Seawater moves through the sea and normal circulation patterns transport most of the water. Seawater is moved along the deep sea like a conveyor belt and takes hundreds of years which is why its called the Global Conveyor Belt.
Differences in temperature and salinity changes the density of the seawater which slows down the process of the global conveyor belt.
The cause of deep ocean and surface currents is when dense water pushes deeper water out of its way and that water moves along the bottom of the ocean. This deep water mixes with less dense water as it flows. The surface currents move water into the space vacated at the surface where the dense water has sank. Water also sinks into the deep ocean off the Antarctica.
Upwelling occurs along the coast when the upwelling forces denser water from below to take the place of less dense water at the surface that is pushed away by the wind and blows water strongly away from the shore.
La Niña is sometimes referred to as the cold phase and El Niño as the warm phase. This effects ocean temperatures and global weather.