Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 105

when reaching these latitudes, and then traditionally paraded a straw
horse effigy on deck—then threw it overboard—to celebrate.
Partly because the winds are calm, the waters are too. The Sar-
gasso Sea is the best-known part of a wider stretch of more or less
becalmed water called the North Atlantic Gyre. Fast-moving oceanic
currents encircle the Gyre: to the west and north, the Gulf Stream
and its continuation the North Atlantic Drift, flowing eastwards and
northwards; and to the east the Canary Current, flowing southwards
to join the North Equatorial Current south of the Gyre and flowing
westwards, back into the source of the Gulf Stream. Held within this
circulatory system, the Gyre itself very slowly rotates, under the
influence of the ever-present Coriolis effect. There are other gyres:
two each, north and south, in the Atlantic and Pacific oceans respec-
tively, and one in the Indian Ocean, each also surrounded by faster-
moving currents.
This gyre-plus-current combination forms a complex system,
which has been more clearly revealed by chemical clues such as analyses
of salt content than by physical trackers such as floating beacons. The
floating markers, infuriatingly, did not move in the orderly pattern
predicted, but rather took off in myriad mutually inconsistent and
incomprehensible directions. What was happening? The arrival of
satellite images showed why. On these images, near-circular 'mesos-
cale eddies' can clearly be seen, 100 kilometres or so across, continu-
ally spinning off from the boundary between the slow-moving gyres
and the speeding current systems that encircle them. The rotating
eddies persist for a year or two, slowing, before merging with the
mass of calm water in the gyre.
For the most part the great oceanic gyres are regions of low nutri-
ents and low biological productivity (the Sargasso Sea is a bit of an
exception in this). They are far from land-based, river-fed nutrient
sources, and in such calm waters it is hard to stir nutrients from the