You'd only think that if you're picturing the oceans as a simple...

You'd only think that if you're picturing the oceans as a simple monolithic mass. They're not: there's a constant, dynamic interplay between multiple different factors dictating the rate and pattern of convection (i.e. vertical overturning circulation). Evaporation and/or cooling increase local density and cause water to sink; more evaporation due to warm winds causes faster sinking. Strong sustained winds in one direction can cause an entire ocean to "slosh" so that the warm water piles up on one side then gets forced down and under, to be replaced by cold water upwelling on the other side. Lots of dynamic behaviour, most of which has been understood (at least in principle) for decades - which is why climate scientists have been saying that it's a bad idea to read too much into short-term noise.

Always remember that if you were to liquefy the atmosphere it would only be a few metres deep. The oceans, at kilometres deep, have a huge influence.