Ocean currents are continuous movements of seawater driven by several interacting forces. Wind drives much of the circulation near the surface, while differences in water density caused by temperature and salinity contribute to circulation throughout the ocean, including the deep ocean. Tides also create currents, while Earth's rotation and the shape of ocean basins and coastlines influence how moving water flows.
What Is an Ocean Current?
An ocean current is the movement of seawater from one place to another. Currents exist at very different scales, from local flows along a coastline to enormous systems that move water across entire ocean basins.
Some currents are relatively fast and concentrated. Others are broad, slow movements of water that form part of the global ocean circulation system.
Currents are also different from waves. A wave primarily transfers energy through the water, while a current describes the movement of the water itself.
What Causes Ocean Currents?
There is no single cause of all ocean currents. Different parts of the ocean are moved by different forces, and those forces often interact.
The main drivers and influences include:
- Wind: transfers momentum into the ocean surface and drives much of the large-scale surface circulation.
- Temperature: affects the density of seawater, with colder water generally being denser than warmer water.
- Salinity: also affects density, with saltier seawater generally being denser than fresher seawater at the same temperature.
- Tides: create horizontal movement of seawater known as tidal currents.
- Earth's rotation: influences the direction of large-scale moving water through the Coriolis effect.
- Ocean and coastal geography: continents, coastlines and the seafloor constrain, concentrate and redirect moving water.
How Does Wind Cause Ocean Currents?
Wind moving across the ocean transfers momentum into surface water. Over large distances and long periods, persistent global wind systems move enormous amounts of water and help establish major surface currents.
The winds themselves ultimately develop from differences in atmospheric pressure created in part by the uneven heating of Earth. That wider atmospheric mechanism is explained in What Causes Wind?.
Ocean currents do not simply follow the wind in a perfectly straight line. Earth's rotation influences the path of moving water, while continents and ocean-basin boundaries redirect the flow.
The result is a network of large-scale surface currents rather than water simply moving in the same direction as the wind everywhere.
What Are Surface Ocean Currents?
Surface currents occur in the upper part of the ocean and are strongly influenced by global wind patterns. They form part of large circulation systems that can transport water across thousands of kilometres.
On an ocean-basin scale, these currents can form large rotating systems known as gyres. The combination of prevailing winds, Earth's rotation and continental boundaries helps determine their direction and shape.
Surface currents are important because they redistribute heat around the planet, moving warm and cold water between different regions.
How Does Earth's Rotation Affect Ocean Currents?
Earth's rotation affects moving air and water through the Coriolis effect. On a rotating Earth, large-scale moving water is deflected relative to the surface below it.
In the Northern Hemisphere, this deflection is toward the right of the direction of motion. In the Southern Hemisphere, it is toward the left.
The Coriolis effect does not create the original movement of the water. Instead, it influences the path taken by water that has already been set in motion by forces such as wind or pressure and density differences.
What Causes Deep Ocean Currents?
Deep-ocean circulation is influenced by differences in the density of seawater. Two of the most important controls on seawater density are temperature and salinity.
Cold seawater is generally denser than warm seawater. Increasing salinity also generally increases density. In regions where seawater becomes sufficiently dense, it can sink and contribute to large-scale overturning circulation.
Density-driven circulation associated with temperature and salinity is commonly described as thermohaline circulation, although the real global circulation system also depends on wind, mixing, pressure gradients and the geometry of ocean basins.
What Is Thermohaline Circulation?
Thermohaline circulation refers to ocean circulation associated with differences in seawater density caused by temperature and salinity. Cooling or increasing salinity can make seawater denser, allowing it to sink beneath less dense water under suitable conditions.
These density differences help connect surface and deep waters across the global ocean. Dense water forms and sinks in certain regions, while deep water is transported through ocean basins and can eventually return toward the surface through mixing and upwelling.
The global circulation system is more complex than a single conveyor belt. Wind-driven circulation, mixing, basin geometry and interactions between different water masses all contribute to the movement and exchange of seawater.
How Does Salinity Affect Ocean Currents?
Salinity changes the density of seawater. At the same temperature, water with a higher concentration of dissolved salts is generally denser than water with lower salinity.
Processes such as evaporation, rainfall, river input, melting ice and sea-ice formation can therefore change the density of seawater. Where these density changes are large enough, they can contribute to sinking, rising and the movement of water masses.
Do Tides Cause Ocean Currents?
Yes. Tides create horizontal movement of seawater known as tidal currents. These can be particularly noticeable near coastlines, in bays, estuaries and narrow channels.
A tide and a tidal current are related but are not the same thing. Tide describes the periodic rise and fall of water level, while a tidal current describes horizontal movement associated with the tide.
The underlying mechanism behind the tides is explained in How Do Tides Work?. The size of the vertical difference between high and low water is explored separately in What Is Tidal Range?.
Why Are Ocean Currents Different Near the Coast?
As moving water approaches land, it is constrained by coastlines, changes in water depth and features of the seafloor. Narrow channels can concentrate flow, while headlands, bays and underwater topography can redirect it.
Local winds, tides and waves can also generate coastal currents on much smaller scales than the major circulation systems of the open ocean.
This means the current experienced at a beach may be controlled by very different processes from a current transporting water across an entire ocean basin.
Are Rip Currents Ocean Currents?
Rip currents are a specific type of nearshore current rather than part of the major circulation systems that move water around ocean basins. They flow away from the shore through the surf zone and are closely related to breaking waves and local coastal conditions.
Because they can create a direct hazard to swimmers and other water users, they require a different type of understanding from large-scale ocean circulation. See How to Spot and Escape a Rip Current for practical safety guidance.
Why Are Some Ocean Currents Warm and Others Cold?
Ocean currents transport water and heat between different regions. Currents moving away from lower latitudes can carry relatively warm water toward cooler regions, while currents associated with higher-latitude or upwelled water can bring colder conditions into other areas.
This redistribution of heat is one reason ocean circulation has such an important influence on regional climate and coastal water temperatures.
Do Ocean Currents Affect Coastal Water Temperature?
Yes. Currents can transport relatively warm or cold water into a region and therefore influence the temperatures experienced along a coastline. Local processes such as wind-driven upwelling can also bring colder water from depth toward the surface.
Water temperature at a particular coast is not controlled by currents alone. Latitude, season, solar heating, water depth, mixing and local weather also matter.
The Atlantic coast around Big Bay, Cape Town provides a useful example. This part of southern Africa lies within the wider Benguela system, where wind-driven coastal upwelling can bring cold, deeper water toward the surface. It helps explain why coastal water temperatures can be much cooler than latitude or warm air temperatures alone might suggest.
What Is Upwelling?
Upwelling occurs when deeper water rises toward the ocean surface. Along some coastlines, winds and Earth's rotation combine to move surface water away from the coast, allowing deeper water to rise and replace it.
That deeper water is often colder and rich in nutrients. As a result, upwelling can strongly affect coastal water temperature and marine ecosystems.
The Benguela system along southwestern Africa is a major real-world example. Prevailing winds can move surface water offshore, allowing colder, nutrient-rich water to rise along the coast. The result demonstrates how wind, Earth's rotation and coastal geography can combine to create conditions that are experienced directly at the shoreline.
Why Do Ocean Currents Matter?
Ocean currents redistribute heat, nutrients, oxygen and water around the planet. They influence marine ecosystems, weather and climate, navigation and the conditions experienced along individual coastlines.
For someone standing at the coast, that global circulation can feel distant. But ocean circulation helps determine something very immediate: what kind of water reaches a coastline, where that water has come from and how warm or cold the sea can be.