Geoscience Reference
In-Depth Information
al convergence zone, at the thermal equator, with an associated strengthening of the north-
east trade wind. This can intensify the North Atlantic circulation gyre in mid-latitudes.
During the most recent ice age in the North Atlantic the over-turning, thermohaline
circulation, also referred to as the global ocean conveyor belt, was partially shut down. The
northern North Atlantic was much cooler than at present. The northward transport of heat
and salt of the North Atlantic current (also referred to as the North Atlantic Drift), being
the extension of the always existing Gulf Stream, was much reduced. During the cooling
period of ice formation and low deep water generation the salt export from the northern
waters was decreasing, or was weak. The inflow of low-saline melt water from the Arctic
Basin was also weak. These together implied a gradual increase of the surface layer salin-
ity and density, decreasing the difference between the top layer and the deep water. In the
polar, high-latitude regions the buoyancy provided by inputs from several sources also play
an important role in ocean forcing, together with the large-scale wind forcing. The process
may continue until a critical density difference is reached, allowing the deep water forma-
tion to start or increase, reactivating the conveyor belt. This allows for gradually increasing
inflow from the south of relatively warm and saline water.
The NAO will then gradually increase due to the warming, strengthening the wester-
lies and weakening the northward penetration. In parallel, the density difference between
surface layer and deep water increases due to the warming and inflow of low saline water
from the Arctic Basin. The tidal energy for the interior mixing remains the same. The out-
lined conceptual cyclic process may thus continue.
The northward transfer of heat driven by the thermohaline circulation in the North At-
lantic provides a warming of the present climate of north-western Europe by 5 to 10 °C
(North and Duce,
2002
). Data from ice cores as well as sedimentary records confirm that
age several flips in the circulation occurred, causing major climate change in this region,
now referred to as Dansgaard-Oeschger events. Cold climate episodes could start with a
temperature drop of over 5 °C over Greenland over a few decades or less, but persisting for
centuries.
