Answer:
Upwelling and Downwelling
Step-by-step explanation:
The Earth's oceans have differing temperatures and sediment levels. Water with more sediment is usually colder, and thus is more dense and sinks. Water with less sediment is usually warmer, and thus is less dense and floats. Through a global conveyor belt of water, cold water starts at the the poles, cycles to the equator, where it is warmed, and then cycles back to the poles.