Earth and Its Orbit - The Science of Navigation - page 30

Ocean Currents
The details of ocean current form, strength, and evolution are complex. These
depend on the earth's rotation, the topography of the ocean floors, water
salinity and heat content, and the prevailing winds. Much of the detail is not
well understood and consequently is not predictable, but certain trends are
clear. There is, for example, a global conveyor belt of heat—warm surface
water in the tropics that is driven north and south by winds, away from the
equator. These water bodies then cool, sink, and drift back toward the equator
at depth.
Some currents are strong and permanent. The Gulf Stream is a warm
surface current flowing north from Florida to Newfoundland before splitting
in mid-Atlantic into a western European stream and a West African stream.
This current warms the east coast of North America, and Western Europe.
The California current is cool and shallow; it moves south from British Colum-
bia to Baja California. One consequence of this current is the cool waters off
Hawaii. The Brazil current was historically important because, during the Age
of Exploration, it took Portuguese ships off the west coast of Africa and car-
ried them southward to Brazil—leading to the Europeans' discovery of that
vast region. The South Atlantic current then brought them eastward back to
southern Africa.
Navigators learned to trust these currents. Even today, currents are impor-
tant to shipping, if not to navigation, because a judicious choice of route that
takes currents into account can significantly reduce transport costs.
increases air pressure. This basic circulation pattern is responsible for the
trade winds : air at the surface flows from regions of high pressure (horse
latitudes) to low pressure regions (the equator). They get deflected by the
Coriolis force, resulting in winds that are from the northeast in the north-
ern hemisphere and from the southeast in the southern hemisphere. At the
equator, the winds are less reliable; they are light and variable, and we have
the doldrums . 26
North and south of the horse latitudes, the pattern of high and low
26. The origin of the phrase horse latitudes is uncertain; tradition has it that Spanish
sailors, on their way to their Caribbean colonies, became becalmed and threw their horses
overboard to save on drinking water. Trade winds are so called because, being strong and
reliable, they greatly aided the passage of merchant ships across oceans. Sailing vessels were
becalmed in the doldrums ; later, the word took on a more general meaning of lethargy.