Answer:
Thermohaline circulation occurs due to density differences caused by temperature (term) and salinity (halina) variations of waters. Due to their physical properties, colder waters are denser and therefore sink so that the space released in the shallows is occupied by warmer, less dense waters. In addition to temperature, salinity also changes water density: the greater the amount of salt present in a given body of water, the denser it becomes.
The difference in density of the water occurs both vertically and horizontally in the oceans. This is because in polar regions, the surface water of the oceans loses heat to the atmosphere because the ocean is warmer than the air. When losing heat to atmosphere, the waters cool. In addition, the process of sea ice formation expels salt, releasing amounts of salt into the water column and thereby increasing its salinity and density.
In this way, the cold and saline waters of the surface layers of the polar oceans sink toward the ocean floor, "pushing" layers of water further down. When it reaches great depths, this dense, cold and high salinity water starts to travel along the seabed towards the tropics. As latitude decreases, the water gets hotter and becomes less dense until it rises to the surface, continuing the flow of motion.
This is how the oceans, through thermohaline circulation, play a fundamental role in controlling the climate of planet Earth, acting in the heat transport of equatorial regions to higher latitudes.