The circulation of the water is caused by temperature and salinity changes. Remember that salinity is the measure of dissolved salt in the water. water circulation is called thermohaline circulation.
A main characteristic of thermohaline circulation is that cold, dense and more saline water is place deeper and moves towards the south. On the other hand, warm and less saline water is place near the surface and moves towards North Pole. All this causes thermohaline circulation.
