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 frigid water that propels today's oceanic thermohaline circulation is produced in vast quantities. This water is highly saline (salty), because when seawater freezes, its salt is excluded. The sea ice therefore contains almost no salt, but the ocean around it is highly saline. This makes the water quite dense, and its frigidity makes it denser still. Being denser than the less saline, less frigid northern surface waters, this water drops to the floor of the ocean. This water is known to oceanographers as North Atlantic Deep Water (NADW).
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, then, still on the surface (though it extends
a kilometer and a half -- almost a mile -- deep), moves up along the East Coast
of North America, and on across to the coast of Scandinavia, which it helps
protect from the extreme cold of northern winters. When this warmer 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)