You assume too much. First, colder artic water migrates south in the winter so that the temperature difference is relatively the same. Cooler water might be moving north from the equator, but at the same time extremely cold water is moving south from the artic.

Second, some temperature difference would shut it down. If water moving up from the equator became nearly as cold as the artic water, then the system would stop working. Now, that’s an extreme and I'm not saying that’s going to happen. What I'm saying is that a temperature change of 10 degrees could very well alter it significantly in direction and strength, a change that might not correct itself after the temp's are back to normal.