[jambutty]

As the NAD progresses northwards it meets the cold water of the North Atlantic, cools and becomes more dense than the water it meets. This causes it to sink and underwater currents take it back towards the Mexican Gulf.

If or maybe rather when the North polar ice cap melts it chucks squillions of tons of very cold water into the North Atlantic and it travels southwards and meets the NAD further south than before. Thus the above process happens further south than normal.

The more melt there is from the North polar ice cap the further south the GS sinks.