Surface Currents
Wind that blows over the ocean water creates waves. It also createssurface currents,
which are horizontal streams of water that can flow for thousands of kilometers and can
reach depths of hundreds of meters. Surface currents are an important factor in the ocean
because they are a major factor in determining climate around the globe.
Causes of Surface Currents
Currents on the surface are determined by three major factors:the major overall global wind
patterns, the rotation of the Earth, and the shape of ocean basins.
When you blow across a cup of hot chocolate, you create tiny ripples on its surface that
continue to move after you’ve stopped blowing. The ripples in the cup are tiny waves,
just like the waves that wind forms on the ocean surface. The movement of hot chocolate
throughout the cup forms a stream or current, just as oceanic water moves when wind blows
across it.
But what makes the wind start to blow? When sunshine heats up air, the air expands, which
means the density of the air decreases and it becomes lighter. Like a balloon, the light warm
air floats upward, leaving a slight vacuum below, which pulls in cooler, denser air from the
sides. The cooler air coming into the space left by the warm air is wind.
Because the Earth’s equator is warmed by the most direct rays of the Sun, air at the equator
is hotter than air further north or south. This hotter air rises up at the equator and as
colder air moves in to take its place, winds begin to blow and push the ocean into waves and
currents.
Wind is not the only factor that affects ocean currents. The ’Coriolis Effect’ describes how
Earth’s rotation steers winds and surface currents (Figure14.15). The Earth is a sphere
that spins on its axis in a counterclockwise direction when seen from the North Pole. The
further towards one of the poles you move from the equator, the shorter the distance around
the Earth. This means that objects on the equator move faster than objects further from
the equator. While wind or an ocean current moves, the Earth is spinning underneath it. As
a result, an object moving north or south along the Earth will appear to move in a curve,
instead of in a straight line. Wind or water that travels toward the poles from the equator
is deflected to the east, while wind or water that travels toward the equator from the poles
gets bent to the west. The Coriolis Effect bends the direction of surface currents.
The third major factor that determines the direction of surface currents is the shape of ocean
basins (Figure14.16). When a surface current collides with land, it changes the direction
of the currents. Imagine pushing the water in a bathtub towards the end of the tub. When
the water reaches the edge, it has to change direction.