Wind is one major cause of ocean water circulation. Surface currents follow the pattern of wind above the ocean.
Worldwide, winds near the equator blow from the east, causing currents to travel westward. When the surface currents approach land masses, they turn north or south in deflected currents.
Deflected currents are very strong because they carry the force of water that has traveled all the way across the ocean uninterrupted. The winds about halfway between the equator and North and South Poles blow from the west—the opposite direction from equatorial winds.
The currents again traverse the oceans until they reach land masses and are deflected back toward the equator. The whole route of these surface currents forms a circular pattern called a gyre. The Indian Ocean has a smaller gyre, because it is a smaller ocean.
The difference in water temperatures is another cause of water circulation far beneath the surface. Equatorial water is much warmer than polar water because the Sun heats the water at the equator more than at the North and South Poles.
Because warm water floats on top of cold water, the colder water at the poles sinks below the warmer currents circulating from the equator. Deep in the ocean, the cold water flows back toward the equator, causing the warmer water there to rise.
Warm water rising to the surface is called upwelling. Currents that result from the difference in temperatures are called convection currents.
The circulation of ocean water happens slowly. Surface currents take about 10 years to complete their cycle. Deep underwater currents travel from the poles to the equator and back in about 1,000 years.
Volcanic activity can cause ocean water to boil and vaporize.
Upwelling occurs when warm, often nutrient-rich water rises to the surface.