DEEP-OCEAN CIRCULATION
Methods devised (escogitati) to determine deep-ocean circulation have
met with varying success, but all point to a quite complex pattern of subsurface
currents. The deep-ocean currents differ from surface currents in that they
(1) are density driven,
(2) are much slower,
(3) move in a predominantly north-south direction, and
(4) they cross the equator.
 The
deep-ocean circulation is often referred to as a thermohaline
circulation, because the circulation is controlled by differences in temperature
and salinity. Varying combinations of temperature and
salinity produce water of varying densities, and it is these density differences
that produce the deep-ocean circulation. Since the majority of the world’s
water masses are formed at the surface, our discussion of the deep-ocean
circulation must start here. We will move through the circulatory pattern,
beginning and ending with the surface waters around Antarctica. As the high
density surface water around Antarctica sinks, it mixes with the warmer, more
saline circumpolar water to form Antarctic bottom water. See figure 1-2-5.
Because Antarctic bottom water is the most dense water found in the ocean, it
sinks to the ocean floor and spreads, or flows, northward into the deep-ocean
basins of the Atlantic, Pacific, and Indian Oceans. This water mass has been
tracked as far north as the 35th parallel of the Northern Hemisphere.
In the sub-Arctic regions of the Northern Hemisphere, the same type of
process occurs. The cold,  dense
surface water sinks and forms North
Atlantic deep and bottom water. This water mass spreads southward and is
in contact with the bottom, except where it encounters Antarctic bottom water. (See
figure 1-2-6.) Being less dense than Antarctic
bottom water, it is found above Antarctic bottom water wherever the two
exist together. The North Atlantic deep and bottom water eventually makes its
way back to the Antarctic Ocean, where it mixes with intermediate water masses
and Antarctic bottom water to form Antarctic
circumpolar water. Here, the cycle begins again as the cold, dense
surface water of Antarctica sinks and mixes with the circumpolar water. Above
the deep and bottom waters, the intermediate water masses also show a basic
equatorward movement. Antarctic intermediate water actually crosses the equator
and moves as far north as 20° to 35°N.
Its Northern Hemisphere counterpart, Arctic intermediate water, moves south
but does not cross the equator. Mediterranean and Red Sea water both cross the
equator, and have been identified far into the Southern Hemisphere. The Central
and Equatorial water of low and middle latitudes move poleward in their
respective hemispheres, while in high latitudes the near-surface waters move
toward the equator. The Atlantic circulation is considered much more vigorous
than that of the Pacific, because surface-density contrasts are much greater.
However, even with the greater surface-density contrasts, the circulation is
SLOW—VERY SLOW.
The deep-sea currents associated with the deep-ocean circulation flow at a
rate of a few centimeters per second or less. If we were able to free float a
bottle at a designated depth, this rate of speed would equate to the bottle
moving less than 2 degrees of latitude (120 nmi) in a year, or 0.06 nmi/hr. In
summary, and in its simplest form, we can say that the deep-ocean circulation
consists primarily of
(1) equatorward-flowing subsurface water, which moves at an extremely slow
rate of speed and
(2) the much faster poleward-flowing surface water.
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