But in the ocean it often happens that the reverse happens: so we might get something that's heavy, heavier (like) more dense waters on top of lighter water. And when that happens, we say that the ocean is unstable and we have "overturning". That means that the waters that are heavy come down to the bottom part of the ocean and those that are light need to go up. And that happens when waters are cooled and it also happens when the upper waters get saltier because of evaporation, etc.
So those density differences mean that the ocean is going to circulate: it's going to move. And those density differences can come about because of temperature changes or salinity changes. Credit: Susan Lozier.