the global ocean is a single body of water though.

Just as the atmosphere is a single body of air. Yet both have currents (or ‘winds’), seasons, regions, hemispheres and all sorts of details.

One reason for the special relevance of the North Atlantic is the Atlantic meridional overturning circulation (AMOC).

Severe weakening of the AMOC has the potential to cause an outright collapse of the circulation, which would not be easily reversible and thus constitute one of the tipping points in the climate system.[10] A shutdown would have far greater impacts than a slowdown on both the marine and some terrestrial ecosystems: it would lower the average temperature and precipitation in Europe, slashing the region’s agricultural output,[11] and may have a substantial effect on extreme weather events.[12]