Below about 1,000m the temperature and salinity are pretty uniform. Subtle variations in T and S in the deep ocean reflect and identify different water masses. Controls on the density of sea water. We've discussed this in several previous lectures. But now the issue is more important because density dictates whether surface waters will sink and become part of the deep-ocean layer and deep circulation.
Temperature is the most important factor controlling density of surface waters in the open ocean.
Cold waters formed during seasonal cooling at high latitudes tend to sink into the deep ocean. Salinity is an important influence on density at high latitudes in the open ocean where surface waters are uniformly cold. Small increases in salinity, such as during seasonal sea-ice formation, controls which cold surface waters will actually sink. Because water and sea water are slightly compressible, density increases with pressure. So, as sea water sinks, its density increases.