In the ocean as a whole, salt water is on the surface - because it is hotter than water with less salt. These two regions do not mix except in some special cases, as happens in the thermoaline circulation.
The part containing the highest salt concentration is denser than the part containing the least salt. Thus, when a part of the ocean that contains higher salinity comes in contact with a part with lower salinity, a current forms. The region of higher density (with higher salt concentration) is swallowed and submerged by the region of lower density (with lower salt concentration). This submersion creates a very large and slow current called thermoaline circulation.
Thus we can say that thermohaline circulation repeats in a cycle because the new water cools and sinks.
"Surface ocean currents are primarily driven by winds. Deep ocean currents, on the other hand, are mainly a result of density differences. The thermohaline circulation<span>, often referred to as the ocean's "conveyor belt", links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans." </span>
