Why the atmosphere? Deep ocean water is already super cold right? Plus water does a good job of absorbing heat.basically transfer the surface heat energy to deep waters,that way surface level glaciers are kept frozen?
Water is a good insulator, but not a perfect insulator. If the temperature difference between the bottom and top changes so does the heat transfer and given time the effects will spread. But most important is, warm water/moisture is less dense than cooler water, that means large heat transfer by convection currents in addition to the low conduction (inverse of insulation).