Geoscience Reference
In-Depth Information
The problem now is that the increase of GHG in the atmosphere leads to enhanced
melting of ice in the north, including in the Arctic Basin, generating a decrease of the sur-
face layer density. With the energy available for surface layer and interior mixing being
essentially the same as before, this implies a possible decrease of the deep water formation
and an associated decrease of inflow from the south of warm and saline water. This, then,
counteracts the warming in the north. The critical numbers in this conjecture are the dens-
ity difference between surface and deep waters and the amount of energy available for the
interior mixing. If this is only provided by the tidal motion it may be assumed to be essen-
tially constant.
Strong variations of the North Atlantic circulation have been observed during past
decades. These appear coupled with regular low frequency changes of the NAO, which
shows an overall increase since the early 1960s. Sedimentary records suggest that the flow
of warm high-salinity water presently reaching the northern North Atlantic was practic-
ally cut off during the last glacial maximum. Related oceanic and atmospheric fronts were
then pushedsouthwards, andthe NorthAtlantic current was diverted towards Portugal. The
formation of deep water in the North Atlantic was then reduced, leading to a related de-
crease in heat transfer to the north - a positive feedback.
Ocean circulation and climatic conditions are closely linked. The ocean has a much
longer 'memory' than the atmosphere. Researchers at the Geological Survey of Denmark
and Greenland and the Universities of Aarhus and Gothenburg (Kuijpers, personal commu-
nication 2010) have found a link between sea surface warming in the North Atlantic and
cold winters in Europe. The periods of cold winters may be linked to the variability of SST
in the North Atlantic called the Atlantic Multidecadal Oscillation. The multidecadal data
indicate that we may be entering a similar climate regime to that in the 1940s, with notice-
ably colder winters than in previous decades. The multidecadal oscillation of the SST does
fit with the low frequency changes in the NOA and variations of the North Atlantic circu-
lation observed during the last couple of decades.
The observation that warm SST in the North Atlantic appears linked with cold winters
in Europe seems to fit with the observation of opposite oscillation between the Labrador
