Models for climateprediction 99The ocean’s deep circulation
For climate change over periods up to a decade, only
the upper layers of the ocean have any substantial
interaction with the atmosphere. For longer periods,
however, links with the deep ocean circulation be-
come important. The effects of changes in the deep
circulation are of particular importance
Experiments using chemical tracers, for in-
stance those illustrated in Figure 5.20 (see next
box), have been helpful in indicating the regions
where strong coupling to the deep ocean occurs. To
sink to the deep ocean,water needs to be partic-
ularly dense, in other words both cold and salty.
There are two main regions where such dense
water sinks down to the deep ocean, namely in the
north Atlantic Ocean (in the Greenland sea between
Scandinavia and Greenlandand the Labrador sea
west of Greenland) and in the region of Antarctica.
Salt-laden deep water formed in this way con-
tributes to a deep ocean circulation that involves
all the oceans (Figure 5.18) and is known as the
thermohaline circulation(THC).
In Chapter 4 we mentioned the link between
the THC and the melting of ice. Increases in the ice
melt can lead to the ocean surface water becoming
less salty and therefore less dense. It will not sink so
readily, the deep water formation will be inhibited
and the THC is weakened. In Chapter 6, the link be-
tween the THC and the hydrological (water) cycle
in the atmosphereis mentioned. Increased precipi-
tation in the north Atlantic region, for instance, can
lead to a weakening of the THC.DEEPSALTYCURRENTFigure 5.18Deep water formation and circulation. The deep salty current originates in the north
Atlantic. Northward flowing water near the surface that is unusually salty becomes cooler and even saltier
through evaporation, so increasing its density and causing it to sink.