Geography Reference
In-Depth Information
sea creatures extract dissolved calcium from the water to build their
shells and when they die this calcium drifts onto the ocean floor. The
balance of inputs from rivers and losses onto the ocean floor largely
controls the chemical composition of the oceans. The concentration
of salts in the ocean therefore varies depending on location and time.
Surface water salinity can be diluted by melting ice or rainwater, or it
can be concentrated by evaporation. The salinity of the ocean water
around the dry subtropics of 20° to 30° north or south tends to be
greater than elsewhere, because evaporation is greatest here.
(ii) In terms of water temperature, the surface of the ocean
absorbs the Sun's energy and gains heat. More heat is gained than
lost in the low latitudes and more is lost than gained in the high
latitudes. Just as with the atmosphere there is a tendency for warm
surface water to move polewards. This transport of heat from the
equator towards higher latitudes provides good regulation of the
Earth's climate system otherwise the poles would be even colder
and the tropics even hotter than they are at present. Water pro-
vides an excellent long-term store of heat for the Earth, as it takes
more energy to heat water by 1°C (and more energy is released
when water cools by 1°C) than for any other substance.
Both temperature and salinity control the density of seawater. If
the density of seawater increases with depth then the water is said to
be vertically stable. If, however, there is more dense water on top of
less dense water then vertical mixing of water will take place. This
means that if there is a warm, strong wind encouraging evaporation at
the ocean surface then this would lead to more salty and dense surface
waters which leads to instability and mixing of water.
(iii) The surface currents of the ocean as shown in Figure 1.3 are
driven by the surface winds. For example, the trade winds (shown
in Figure 1.2) drive the northern and southern equatorial currents
moving in a westerly direction parallel to the equator.
(iv) The surface currents are deflected by the continents and the
Coriolis effect (to the right in the northern hemisphere and to the
left in the southern hemisphere). This creates warm currents along
the eastern coasts of the Americas, Australia, Asia and Africa. In the
North Atlantic this warm current is called the Gulf Stream which
brings warm conditions to north-west Europe. The Gulf Stream
forms the western and northern parts of the North Atlantic sub-
tropical
gyre
. The five subtropical gyres (see Figure 1.3) are the
