Answer:
In the Earth's polar regions ocean water gets very cold, forming sea ice. As a consequence, the surrounding seawater gets saltier, because when sea ice forms, the salt is left behind. As the seawater gets saltier, its density increases, and it starts to sink.
Step-by-step explanation:
Salt is forced out of the ice crystals when sea ice forms, causing the surrounding water to become saltier. This saltier water is denser and therefore sinks. Surface water is pulled in to replace the sinking water, which in turn eventually also becomes cold and salty enough to sink.