Ocean currents that occur at 328 feet (100 meters) deep or above usually are classified as surface currents. Surface currents, which include coastal currents and surface ocean currents, are driven primarily by winds.
You're likely familiar with coastal currents if you've ever gone to the beach. These surface currents also affect wave and land formations. In order to better understand coastal currents, it helps to first understand waves.
As winds blow across the ocean, they pull on the water's surface, and the buildup of energy forms waves. The speed of the wind, the distance it blows and the length of time that it blows all affect the size of waves. If the wind blows fast, for a long time and for a long distance in the same direction, large waves form. Waves break when their bases encounter the sea floor and they become unstable, toppling over onto the shore.
The energy released when waves break on the beach creates longshore currents. When waves approach the beach at an angle rather than head on, part of the wave's energy is directed perpendicular to the shore and part of it is directed parallel to the shore. The parallel energy generates the longshore current, which runs along the shoreline. If you've ever been swimming in the ocean and felt the ocean tugging you farther down the shore, then you've felt the impact of a longshore current.
As these currents travel, they pick up sediment and transport it down the beach in a process known as longshore drift. Longshore drift can form long, narrow outcroppings of land called spits, as well as barrier islands, long islands located parallel to the coast. Barrier islands constantly change as longshore currents keep picking up, moving and redepositing sand.
Rip currents are another type of coastal current that form where underwater land formations prevent waves from flowing straight back out to sea. You've probably seen signs posted at the beach, warning of rip currents. They result from spent waves (or waves that have already crashed) funneling out of a narrow opening, like a break in a sandbar, with great force. Imagine the great volume of water that rushes out of the tub when you open the small drain, and you get the general idea of a rip current. You can learn all about rip currents in "How Rip Currents Work."
Yet another type of coastal current called upwelling occurs when winds displace surface water by blowing it away and deeper water rises up to replace it. The opposite process, downwelling, occurs when wind blows surface water towards a barrier, like the coastline, and the resulting accumulation of water forces the water on top to sink. Both of these processes can occur in the open ocean as well.
Upwelling and downwelling are crucial to the cycling of nutrients in the ocean. The cold, deeper layers of water are rich in nutrients and carbon dioxide, while the warmer surface waters are rich in oxygen. When the layers trade places, the nutrients and gases do too.
Downwelling prevents dissolved oxygen from being used for the decay of organic matter at the surface, which could lead to a bloom of anaerobic bacteria and a buildup of toxic hydrogen sulfide. Upwelling, meanwhile, enables ecosystems to flourish where they otherwise would not. The influx of nutrients from deeper colder waters nourishes a wide variety of life in unlikely places, such as Antarctica.
While coastal currents are caused by local winds, surface currents in the open ocean originate from global wind patterns. On the next page, you'll learn about these currents.