Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 96

polar regions. Each year the cold surrounding seas form a carapace of
sea ice that crystallizes from the surface ocean waters. The sea ice itself
is pure water ice, without salt. The water left out of the freezing proc-
ess therefore contains the excluded salt: that water is dense because of
its added salt content, and also because it is cold—and so it sinks. The
tail ends of originally warm currents (such as the Gulf Stream) might
enter these polar regions too. These are saltier because so much water
has evaporated from them on their journey. As these dense, salty
waters cool in the frigid polar regions they, too, begin to sink.
These descending masses of cold, salty, dense water are today the
main powerhouses of the Earth's oceanic circulatory system. Much of
the world's deep water originated from the fringes of the Antarctic, but
a classic example lies in the North Atlantic. It is called the North Atlan-
tic Deep Water, and is so often cited in discussions of ocean currents
that it has its own acronym, the straightforward but uneuphonious
NADW. From its origins, the NADW flows around the southern end of
Greenland, before being pushed against the eastern margin of the
Americas, both north and south, by the Coriolis effect.
It is quite a current. Oceanographers describe the scales of these
flows in units called sverdrups, named after the pioneering oceanog-
rapher Harald Sverdrup. A sverdrup is a million cubic metres a second,
and the NADW flow approximates to 10 sverdrups, which is more
than five times greater than the total flow of all of the world's rivers.
It flows along, rather than down, the lower part of the continental
slope and the adjoining part of the deep ocean floor as a contour
current (because it flows along the contours, rather than across them).
Contour currents are usually quite gentle, moving at a few centi-
metres a second, but now and again there are dramatic increases in
current velocity up to, and sometimes exceeding, a metre a second
during 'benthic storms'—some of which are perturbations from
surface storms that are transmitted down into deep water.