Sailors have long been aware of currents on the ocean
surface that could carry them huge distances off course if they
did not take precautions. But it is only in the last half-century that
oceanographers have become aware that these currents are simply
part of a gigantic circulation that extends through the depths of
the ocean on a global scale.
Ocean circulation is driven by persistent winds at the surface
and density differences lower down. The pattern of wind-surface
currents is dominated by five great loops or ‘gyres’: two in the
Atlantic, two in the Pacific, and a fifth in the Indian Ocean.
Pulled by the prevailing winds, they are curled into loops firstly
by running up against land barriers and secondly by the effect
of the Earth’s rotation.
The Earth’s rotation is crucial because, as it spins, wind and water
moving across its surface are deflected from their courses – a
phenomenon known as the Coriolis effect. As they are blown
by the wind, currents are deflected to the right in the northern
hemisphere and to the left in the south. The deflection increases as
you go deeper, creating a spiral of deflection known as the Ekman
spiral, after the Swedish oceanographer Vagn Ekman (1874–1954).
On average, the current flows at right angles to the prevailing wind.
The effect creates clockwise-circulating gyres in the northern
hemisphere and counterclockwise gyres in the south. Driven
westward in the tropics by easterly trade winds, currents turn to
flow away from the tropics up the western edges of the oceans as
warm ‘western boundary currents’. Then, driven back east across
the ocean by westerly winds in the mid-latitudes, they turn again
to flow back to the tropics down the eastern edge of continents as
cool ‘eastern boundary currents’.
Western boundary currents are narrow, fast-flowing rivers in
the sea. The Gulf Stream is 100 km (60 miles) wide and about
1,000 m (3,280 ft) deep, and reaches speeds of 2.5 m (8 ft) a
second. As it passes Newfoundland, it carries 150 million cu. m
(196 million cubic yards) of water every second – about 250 times
as much as all the rivers emptying into the Atlantic combined.
Such vast currents carry huge quantities of heat from the tropics
to the mid-latitudes, and play a big part in making these regions
habitable for wildlife, and for humans, too. The British Isles,
for instance, owes its mild climate, which would otherwise be
Siberian, to the Gulf Stream.
Eastern boundary currents, such as the Canary Current and the
California Current, by contrast, are much broader and weaker.
However, they generate upwellings of nutrient-rich water from
the deeps in eddies around capes and promontories. The feast
these upwellings provide attracts huge numbers of fish, making
eastern boundary currents some of the world’s richest fisheries.
Deep water circulation
In the ocean deeps, water is moved by variations in water density.
Because these density variations depend on heat and salt, it is
known as the thermohaline circulation. As water cools or gets
saltier, it becomes denser and sinks. But this simple process
generates movement on a massive scale, and it moves water
right around the world in what is sometimes known as the
Great Ocean Conveyor.
In polar regions, dense water tends to sink into the deep ocean
basins. Polar water is not just denser because it is cool but because
when it freezes into ice sheets, it leaves the salt behind in the
unfrozen water. Even the warmer water brought in by western
boundary currents is dense, because evaporation leaves it saltier.
In the North Atlantic, this dense water, sometimes known as
North Atlantic Deep Water (NADW) slowly sinks and starts to
push dense water southwards ahead of it. It flows deep down
through the Atlantic right across the equator to meet the even
denser Antarctic Bottom Water (AABW). The circulation moves
slowly eastwards deep in the Southern Ocean, past South Africa
and Australia, to run through the Pacific before finally rising
to the surface and warming up to flow westwards through the
Pacific and Indian oceans into the Atlantic in the top layers of the
ocean. Finally, it heads north through the Atlantic to Greenland
to start the cycle again. It takes a millennium or more for water to
go right around – so water coming back to Greenland now began
its journey in the time of the Vikings.