In general at high enough depth the ocean temperature is constant[0]. At high latitude like the southern ocean it's constant at whatever depth. I think the surface ambient temperature is below(cooler) the temperature where the water density is the highest around 4 degrees for pure water(water has negative thermal expansion which causes it to expand and float!), for southern ocean salinity is between 33-34 and maximum density is below 0[1] but still ambient might be lower which means the colder water is lighter.

https://en.wikipedia.org/wiki/File:ThermoclineSeasonDepth.pn...

https://en.wikipedia.org/wiki/File:T-S_diagram.pdf