Density currents
Density currents are vertical currents of ocean water that result from density differences among water masses. Denser water sinks and slowly spreads out beneath the surface. An increase in seawater density can be caused by a decrease in temperature or an increase in salinity. Processes that increase the salinity of water include evaporation and the formation of sea ice. Process that decreases the salinity of water are precipitation, runoff from land, icebergs melting, and sea ice melting.
High latitudes
Deep-ocean density currents usually begin in high latitudes at the surface where surface water becomes cold and salinity increases as sea ice forms. As it becomes dense enough, it sinks, initiating deep-ocean density currents. When the water sinks, it's removed from the physical processes that increased the density in the first place. During this time in the deep ocean, the temperature and high latitudes remain unchanged. It's why oceanographers are able to track the movements of density currents in the ocean.