Abstract
In addition to surface currents linked to wind stress and the Coriolis force, ocean circulation is characterized by the worldwide transport of deep water masses via the thermohaline circulation or meridional overturning circulation, two simplified names that do not fully capture the complexity of this system. In the North Atlantic, salt water of subtropical origin gradually cools and plunges deep into several areas of the Nordic Seas and the Labrador Sea. These plunges are due to the increased density of seawater, but the upwelling in other basins is linked to the mechanical energy of wind and tides. Today, the Atlantic Meridional Overturning Circulation (AMOC) transports around 15 million cubic meters per second (15 Sv), leading to a northward heat transport of around 1 petawatt (1 PW = 10th W). This oceanic heat has a major influence on atmospheric temperatures at high latitudes in the North Atlantic and Northern Europe. Studying the current phenomenon is complex, however, as recent warming has probably had an influence on the AMOC, whose natural equilibrium has been disturbed.
A comparison of postglacial temperature variations for the different latitude bands reveals systematic contrasts, with a delay in warming in the Northern Hemisphere, which is also marked by two transient reversals at around 16 000 and 12 000 years BP. This contrast becomes evident in the curves representing the thermal contrast between the two hemispheres or the decomposition into principal components. The two increases in the inter-hemispheric gradient and in the second principal component correspond to two minima in the intensity of the Atlantic deep ocean circulation, highlighted by geochemical indicators measured in marine sediments.
