Thermohaline circulation
The water of the great world ocean is, like its crust and interior,
constantly in motion. Currents carrying colossal amounts of water
transport it around the globe, by what has been named "The
Great Ocean Conveyor" (Broecker, 1991). Oceanic thermohaline
-- so named because it involves both heat, hence "thermo,"
and salt, hence haline, for common table salt (halite) -- circulation
is what drives the Conveyor. The two attributes, temperature and
salinity, determine the density of seawater, and the differences
in density between the water masses in the world's oceans causes
the water to flow. Thermohaline circulation -- the Great Ocean
Conveyor -- thereby produces the greatest oceanic current on the
planet. It works in a fashion similar to a conveyor belt -- hence
the name -- transporting enormous volumes of cold, salty water
from the North Atlantic to the Northern Pacific, and bringing
warmer, fresher water in return.
Descriptions of the working of the Conveyor usually start with what happens in the North Atlantic, under and near the polar region sea ice. There warm, salty water that has been transported north from tropical regions is rapidly cooled, forming frigid water in vast quantities. When this seawater freezes, its salt is excluded (sea ice contains almost no salt), increasing the salinity of the remaining, unfrozen water. This salinity makes the water quite dense, and its frigidity makes it denser still. Being denser than the less saline, warmer surface waters moving in from the south, this water drops to the floor of the ocean. This water is known to oceanographers as North Atlantic Deep Water (NADW), and it propels today's oceanic thermohaline circulation.
In the northernmost reaches of the North Atlantic, this water begins a great circuit through the world's oceans. First it moves south through the North Atlantic, then south through the South Atlantic, rounding Brazil, and then encounters great masses of similarly frigid and saline water coming from under the sea ice area surrounding Antarctica (called Antarctic Bottom Water [AABW] or Antarctic Deep Water [AADW]), hugging the ocean bottom as it flows. This greatest of ocean currents then moves east, well north of the Antarctic mainland but well south of Africa (where, past the Cape of Good Hope, a branch pushes northward along the east African coast) and continues east across the entire breadth of the Indian Ocean north of Antarctica, swings around south of Australia and far into the Pacific. As it continues on its submarine migration, the current mixes with warmer water, warms, and rises, until finally, in the northern Pacific, it dissipates as a coherent entity.
In the Pacific, however, a warm, shallow-sea
counter-current has been generated. This counter-current moves
south and west, wends its way through the Indonesian archipelago,
across the Indian Ocean, still heading west, and rounds southern
Africa just off the Cape of Good Hope. It crosses though the South
Atlantic, still on the surface (though it extends a kilometer
and a half -- almost a mile -- deep), where tropical warmth increases
evaporation and leaves the counter-current saltier. It then moves
up along the East Coast of North America, and on across to the
coast of Scandinavia, which its warmth helps protect from the
extreme cold of northern winters. When this warmer, saltier water
reaches high northern latitudes, it chills, and eventually becomes
North Atlantic Deep Water, completing the circuit.
Although this global thermohaline circulation has been vigorous since the end of the ice age, the global conveyor is vulnerable to significant and rapid changes. Because the circulation is driven by the varying densities of water, it can become sluggish, or perhaps even stagnant, when those densities change. At times, frigid deep water from Antarctica has been the dominant driver of the world's circulation, which results in the cooling of surface waters in the North Atlantic and lower temperatures for coastal North America and northern Europe (Broecker, 2001). During the Ice Age, the primary driving force of global thermohaline circulation may have switched back and forth between the waters of the North Atlantic and those of Antarctica. As the deep water driver seesawed between north and south, it produced rapid shifts of temperature for the North Atlantic region and significant climate instability (Broecker, 1997).
The density of tropical water is also quite high, though not as that of polar water. It owes this high density to the elevated rate of evaporation in tropical areas. As with the freezing of polar sea ice, evaporation leaves the salt behind in the remaining seawater. (Both freezing and evaporation therefore produce higher salinity in surface water. Being denser than the surrounding water, tropical water sinks, though because polar water is much colder and therefore denser, it sinks faster and deeper than tropical water.)
During warming episodes in Earth's history, polar regions tend to warm proportionately more than tropical regions do, and the temperature difference between the waters of the two regions decreases. Less water is frozen into ice, and salinity declines. Polar water also becomes warmer, and therefore even less dense. Conversely, tropical water, its salinity increased by greater evaporation, becomes more dense. The density difference between polar and tropical waters decreases. This can slow, and perhaps stop, thermohaline circulation.
(The major European and Asian rivers
that empty into the Arctic Ocean have been increasing their flows
[Peterson, 2002], apparently as a result of increased high latitude
precipitation due to global warming. As a consequence, deep water
in Arctic seas has freshened during the past 40 years. Computer
modeling indicates that the warming-induced precipitation increase
can be traced only to human releases of greenhouse gases, not
to natural variations in the rain cycle [Wu, 2005].)
One extremely important attribute of
thermohaline circulation is that it carries oxygenated water to
the deep ocean. The polar seas (the North Atlantic and the Southern
Ocean) that produce the frigid water which drives the Great Ocean
Conveyer are storm-swept, especially in winter. This turbulence
oxygenates the water, and its frigidity (like a frigid can of
soda) allows it to carry lots of dissolved gas. Descending to
the ocean floor, this frigid water thereby oxygenates the deep
sea. Without this input of highly oxygenated water, the deep ocean
would become anoxic. (The activity of phytoplankton only provides
oxygen to the ocean's surface.) A vigorous thermohaline circulation,
therefore, translates into a well-oxygenated ocean, whereas a
weak thermohaline circulation results in ocean stratification
(separation into distinct deep ocean and surface ocean layers,
with little mixing between them) and deep water anoxia.
During earlier periods of Earth's history, it is likely that something
similar to today's thermohaline circulation occurred in the planet's
oceans, especially when global climate was cool. Certainly the
density variations produced by temperature and salinity differences
would have existed, even if those differences were more muted.
High global temperatures may have been capable of slowing, or
even possibly stopping, thermohaline circulation, however. And
the nature of the circulation would have been dependent on the
configuration of the oceans and continents, which changed with
time. Nonetheless, and perhaps surprisingly, those folks known
as paleoceanographers apparently can use general principles of
oceanic circulation, coupled with data on the ancient positions
of oceans and continents, to make reasonably good determinations
of what ocean circulation would have looked like in ages past.
More detailed global oceanic circulation
maps:
The maps below are provided to
give the interested reader a more detailed view of global oceanic
circulation. The first map is actually a schematic representation
of the working of the great oceanic conveyer, but from the perspective
of the Southern Ocean. The Southern Ocean is placed at the center
of the diagram because it is the only place in the world where
an ocean is unobstructed by a continental land mass, and currents
can move freely around the entire globe. This allows wind and
wave to build to gale conditions and has earned the name "the
screaming sixties" for the Southern Ocean's location at about
60°S latitude. Because the Southern Ocean lies just off the
coast of Antarctica, both wind and wave are also frigid. The Southern
Ocean makes a significant contribution to global thermohaline
circulation via the fresh, frigid water (Antarctic Deep Water
or Antarctic Bottom Water) that pours from under the sea ice and
the huge floating ice shelves that cut deeply into the continent.
(An additional, somewhat warmer contribution is in the form of
what is called Antarctic Intermediate Water.)
The diagram depicts the Atlantic, Pacific and Indian Oceans essentially as continent-enclosed arms radiating from the central Southern Ocean. Although the diagram is schematic rather than a realistic map, it nonetheless conveys basic information about the global thermohaline circulation, and in greater specificity than the map above. The currents depicted are color-coded: purple for surface currents, red for those at intermediate levels and for Antarctic subsurface water, green for deep currents, and blue for those which hug the ocean bottom (From Siedler, 2001, figure 1.2.7, as taken from Schmitz, 1996).
The second map provides the details of the major surface currents of the world (note that the colors do not have the same meaning as in the schematic above). Note, in particular the Benguela Current off the southwest coast of Africa. This cold current sends chill, oxygenated, nutrient-laden water upwelling along the coast. But this water is so hospitable to marine plankton that there is a constant rain of dead organisms and organic debris into the depths. As this debris is decomposed, anoxic conditions result, and sulfur-reducing bacteria generate toxic hydrogen sulfide, killing aerobic marine organisms. See discussion in the Methane Catastrophe section. (From Siedler, 2001)
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