Deep ocean currents are a result of varying densities of the water. The density of the water can vary due to temperature and salinity; in other words, the deep ocean currents are a result of thermohaline circulation.
At the earth's poles, the water that freezes cools the water around it. Also, salt doesn't freeze with it, so what you get is cold, salty water that gets pushed to the bottom. Heating at the earth's equator causes this water to float back up, an upwelling. These processes are the sole drivers of water around the globe. This flow goes at a few centimeters per second, and it takes around 1,000 years in order for one section of the belt in order to make it around the globe once.
