The oceans contain approximately 97% of the water on Earth, and play a very important role in storing and circulating heat around the globe. The surface of the ocean receives heat from the sun, and this warmer water is less dense than colder water, so it sits at the top of the water column, above the colder, denser masses of water. Winds are the primary driver of ocean circulation at the surface, and as they push the warm surface water, the movement carries down to the layers below, and so the water at all levels is constantly in motion. In the tropics, the warm water at the surface tends to be pushed by the winds to the centers of large rotating gyres of water (see image below), but some of it moves toward the poles. When this warm water approaches the poles, its temperature drops, and as it becomes denser it begins to sink. It then begins to move back toward the equator, sliding underneath the now warmer and less dense surface waters. At higher latitudes, such as around northern Europe, when the warm water begins to cool, its heat is lost to the atmosphere and contributes significantly to warming the air.
