Then, in the Indian Ocean, there is still another current which
changes its direction twice yearly as the monsoons (seasonal winds)
reverse each April and October. This gives us a clue to the cause of these
ocean currents. If a current changes with each shift of prevailing winds
then the winds must be responsible for the flow of the current.

We know that wind is air in motion and that this motion comes
from unequal heating of air over the earth's surface. Air heated at the
equator expands and rises, and is replaced by cooler air from outside
areas. Since the tropics, especially over ocean regions, are always warm
there are strong winds blowing continuously toward the equator from
each of the temperate zones. These winds are called the trade winds.
North of the equator they blow from the northeast and are, therefore,
called the Northeast trades. South of the equator they blow from the
southeast and, hence, are known as the Southeast trades. Unless
interrupted by storms or other atmospheric disturbances these trade
winds blow day and night the whole year around.

A wind, blowing steadily over a great expanse of water, causes
the surface of that water to move. There is considerable friction between
the molecules of water and, as the surface water is pushed forward, this
motion is transferred to the water beneath. If the wind keeps blowing
long enough, the whole body of water, from top to bottom, will be set in
motion. Something like that, scientists say, is happening near the
equator.