum (wind stress), heat and fresh water.
The resulting circulation is thus intimately coupled
to the atmosphere. Here we introduce first the
upper ocean circulation, which is often thought of
as being dominated by the wind forcing, and then
the global thermohaline circulation. The two are
intimately linked and are only described separately
here for the sake of clarity.
1.2.4.1 The wind-driven circulation
The upper layer circulation in each ocean basin is
driven by the large-scale distribution of wind stress
or, more exactly, by the curl of the wind stress.
1.2 Ocean Processes and Climate Phenomena
17
Clarke, Church and Gould
cover, solar radiation is able to penetrate into and
through sea ice. In doing so it heats both the interior of the ice and the water column below (Holland
et al., 1997). Leads are also important in allowing
radiation to penetrate into the ocean thus heating
the water column. The sea ice is then melted from
below.
1.2.3 Ocean storage of heat and
fresh water
The ocean’s central role in the climate system
comes from its ability to store and transport heat,
fresh water (and carbon) over a wide range of time
and space scales. A simple indication of the ocean’s
importance is that 3 m of seawater has about
the same heat capacity as the whole atmospheric
column above it (Gill, 1982). On the diurnal time
scale, the ocean stores heat during the day and
releases it to the atmosphere at night. Under calm
conditions, solar heating can warm the upper few
metres by about a degree, provided the water column is not too clear. Heat loss during the following night will cool the surface, making the water
denser. The resulting convection will develop a
shallow diurnal mixed layer a few metres to tens of
metres thick.
Beyond the diurnal time scale, kinetic energy
from wind and waves together with current shear
at the base of the surface layer sustains an upper
mixed layer a few tens of metres in thickness. The
daily net heating (or cooling) of the ocean is averaged over such a mixed layer. In summer, net heating adds buoyancy to the surface waters, increasing
the stratification. This, coupled with generally
weaker winds in summer, makes for shallower
mixed layers. The stability of the water column
can be further increased by the addition of fresh
water to the surface ocean through rainfall, river
discharge, spring runoff from snow melt on land
and through the melting of sea ice. Because these
salinity-stabilized surface mixed layers are very
shallow, they are able to warm up quicker in
spring than they would if they were stabilized by
thermal effects alone.
The depth of the winter mixed layer represents
the water depth that participates directly in the
seasonal cycle of storage and release of heat, fresh
water and gases. The depth of this layer can reach
a few hundred metres in parts of the subtropical
and higher-latitude ocean (Fig. 5.4.2, Hanawa and
Talley, Chapter 5.4), greatly slowing the daily,
weekly and monthly change in temperature. These
deep layers are formed on the equatorward (or
warm) side of major currents such as the eastward
extension of western boundary currents and the
Antarctic Circumpolar Current, particularly where
frequent outbreaks of cold continental air masses
result in large heat loss to the atmosphere. These
sites are the locations for the formation of Mode
waters (Fig. 5.4.3, Hanawa and Talley, Chapter
5.4), which are subsequently advected into the ocean
interior. Anomalies in the heat content of these
Mode waters may persist for several years despite
being exposed to the atmosphere each winter when
the shallow seasonal thermocline is removed (Sutton
and Allen, 1997). Such ocean anomalies may be
important in determining the predictability of interannual climate anomalies (Rodwell et al., 1999).
Heat loss in a few special regions (e.g. the
Labrador Sea, the Golfe de Lyon in the western
Mediterranean Sea and in the high-latitude
Southern Ocean) can be intense, reaching several
hundred watts per square metre over areas a few
hundred kilometres across during particularly
intense winters. The winter mixed layers so formed
extend to over a thousand metres and sometimes to
the seafloor (Lazier et al., Chapter 5.5). Once
water enters this deep layer, it remains there for
centuries before returning to the surface and
contact with the atmosphere. The volume of this
layer is some three to four times the volume of the
thermocline. It is through these regions that the
deep ocean is ventilated and renewed.
1.2.4 Ocean circulation
The ocean circulation is driven by air–sea fluxes of
momentum (wind stress), heat and fresh water.
The resulting circulation is thus intimately coupled
to the atmosphere. Here we introduce first the
upper ocean circulation, which is often thought of
as being dominated by the wind forcing, and then
the global thermohaline circulation. The two are
intimately linked and are only described separately
here for the sake of clarity.
1.2.4.1 The wind-driven circulation
The upper layer circulation in each ocean basin is
driven by the large-scale distribution of wind stress
or, more exactly, by the curl of the wind stress.
1.2 Ocean Processes and Climate Phenomena
17
Clarke, Church and Gould