The oceans are in constant motion. Ocean currents are the horizontal and vertical circulation of ocean waters that produce a steady flow of water in a prevailing direction. Currents of ocean water distribute heat around the globe and help regulate Earth's climate, even on land. Currents carry and recycle nutrients that nourish marine (ocean) and coastal plants and animals. Human navigators depend on currents to carry their ships across the oceans. Winds drive currents of surface water. Differences in temperature and salinity (saltiness) cause water to circulate in the deep ocean. The rotation of the Earth, the shape of the seafloor, and the shapes of coastlines also determine the complex pattern of surface and deep ocean currents.

Ocean water is layered. The shallowest water, called surface water, is warmer, fresher, and lighter than deep water, which...