Ocean. It is also observed in the warm current of
Kuro-Shivo in the North Pacific and in the Agulhas Current
which runs along the South African coast in the southern
hemisphere. This phenomenon is a complex result of the
increasing strength of the Coriolis force with distance from
the equator.
Differences in density also lead to large movements of the
oceanic water masses. The densest waters are in the polar
regions where sea ice forms. Already dense because of their
low temperature, the waters receive a further load from two
salt inputs: one coming from the flow of currents from the
subtropics, where evaporation is intense and the other from
salt released during sea ice formation. These dense waters
tend to drop due to gravity below the warmer and less salty
waters, then to spread out at the bottom of the ocean where
the temperature, around 0–2 °C, varies little from the poles
to the equator. These dense water masses are the starting
point of the great global circulation loop of the ocean, called
thermohaline, since denser waters tend to sink below less
dense waters and the density of water masses is dependent
only on temperature and salinity. This mechanism plays an
important role in ocean circulation, as it contributes over
75% to the formation of all the masses of deep waters of the
world ocean. Paradoxically, this process of downwelling
deep water occurs only in a very small fraction of the surface
of the oceans: in the Labrador Sea, the Norwegian and
Greenland Seas and in some regions of North Atlantic as
well as at the edge of the Antarctic continent, particularly in
the Weddell Sea. In overall, the combination of the thermohaline circulation and the circulation caused by winds
makes up the ocean meridional overturning circulation.
Downwelling waters in the North Atlantic descend to a
depth of 2000–3000 m up to latitude of around 60° south,
where water masses undergo a slow movement of ascent to
the surface. Carried by the Antarctic Circumpolar Current
that runs from west to east around the southern polar continent, the deep waters from the North Atlantic then spread out
in the South Pacific and the Indian Ocean. The return part of
this great circulation loop occurs through warm currents near
the surface. They pass between the Indonesian islands, cross
the Indian Ocean, circumnavigate Africa by the Agulhas
Current, and then up towards the North Atlantic with the Gulf
Stream and the North Atlantic Drift. But, while the return by
the warm currents takes a few decades or even up to a hundred years, it takes several hundred to a thousand years from
the time the cold waters sink in the North Atlantic to their
arrival in the center of the Pacific, showing the slowness of
this gigantic mixing achieved by the deep circulation.
Reconstructing Ocean Circulation in the Past
Paleoceanography reconstructs past ocean circulation by
analyzing sediments that have settled in more or less regular
layers on the ocean floor, the uppermost layers corresponding to the most recent deposits. This discipline has
flourished thanks to several scientific and technical developments. One of these developments is in the methods of
coring and drilling which allow cores to be brought to the
laboratories which have relatively undisturbed sediments
reliably recording the conditions in the ocean at the time the
sediment was deposited. Furthermore, in-depth analysis of
fauna (foraminifera) and flora (diatoms, coccoliths) fossils
that lived in the illuminated area or in the water column
permit the reconstruction of the temperatures of surface
waters (see Chap. 21). Finally, new geochemical methods
based on the analysis of stable and radioactive isotopes of
elements present in marine sediments have provided dating
methods, stratigraphic markers and tracers of large marine
currents (see Chaps. 4, 6 and 7, Volume 1).
It is now possible to draw up temperature maps of surface
waters of the ocean at critical times of the history of climate
on Earth, such as during the Last Glacial Maximum with a
14 C age close to 18,000 years (in other words, a calendar age
of about 20,000 years, Chap. 2) or during interglacial periods of the Quaternary. Paleoceanography also allows the
reconstruction of the movements of fronts separating surface
water masses with very different characteristics: a descent of
polar waters in lower latitudes, driving out temperate waters,
is accompanied by a strong cooling also felt by adjacent
coastal areas. Conversely, their retreat is accompanied
immediately by a significant warming.
Marine sediments also contain markers for the conditions
that prevailed at depths, at the water-sediment interface. The
most important of these are benthic foraminifera, microscopic animals with a calcareous shell whose isotopic
composition is a particular reflection of the temperature and
dissolved carbon dioxide content of the deep waters of the
ocean. Measurements by oceanographers show that at a
certain depth in the ocean, the physical and chemical characteristics of the waters are almost constant over a distance
of several dozen kilometers. By taking sediment cores at
different depths in an ocean basin, it is possible to reconstruct the features of large deep water masses, to deduce the
main features of their circulation within a given period
and to track their variations over time. Paleoceanography
therefore allows the reconstruction of the main features of
changes in the ocean in three dimensions.
1 The Climate System: Its Functioning and History
15
Kuro-Shivo in the North Pacific and in the Agulhas Current
which runs along the South African coast in the southern
hemisphere. This phenomenon is a complex result of the
increasing strength of the Coriolis force with distance from
the equator.
Differences in density also lead to large movements of the
oceanic water masses. The densest waters are in the polar
regions where sea ice forms. Already dense because of their
low temperature, the waters receive a further load from two
salt inputs: one coming from the flow of currents from the
subtropics, where evaporation is intense and the other from
salt released during sea ice formation. These dense waters
tend to drop due to gravity below the warmer and less salty
waters, then to spread out at the bottom of the ocean where
the temperature, around 0–2 °C, varies little from the poles
to the equator. These dense water masses are the starting
point of the great global circulation loop of the ocean, called
thermohaline, since denser waters tend to sink below less
dense waters and the density of water masses is dependent
only on temperature and salinity. This mechanism plays an
important role in ocean circulation, as it contributes over
75% to the formation of all the masses of deep waters of the
world ocean. Paradoxically, this process of downwelling
deep water occurs only in a very small fraction of the surface
of the oceans: in the Labrador Sea, the Norwegian and
Greenland Seas and in some regions of North Atlantic as
well as at the edge of the Antarctic continent, particularly in
the Weddell Sea. In overall, the combination of the thermohaline circulation and the circulation caused by winds
makes up the ocean meridional overturning circulation.
Downwelling waters in the North Atlantic descend to a
depth of 2000–3000 m up to latitude of around 60° south,
where water masses undergo a slow movement of ascent to
the surface. Carried by the Antarctic Circumpolar Current
that runs from west to east around the southern polar continent, the deep waters from the North Atlantic then spread out
in the South Pacific and the Indian Ocean. The return part of
this great circulation loop occurs through warm currents near
the surface. They pass between the Indonesian islands, cross
the Indian Ocean, circumnavigate Africa by the Agulhas
Current, and then up towards the North Atlantic with the Gulf
Stream and the North Atlantic Drift. But, while the return by
the warm currents takes a few decades or even up to a hundred years, it takes several hundred to a thousand years from
the time the cold waters sink in the North Atlantic to their
arrival in the center of the Pacific, showing the slowness of
this gigantic mixing achieved by the deep circulation.
Reconstructing Ocean Circulation in the Past
Paleoceanography reconstructs past ocean circulation by
analyzing sediments that have settled in more or less regular
layers on the ocean floor, the uppermost layers corresponding to the most recent deposits. This discipline has
flourished thanks to several scientific and technical developments. One of these developments is in the methods of
coring and drilling which allow cores to be brought to the
laboratories which have relatively undisturbed sediments
reliably recording the conditions in the ocean at the time the
sediment was deposited. Furthermore, in-depth analysis of
fauna (foraminifera) and flora (diatoms, coccoliths) fossils
that lived in the illuminated area or in the water column
permit the reconstruction of the temperatures of surface
waters (see Chap. 21). Finally, new geochemical methods
based on the analysis of stable and radioactive isotopes of
elements present in marine sediments have provided dating
methods, stratigraphic markers and tracers of large marine
currents (see Chaps. 4, 6 and 7, Volume 1).
It is now possible to draw up temperature maps of surface
waters of the ocean at critical times of the history of climate
on Earth, such as during the Last Glacial Maximum with a
14 C age close to 18,000 years (in other words, a calendar age
of about 20,000 years, Chap. 2) or during interglacial periods of the Quaternary. Paleoceanography also allows the
reconstruction of the movements of fronts separating surface
water masses with very different characteristics: a descent of
polar waters in lower latitudes, driving out temperate waters,
is accompanied by a strong cooling also felt by adjacent
coastal areas. Conversely, their retreat is accompanied
immediately by a significant warming.
Marine sediments also contain markers for the conditions
that prevailed at depths, at the water-sediment interface. The
most important of these are benthic foraminifera, microscopic animals with a calcareous shell whose isotopic
composition is a particular reflection of the temperature and
dissolved carbon dioxide content of the deep waters of the
ocean. Measurements by oceanographers show that at a
certain depth in the ocean, the physical and chemical characteristics of the waters are almost constant over a distance
of several dozen kilometers. By taking sediment cores at
different depths in an ocean basin, it is possible to reconstruct the features of large deep water masses, to deduce the
main features of their circulation within a given period
and to track their variations over time. Paleoceanography
therefore allows the reconstruction of the main features of
changes in the ocean in three dimensions.
1 The Climate System: Its Functioning and History
15