Effect on Global Climate
Surface currents play a large role in determining climate. These currents bring warm water
from the equator to cooler parts of the ocean; they transfer heat energy. Let’s take the Gulf
Stream as an example; you can find the Gulf Stream in the North Atlantic Ocean inFigure
14.15. The Gulf Stream is an ocean current that transports warm water from the equator
past the east coast of North America and across the Atlantic to Europe. The volume of
water it transports is more than 25 times that of all of the rivers in the world combined, and
the energy it transfers is more than 100 times the world’s energy demand. It is about 160
kilometers wide and about a kilometer deep. The Gulf Stream’s warm waters give Europe
a much warmer climate than other places at the same latitude. If the Gulf Stream were
severely disrupted, temperatures would plunge in Europe.
Deep Currents
Surface currents occur close to the surface of the ocean and mostly affect the photic zone.
Deep within the ocean, equally important currents exist that are calleddeep currents.
These currents are not created by wind, but instead by differences in density of masses of
water. Density is the amount of mass in a given volume. For example, if you take two full
one liter bottles of liquid, one might weigh more, that is it would have greater mass than
the other. Because the bottles are both of equal volume, the liquid in the heavier bottle is
denser. If you put the two liquids together, the one with greater density would sink and the
one with lower density would rise.
Twomajorfactorsdeterminethedensityofoceanwater: salinity(theamountofsaltdissolved
in the water) and temperature (Figure14.17). The more salt that is dissolved in the water,
the greater its density will be. Temperature also affects density: the colder the temperature,
the greater the density. This is because temperature affects volume but not mass. Colder
water takes up less space than warmer water (except when it freezes). So, cold water has
greater density than warm water.
More dense water masses will sink towards the ocean floor. Just like convection in air, when
denser water sinks, its space is filled by less dense water moving in. This creates convection
currents that move enormous amounts of water in the depths of the ocean. Why is the water
temperature cooler in some places? Water cools as it moves from the equator to the poles
via surface currents. Cooler water is more dense so it begins to sink. As a result, the surface
currents and the deep currents are linked. Wind causes surface currents to transport water
around the oceans, while density differences cause deep currents to return that water back
around the globe (Figure14.18).