How Ocean Currents Function

If you've ever lost a hat or a pair of sunglasses at sea, you're familiar with how dynamic the ocean is. If you didn’t grab the item right away, it was probably on its way around the world, carried by ocean currents.

When we refer to water as a 'current,' we mean the movement of the water itself. Currents can occur in rivers, lakes, marshes, and even pools. But none of these bodies of water have the complex current systems that oceans do. From the predictable flow of tidal currents to the unpredictable nature of rip currents, ocean currents can be driven by factors like tides, winds, or water density differences. They have a huge impact on weather patterns, marine travel, and nutrient cycles.

So, how do ocean currents affect us? For one, they help maintain warmer climates in Western Europe, support abundant plant and animal life in Antarctica, and their disruption may have triggered a mass extinction, wiping out 95% of marine species 250 million years ago [source: NOAA: "Ocean"]. Some ocean currents even move entire oceans, flipping their waters upside down about every 1,000 years [source: NOAA: "Ocean"].

Understanding ocean currents is critical for sectors like shipping and fishing, and also plays a key role in search-and-rescue efforts, hazardous material cleanups, and activities like swimming and boating. By using both forecasted and real-time data on current patterns, boaters can safely dock and undock their vessels, rescuers can predict the likely drift of a missing person, cleanup teams can forecast the movement of spills, and surfers can position themselves to ride the best waves.

If you're interested in learning about the local currents that drag you out to sea when you hit the beach, or the global currents that circle the entire planet, this article will answer all your essential questions about ocean currents. What causes them? What shapes do they take? How do they influence ecosystems? On the next page, you'll explore the currents that occur at the ocean's surface.

Types of Ocean Currents: Surface Currents

Ocean currents found at depths of 328 feet (100 meters) or less are typically classified as surface currents. These currents, which include coastal currents and surface ocean currents, are primarily driven by wind patterns.

If you've spent time at the beach, you're probably familiar with coastal currents. These surface currents influence not only the movement of water but also wave action and the shaping of the coastline. To truly understand coastal currents, it’s helpful to first understand waves.

Winds blowing across the ocean exert a pull on the water's surface, causing energy to build up and form waves. Factors like wind speed, the distance it travels, and the duration of the wind all determine wave size. Strong winds that blow for a long time and over vast distances can produce large waves. Waves break when they reach the ocean floor, become unstable, and topple over onto the beach.

The energy released when waves crash onto the shore gives rise to longshore currents. When waves approach the coastline at an angle rather than directly, part of the wave’s energy moves perpendicular to the shore, while the rest moves parallel. This parallel energy creates the longshore current, which flows along the shoreline. If you’ve ever been in the ocean and felt the current tug you down the beach, you’ve experienced the effect of a longshore current.

As these currents move, they carry sediment along with them, a process called longshore drift. Longshore drift can create narrow land formations called spits and barrier islands, which are long islands running parallel to the coast. These islands are constantly reshaped as longshore currents continue to move, collect, and redistribute sand.

Rip currents are a type of coastal current that occur when underwater land features block waves from flowing back to the ocean. You’ve likely seen warnings on the beach about rip currents. They form when waves, having already crashed, funnel out of a narrow gap, such as a break in a sandbar, with significant force. Think of how water rushes out of a bathtub drain when you open a small outlet—this is similar to a rip current. For more details, you can explore "How Rip Currents Work."

Another coastal current, upwelling, takes place when wind moves surface water aside, causing deeper water to rise and fill the gap. In contrast, downwelling occurs when wind pushes surface water toward the coast, causing the accumulated water to sink. Both of these processes can also occur in the open ocean.

Upwelling and downwelling are vital to the ocean's nutrient cycles. Deep, cold waters are rich in nutrients and carbon dioxide, while the warmer surface waters contain more oxygen. When these layers swap places, so do the nutrients and gases.

Downwelling stops dissolved oxygen from being used in the breakdown of organic material at the surface, preventing the growth of anaerobic bacteria and the build-up of toxic hydrogen sulfide. On the other hand, upwelling brings a surge of nutrients from colder, deeper waters, supporting ecosystems in unexpected places like Antarctica, where life thrives thanks to the influx of these vital resources.

Coastal currents are driven by local winds, but surface currents in the open ocean are influenced by global wind patterns. On the next page, you'll explore these ocean currents in more detail.

Explore More Surface Currents

By now, you’ve probably noticed that wind and water are deeply connected. To understand surface ocean currents, which occur in the open ocean, it's important to grasp how the winds that drive them work.

Some of these wind patterns are influenced by the Coriolis force. If the Earth didn’t spin, winds would travel in straight lines across the globe. However, because the Earth rotates, winds curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection is called the Coriolis effect.

In the Northern Hemisphere, the powerful trade winds blow from the northeast towards the west, dragging the surface waters of the ocean along with them, particularly near the equator. Due to the coastline and the Coriolis effect, this warm-water current shifts northward, turning at around 30 degrees north latitude. At this point, the westerlies take over and guide the current eastward and southward after it encounters land. Together, these two wind patterns form a continuous circular motion, flowing clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere .

These circular wind patterns generate spiral ocean currents, known as gyres. There are five major gyres located both north and south of the equator: the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Smaller gyres can also be found at the poles, with one circulating around Antarctica. Smaller, short-lived currents often branch off from both large and small gyres.

The Gulf Stream is a powerful current within the North Atlantic gyre that transports warm water northward from the Gulf of Mexico, traveling along the eastern coast of the United States and reaching Western Europe. As a result, people in Florida’s east coast enjoy cooler summers and warmer winters compared to surrounding regions, while Western Europe experiences much milder winters than other areas at the same latitude.

If winds only impact the upper 100 meters (328 feet) of water, how do deeper ocean currents form? Learn more on the next page.

Deep Ocean Currents (Global Conveyor Belt)

Invisible to us land-dwelling beings, an underwater current moves around the globe with a force 16 times more powerful than all of the world’s rivers combined [source: NOAA: "Ocean"]. This deep-water current is called the global conveyor belt and is powered by differences in water density. Movements of water caused by such density variations are also referred to as thermohaline circulation, a term that comes from the combination of 'thermo' (temperature) and 'haline' (salinity), since water density is influenced by both of these factors.

Density describes how much mass an object has per unit of volume, essentially how compact it is. Think of a dense object like a heavy bowling ball compared to a lightweight, air-filled beach ball. With water, the colder and saltier it is, the denser it becomes.

At the poles, when water freezes, the salt doesn’t freeze with it. This leaves behind a large volume of cold, dense saltwater. As this dense water sinks to the ocean floor, more water rushes in to replace it, setting off a cycle. This process continues as the water cools and sinks again, ultimately driving a global current that circulates the world’s oceans.

The global conveyor belt starts in the cold waters of the North Pole, moving southward between South America and Africa, ultimately heading toward Antarctica. As it reaches Antarctica, it is replenished with more cold water and divides into two paths – one leads to the Indian Ocean, while the other flows toward the Pacific Ocean. Near the equator, both sections warm and rise to the surface in a process known as upwelling. When they reach their limits, the sections loop back, returning to the South Atlantic Ocean and then back to the North Atlantic Ocean, repeating the cycle.

The global conveyor belt moves at a much slower pace than surface currents, progressing at a mere few centimeters per second, compared to the faster movement of surface currents, which can travel tens or hundreds of centimeters per second. Estimates suggest that one section of this slow-moving system takes about 1,000 years to complete a full rotation around the globe. Despite its slow speed, it transports an immense volume of water—over 100 times more than the flow of the Amazon River. [source: NOAA: "Currents"].

The global conveyor belt plays a vital role in the world’s food web. As it circulates water across the planet, it enriches nutrient-deficient, carbon dioxide-poor surface waters by transporting them to deeper layers where these elements are abundant. This circulation brings nutrients and carbon dioxide from the deep ocean layers to the surface, aiding the growth of algae and seaweed, which serve as the foundation for all life forms. It also helps in regulating global temperatures.

Continue reading to discover a current that isn't driven by winds or density differences, but rather by forces that are truly beyond our planet.

Tidal Currents

Tidal currents are caused by tides, which are essentially slow, long waves generated by the gravitational pull of the moon and, to a lesser extent, the sun on the earth's surface. Because the moon is much closer to the earth than the sun, its gravitational influence is stronger on the tides.

The moon's gravity causes the ocean to bulge outwards at opposite ends of the earth. This creates higher water levels in areas aligned with the moon and lower levels at points in between. This rise in water level is accompanied by a horizontal movement of water, known as the tidal current.

Unlike other currents, tidal currents do not flow in a continuous stream and shift direction each time the tide moves from high to low. While tidal currents have minimal effect in the open ocean, they can flow rapidly at speeds of up to 15.5 miles (25 kilometers) per hour in narrow regions like bays, estuaries, and harbors. Fast tidal currents can stir up sediment and impact plant and animal life. For example, they might carry fish eggs from an estuary to the open sea or transport nutrients vital for fish from the ocean to the estuary.

The most powerful tidal currents occur at or near the highest and lowest tides. During rising tides, when the current moves toward the shore, it is referred to as the flood current. Conversely, when the tide is falling and the current moves back out to sea, it is called the ebb current. As the positions of the moon, sun, and earth shift at a predictable rate, tidal currents can be forecasted.