Geology Reference
In-Depth Information
North Atlantic
deep water
Pacific
Ocean
Gulf Stream
Pacific
Ocean
Indian
Ocean
Atlantic
Ocean
Figure 1.8
Major routes of the thermohaline circulation that circulates cold water (blue) and warm water (red)
between the deep and surface ocean worldwide. The asterisk marks the location of Odden ice where warm
surface water prevents the expansion of the ice cover. (For color detail, please see color plate section).
during the summer months. Freezing and thawing of the
two primary cryosphere work like a seesaw of oceanic
current—like a narrow board pivoted in the middle (the
equatorial region in this case) so that as one goes up, the
other goes down. Freezing and thawing of annual sea
ice in the primary cryospheres affect the movement of
ocean water in two ways: by rejection of salt to the under-
lying water during sea ice formation and growth and by
melting large volumes of ice that has drifted away from
the polar areas during the summer. The first process initi-
ates what is known as thermohaline oceanic circulation
while the second disturbs it. Thermohaline circulation
(also known as the global ocean conveyor belt) is a large‐
scale pattern of seawater motion around the world driven
by vertical density gradient. The adjective thermohaline
is composed of two syllables:
thermo
, which refers to
temperature, and
haline,
which refers to salt content.
Changes in water density are associated with changes in
its temperature but are caused mainly by the processes of
salt rejection during the initial formation and growth of
ice covers and throughout their lifetime (see Section 2.3.3
for details).
Figure 1.8 shows the major routes of the thermohaline
circulation worldwide. Since the saltier water under the
ice is heavier than the deep water, a density gradient is
developed that causes water to sink at the location of ice
formation and growth. This triggers the circulation in the
oceans around world. The cold and dense polar water
starts to move along the ocean bottom toward the equa-
tor while warm water from middepth to the surface level
travels from the equator back toward the poles. Much of
the world oceans' deep and bottom water is believed to be
formed in polar latitudes as a result of ice formation and
growth. Obviously, major changes in the amount of newly
formed sea ice can disrupt normal ocean circulation
[
Maykut
, 1978;
Carmack
, 1986]. Figure 1.8 also shows the
location of formation of what is known as Odden sea ice
in the Greenland Sea. Odden is a Norwegian word for
headland. This is the crucial location for the initiation of
the thermohaline circulation. The circulation takes the
form of a large tongue that extends over 250,000 km
2
[
Comiso et al.
, 2001], and it can rapidly expand or shrink
over a period of a few days. In some winters it persists for
months, while failing to form in others.
As shown in Figure 1.8, the circulation is closed in the
Greenland Sea as the warm water moving from the
south near the ocean surface level reaches that high‐
latitude area and starts to cool. The cooler water
becomes dense and therefore sinks. The circulation con-
tinues afterward. The cycle may be slowed down if sig-
nificantly less salty water (or freshwater) is provided. In
this case, the sinking of seawater will be activated only
by the colder and denser water at the surface. The cycle
may be interrupted if much less sea ice is formed or the
atmospheric temperature is maintained at the warm
side so that warmer water coming from the south will
not be cooled enough to make it denser. This anomaly
is called the “great salinity anomaly.” This phenomenon
was discovered during the late 1960s and early 1970s in
the Nordic Seas (this includes the Greenland, the
Norwegian and the Iceland Seas). It was later observed
in the 1980s and 1990s. A notable study that addresses
this phenomenon is presented in
Häkkinen
[1999]. The
question that would be raised at this point is: What
would give rise to the major impulse of freshwater that
causes this anomaly? As mentioned before, excessive
rain, snow, or river runoffs are possible answers. Yet,
excessive melting of glaciers and icebergs is a much more
