How does seawater density change when temperature decreases?

In oceanography, the density of seawater is primarily a function of salinity, temperature, and pressure. When considering the effects of temperature and salinity alone, oceanographers often use a unit called sigma-t (σ_t) to represent density.

The density of seawater increases as the temperature decreases. Unlike fresh water, which reaches a maximum density at 4 °C, seawater with a salinity exceeding 24.7 parts per thousand does not pass through a density maximum as it cools. Consequently, as seawater is cooled at the surface, its density continues to increase until it reaches the freezing point. This process drives convective overturn, where the denser, colder surface water sinks, contributing to the thermohaline circulation that replaces deep ocean water with water from the surface.1