Basics of Environmental Science

Earth Sciences / 63

equatorial water flowing poleward passes the eastern coasts, warming them. It is possible to see
from this how a northward deflection of the warm Kuroshio Current, off Japan, could warm the
northern Pacific with climatic consequences felt in California.

There is one important exception to the general rule and it provides another example of the strong
link between ocean currents and climate. In the North Atlantic, the North Equatorial Current flows
westward into the Caribbean, then turns north through the Gulf of Mexico, where it becomes first the
Florida Current, then the Gulf Stream. The Gulf Stream flows north-eastwards across the Atlantic. It
turns south again in the latitude of Spain and Portugal, but a branch continues to flow north-east,
passing the shores of Britain. This is the North Atlantic Drift (or Current) and it gives Britain a much
milder climate (with palm trees growing in western Scotland and almost subtropical conditions in
sheltered places on the Isles of Scilly) than Newfoundland, which in fact is to the south of Britain but
cooled by the Labrador Current. Should the Gulf Stream alter its behaviour so that the North Atlantic
Drift ceased to break away from it and the entire current turned south together, Britain and north-
western Europe would experience a much colder climate.

Atlantic conveyor

Near the edge of the Arctic sea ice, water is close to the temperature at which

it is densest and as ice forms the crystallization process separates water

molecules from salt molecules. This increases the salinity of adjacent water

and, because it contains more salt, the density of the water also increases.

This water sinks all the way to the bottom of the Atlantic, flowing south as the

North Atlantic Deep Water (NADW). It crosses the equator and continues to

the edge of the Antarctic Circle, where it joins the West Wind Drift, or

Circumpolar Current, flowing from west to east. From there the water drives

systems of currents in all the oceans, eventually returning to the North Atlantic.

The NADW removes cold water from the North Atlantic. This water remains

cold until it rises in the South Pacific. During its progress through equatorial

and tropical regions the water warms and it returns as warm water to the area

near Greenland, where it replaces the NADW that is sinking and moving south.

This circulation is of major importance in regulating the climates of the world.

This is believed to have happened about 11000 years ago and, apparently paradoxically, to have
been caused by rapid climatic warming. At that time, the last (Devensian) glaciation was coming
to an end and the ice sheets were melting. The Laurentide ice sheet, which covered much of
northern North America, discharged its water into the North Atlantic, mainly through the Mississippi
River system, releasing a huge amount of very cold fresh water which floated above the denser sea
water. This was part of the reason for the failure of the North Atlantic Drift. The other concerned
the formation of North Atlantic Deep Water (NADW). At the edge of the sea ice, sea water is very
dense. When sea water freezes, its salt is removed as the ice-crystal lattice forms, so adjacent
water is more saline than average. At the same time it is also at about 4°C, the temperature at
which water is densest. The dense water sinks beneath the less dense water and forms a slow-
moving current close to the ocean floor, flowing towards the equator. This is the NADW and the
sinking water is replaced by northward-flowing surface water. It is this system that controls the