The circulation of the water in the oceans is driven by the temperature and the salinity. It is a relatively simple principle that creates the movement of the water and creates the ocean currents.
The cold water is denser, so it sinks deeper. The cold water has lower salinity as well, as the evaporation in the higher latitudes is lower. It moves toward the lower latitudes in the form of deepwater currents, or better said it is moving toward areas with less dense water and pushes the water with smaller density. As this water gets to areas that are warmer it starts to warm up. Becoming warmer and warmer makes it less dense, the salinity constantly increasing because of the evaporation, so it is pushed up toward the surface.
Once it reaches the surface, the water is becoming even warmer and less dense, so in the form of surface currents it starts to move toward the higher latitudes. This continues to happen constantly, with periodic changes in the patterns of the currents, which depends a lot on the global climate and the position of the continental masses.
