AMOC
AMOC (Atlantic Meridional Overturning Circulation) is the component of surface and deep currents in the Atlantic Ocean responisble for regional climate. The flow of warm, salt water from the south is part of the thermohaline circulation . This is an important component of Earth's climate system. The temperature in the northern europe region is warmer, as warm water from the south stays on the surface and makes for a moderate climate. The southern region evaporates water and produces salty water. The salt water is denser and should sink, but the temperature cancels this out.
The gulf stream windws blow the warm water across the atlantic ocean to europe slowly cooling down... Now the water is getting denser, and colder, making it even more dense. Where it reaches the most northern parts it connects to an ice sea and freezes. The water becomes more salty and colder. Resulting in denser water that sinks. The water sinks to the bottom of the ocean (similar to a waterfall).
Disturbance
When the polar ice melts the salinity (saltyness) of the water decreases. making it less effective in sinking. The rising temperature will also release more sweet water, lowering the salinity further. The thermohaline circulation process will slow down. (as worst case scenario it will stop completely.). The northern regions will cool down as delivery of warm water slows down.
The last time this happened was during the Little Ice Age (a period between about 1300 and 1870 during which Europe and North America were subjected to much colder winters). The thermohaline effect will eventually restart.
Reference: water density is a factor of salinity and temperature: warm water is less dense, goes up, salt water is dense and wants to go down. (in the southern regions these two cancel each other out), more north we have cold water and high salinity - the dense water sinks
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