The Earth’s warm, ocean surface currents drift into the North Atlantic region. As the surface waters are exposed to Arctic winds, they super cool and mix with salty water left over from sea ice formation in the region. This now cold, now extra salty body of water begins exhibiting new characteristics, reflective of it’s new density.
Its heavy and it begins sinking. The overall sinking motion of huge masses of surface water draws additional warm surface water northwards. The slow drift of surface waters northward is referred to as “NADC” or the North Atlantic Drift Current. The process of sinking is referred to as “AMOC” or Atlantic Meridinal Overturning Circulation. The overall process is referred to as “THC” or Thermo Haline Circulation because it is driven by differences in temperature and salinity.
The lost heat is transferred as 3-5 degrees additional heat to Western Europe by prevailing winds. The cold sinking water becomes “NADW” or North Atlantic Deep Water, which begins a two and a half year journey along the sea bottom, eventually upwelling in the tropical Pacific and Indian Ocean, moderating surface temperatures.
Climate Change dynamics have accelerated melt rates on the Greenland Glaciers.Â This has resulted in rapid freshening of North Atlantic surface water. Fresh water is less dense than salt water and “floats” on the surface, so the salinity is changing on the surface. This changing density has also changed it’sÂ characteristics. The surface water is no longer dense enough to sink.
This interruption in the NADC mechanism, which has probably been in place since post glacial times, is not yet fully understood. Trend velocity data confirms changes but modeling projections have a wide variation of predictions.
The three part image above illustrates changes in NADWÂ formation in the North Atlantic Basin.
The graph below is from a comprehensive, recent paper on this subject “An Integrated Assessment of changes in the thermohaline circulation”
Till Kuhlbrodt; Stefan Rahmstorf; Kristen Zickfield;Frode Bendiksen Vikebo; Svein Sundby; Matthias Hofmann; Peter Michael Link; Alberte Bondeau; Wolfgang Cramer; Carlo Jaeger.