Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 94

The speed of the Earth's rotation is something that we are normally
wholly unconscious of, but it is nevertheless enormous: a little over
1,600 kilometres an hour at the equator, faster than a commercial
jet plane. As this speed of planetary rotation diminishes to zero at
the poles, anything (for example wind or water currents, an aero-
plane, a bullet) travelling from higher- to lower-speed regions will
'carry' some of its original spin momentum with it—and vice versa,
of course, if moving in the opposite direction—and be deflected
sideways. This is the Coriolis effect, named after the French math-
ematician Gaspard-Gustave de Coriolis (1792-1843), who analysed
the forces involved, not on the Earth, but on rotating water wheels
(he was also to write a treatise on colliding spheres, which became
much celebrated among billiards players of a more analytical
persuasion).
The end result of the Sun's heat and the Earth's spin on winds is the
large-scale cells of atmospheric circulation (Fig. 7), which sailors expe-
rience as more or less predictable wind systems such as the 'trade
winds' and the mid-latitude 'westerlies'. Superimposed on these large-
scale systems are smaller-scale, intermittent weather systems of
cyclones and anticyclones, their characteristic rotary motion also
being a result of the Coriolis effect. The blowing winds tug on the
ocean waters beneath, and so drag the surface layers of water along
with them. The trade winds, for example, drive currents of water
westwards along the equator, with an equatorial counter-current of
returning water to balance the equation.
There are other, more direct means by which the Sun's energy can
make the ocean waters circulate. Subtropical waters, for instance, are
both strongly warmed by the Sun and swept by hot, dry winds (that
are on the descending limbs of the Hadley cells). Evaporation of water
in these regions leaves the surface ocean layers richer in salt, and
hence denser. If sufficiently dense, this surface water can sink to lower