This is something emerging in science the mechanical mixing of the oceans heat content.
Heat will rise, but if it is taken down in water to a lower depth (mixed) and perhaps kept there to some extend by some mechanical motion, it will take a lot of time for the heat to rise. Water is more sluggish than air.
“Kitchen physics” – a watched pot never boils

What that means is it takes a very long time for the heat to permeate through the pot from the bottom and eventually bring the whole thing to a boil, esp if it is a big pot full of water.
RE the oceans, with mechanical motion mixing the warm surface water downward, the resulting (mixed) surface water will not as warm as it would have been without the mechanical mixing. The atmosphere (and sea surface temps) these past 18 years have not been cooling, but remaining stable, which is what one would expect with greater mixing of ocean water thru a mechanical means (like stirring the pot).