Adding heat to the ocean, in contrast, slows down the overturning circulation because ocean currents depend on temperature gradients — moving from warmer locations to cooler locations — that
weaken under global warming as cooler waters heat up.
I also have a hard time understanding how the Gulf Stream — the western boundary current of the North Atlantic gyre — would
weaken much
under global warming, as it is driven by a physical process, gyre circulation, amplified by the presence of the coastline: http://oceanmotion.org/html/background/western-boundary-currents.htm