Geoscience Reference
In-Depth Information
Deep-ocean circulation
The circulation of the ocean is one of the major controls on our global climate. In fact, the
deep ocean is the only candidate for driving and sustaining internal long-term climate
change (of hundreds to thousands of years) because of its volume, heat capacity, and iner-
tia. In the North Atlantic, the north-east trending Gulf Stream carries warm and salty sur-
face water from the Gulf of Mexico up to the Nordic seas. The increased saltiness, or salin-
ity, in the Gulf Stream is due to the huge amount of evaporation that occurs in the Carib-
bean, which removes moisture from the surface waters and concentrates the salts in the sea
water. As the Gulf Stream flows northward, it cools down. The combination of a high salt
content and low temperature makes the surface water heavier or denser. Hence, when it
reaches the relatively fresh oceans north of Iceland, the surface water has cooled suffi-
ciently to become dense enough to sink into the deep ocean. The 'pull' exerted by the sink-
ing of this dense water mass helps maintain the strength of the warm Gulf Stream, ensuring
a current of warm tropical water flowing into the north-east Atlantic, sending mild air
masses across to the European continent. It has been calculated that the Gulf Stream deliv-
ers 27,000 times the energy of all of Britain's power stations put together. If you are in any
doubt about how good the Gulf Stream is for the European climate, compare the winters at
the same latitude on either side of the Atlantic Ocean, for example London with Labrador,
or Lisbon with New York. Or a better comparison is between Western Europe and the west
coast of North America, which have a similar geographical relationship between the ocean
and continent—so think of Alaska and Scotland, which are at about the same latitude.
The newly formed deep water sinks to a depth of between 2,000 m and 3,500 m in the
ocean and flows southward down the Atlantic Ocean, as the North Atlantic Deep Water
(NADW). In the South Atlantic Ocean, it meets a second type of deep water, which is
formed in the Southern Ocean and is called the Antarctic Bottom Water (AABW). This is
formed in a different way to NADW. Antarctica is surrounded by sea ice and deep water
forms in coast polynyas, or large holes in the sea ice. Out-blowing Antarctic winds push
sea ice away from the continental edge to produce these holes. The winds are so cold that
they super-cool the exposed surface waters. This leads to more sea-ice formation and salt
rejection, producing the coldest and saltiest water in the world. AABW flows around the
Antarctic and penetrates the North Atlantic, flowing under the warmer, and thus somewhat
lighter, NADW (see
Figure 29a
)
. The AABW also flows into both the Indian and Pacific
Oceans.
