Thermohaline circulation. Computer artwork showing all 3 layers (shallow, middle and deep) of the large slow global currents driven by thermohaline circulation. Thermohaline circulation is a part of the large-scale ocean circulation that is driven by global density gradients created by surface temperature and salinity (salt content) differences. In certain areas near the polar oceans, the colder surface water also gets saltier due to evaporation or sea ice formation. In these regions, the surface water becomes dense enough to sink to the ocean depths. This pumping of surface water into the deep ocean forces the deep water to move horizontally until it can find an area on the world where it can rise back to the surface and close the current