Environmental Engineering Reference
In-Depth Information
THERMOHALINE (DEEP) CIRCULATION
Surface ocean circulation overrides the vital, slower motion of the deep ocean. The
thermohaline circulation
(THC) moves at speeds of 10-50 km yr
−1
, driven by water
Figure 11.8
Ocean surface current circulation.
masses of different densities determined by temperature and salinity properties. It is
isolated from the surface by the thermocline except at points of formation and has an
average cycling time of 500-2000 years. Strong radiative cooling of warm currents, as
they approach polar oceans, complements dense
bottom water
formation in cold, saline-
enriched sea ice environments at two principal centres. North Atlantic's icy straits pour
North Atlantic Deep Water
(NADW) southwards as a tongue penetrating to 60° S,
between even denser, north-moving
Antarctic Bottom Water
(AABW) and less dense
Antarctic Intermediate Water
(AAIW). AABW is generated beneath the Antarctic
convergence and undercuts NADW to about 40° N, where it becomes trapped in deep
troughs. The same 'pincer movement' cannot occur in the Indian Ocean, and although
AABW also spills into the Pacific there is no equivalent northern Pacific cold outflow.
As a result, warmer, less dense
Pacific and Indian Ocean Common Water
(PICW)
completes the circulation through a sinuous limb into the Atlantic Ocean, rejoining the
Gulf Stream. Subsidence of NADW therefore contributes directly to a convective and
density-driven system which draws a return surface flow of warm water into the North
Atlantic. Involvement of the vast bulk of Earth's 1·4 billion km
3
of ocean water
