Warm water travels northwards close to the surface of the Ocean (light blue). Beneath it there is a slow deep water flow carrying cold dense Arctic waters to Antarctica along the ocean bottom (dark blue). As it travels north, water in the upper current becomes colder and denser.  Winter sea ice forming in the Arctic Ocean leaves salt in the water making it denser still.  At certain places in the North Atlantic the dense water sinks down towards the sea floor and begins to flow South. In this way the circulation ‘overturns’ to form the loop in the conveyor belt between the warm shallow current and the cold deep current.

The currents in the Gulf Stream → are mainly pushed along the surface of the ocean by the wind.  The Gulf Stream originates in the gulf of Mexico.  In the Atlantic, at around 40° North – the latitude of Portugal - it splits in two.  One current turns south and flows back off West Africa, staying on the surface.  The other continues northwards with the north-flowing current in the Atlantic Conveyor.  Here it eventually cools and sinks down to form cold deep water. Only about one-fifth of the Gulf Stream flow contributes to the overturning circulation.

The global ocean circulation is called the thermohaline circulation.  It is given this name because it is driven by density differences arising from heat (thermo-) and saltiness (-haline).  If the density of the water on the surface of the North Atlantic became too low it could stop sinking down to form ‘bottom water’.  This would make the conveyor belt stall.  The Gulf Stream wind would not stop but would turn south at low latitudes.  

We have evidence from geology that the Atlantic conveyor shut down 12,000 years ago.  At the end of the ice age the Greenland ice sheet retreated and a very large volume of melt-water flowed into the North Atlantic.  Despite being cold it was fresh so that it floated on the salty ocean, unable to sink downwards.  Within a few decades Northern Europe lost its central heating system.  The change took place over a few decades and lasted for about a thousand years.  The temperature in Western Europe dropped by about 5°C.