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The circulation of the ocean is usually divided
into two parts, a wind-driven circulation that
dominates in the upper few hundred meters,
and a density-driven circulation that dominates
below. The latter is called the ‘thermohaline’
circulation because of the role of heating, cool-
ing, freshening, and salinification in producing
regional density differences within the ocean.
The thermohaline circulation, for the most
part, is an ‘overturning’ circulation in which
warm water flows poleward near the surface
and is subsequently converted into cold water
that sinks and flows equatorward in the in-
terior. Radiocarbon measurements show that
the thermohaline circulation turns over all the
deep water in the ocean every 600 years or so.
The most spectacular features of the ther-
mohaline circulation are seen in the sinking
phase, in the formation of new deep water in
the North Atlantic and the Southern Ocean.
Large volumes of cold polar water can be
readily observed spilling over sills, mixing
violently with warmer ambient water, and
otherwise descending to abyssal depths. The
main features of the upwelling phase are less
obvious. Most of the gaps in our knowledge
about the thermohaline circulation are due to
uncertainty about where the upwelling occurs
and how upwelled deep water returns to the
areas of deep-water formation. The main new
development in studies of the thermohaline
circulation is the role of Drake Passage and the
Antarctic Circumpolar Current (ACC) in the
upwelling phase.
The thermohaline circulation is an impor-
tant factor in the earth’s climate because it
transports roughly 1015 W of heat poleward
into high latitudes, about one fourth of the total
heat transport of the ocean/atmosphere circu-
lation system. The upwelling branch of the
thermohaline circulation is important for the
ocean’s biota as it brings nutrient-rich deep
water up to the surface. The thermohaline
circulation is thought to be vulnerable to the
warming and freshening of the earth’s polar
regions associated with global warming.
The Cooling Phase - Deep-Water
Formation
The most vigorous thermohaline circulation in
the ocean today is in the Atlantic Ocean where
the overturning is often likened to a giant con-
veyor belt. The upper part of the conveyor
carries warm, upper ocean water through the
tropics and subtropics toward the north while
the deep part carries cold dense polar water
southward through the Atlantic, around the tip
of Africa, and into the ocean beyond. The
Atlantic conveyor converts roughly 15x106
m3s-1 of upper ocean water into deep water.
(Oceanographers designate a flow rate of
1x106m3s-1 to be one Sverdrup, or Sv. All the
world’s rivers combined deliver about one Sv
of fresh water to the ocean.) Most of the 15-Sv
flow of the upper part of the conveyor passes
through the Florida Straits and up the east
coast of North America as part of the Gulf-
stream. The path of the conveyor cuts east-
ward across the Atlantic and continues
northward along the coast of Europe. As the
conveyor moves out across the Atlantic and
northward off Europe it gives up roughly 50 W
of heat per square meter of ocean surface to the
atmosphere. This heat flux is comparable to
Ocean Circulation: Thermohaline Circulation
J. R. Toggweiler, Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, NJ 08542
Robert M. Key, Atmospheric and Oceanic Sciences Program, Department of Geosciences,
Princeton University, Princeton, NJ 08540
