What causes ocean currents?

What are ocean currents?

The World’s Ocean's waters are in constant movement caused by solar radiation. There are three main types of water movements: wind waves, tides, and ocean currents. 

An ocean current is a continuous, directed movement of water that flows throughout the oceans of the Earth. The ocean current is defined as a horizontal flow of sea water through the ocean – just like a massive river flowing in the surface layers of the oceans and seas. 

Currents carry large masses of water, some as big as 100 million m3/s – and that's as much as 100 billion litres per second. Their speed varies; the highest known reaches up to 6.2 mph. 

Currents are caused by prevailing winds, such as trade and anti-trade winds, blowing from one particular direction over on surface of the Earth, depending on their location. Factors that are important in the process of forming an ocean current are also the shape of a coast where the current is forming, and Coriolis force. See how is wind formed for more information. 

Interactions with other currents can influence both the strength and the direction of the currents.

image of a sawfish swimming in the ocean

Types of ocean currents

There are two types of ocean currents based on their temperature: warm water currents and cold water currents. This is pretty straightforward – when the temperature of water in the current is higher than the temperature of the water surrounding it, we speak of warm water currents. These currents flow from the equator towards higher latitudes (e.g., Gulf Stream, North Atlantic Current, Norwegian). And, accordingly, when the temperature of current’s water is lower than that of the surrounding ocean’s, we speak of a cold water current. Those flow from higher latitudes towards the equator (e.g., Humboldt, Kamchatka, Oyashio). 

We can also distinguish types of ocean currents through their position in the water - there are two main types of ocean currents, surface, and deep water currents. 

  • Surface water currents – surface currents are movements of water that take place on the uppermost layer of the ocean - this makes up about 10 percent of the ocean's water. Surface waters are primarily affected by the wind and tend to develop rotational patterns due to the Coriolis force. They are heavily influenced by seasonal patterns. 
  • Deep water currents – deep water currents are water movement patterns that take place more than 400 meters below sea level. These currents and influenced by both temperature changes and water density. Cooler water in cold regions, such as the north Atlantic Ocean, lose heat from the water rapidly to the atmosphere, and then begins to freeze. When the water freezes, it forms sea ice, and salt is left behind, causing the surrounding seawater to become saltier and denser. When warm waters flow into this area of water, the colder dense water sinks to the bottom while warmer waters that have a lower density float to the top. This sinking and rising of water are what causes deep water currents. When dense colder water sinks, the warmer water rushes in to take its place and in turn cools and sinks, creating a continuous cycle. This process is called thermohaline circulation - thermo for temperature and haline - salinity. 

 

What causes ocean currents

Many factors contribute towards formation of ocean currents. These are: winds, water density, tides caused by gravity from the Sun and Moon, and the Earth’s rotation. Below is a list of these factors and how they affect ocean currents. 

Wind

image of a windsurfer

Surface currents are generated mostly by wind, their patterns are dictated by the directions of the winds. Surface wind-driven currents create upwelling currents in conjunction with landforms, creating deep water currents.

Coriolis force

image of whirl clouds

The Coriolis force is generated by the rotation of the Earth and causes a deflection in the winds. Winds in the northern hemisphere deflect to the right and winds in the southern hemisphere to the left. This affects the directions of the currents. 

Tides

image of a wave

The rotation of the Earth and the resulting change in the position of the Sun and the Moon causes differences in gravitational acceleration known as the tidal force. This force is the main cause of cyclical ocean level changes known as tides. Tides occur in following phases: 

  • water level rises – high tide; 
  • water reaches its peak height; 
  • water level drops – low tide; 
  • water reaches its lowest level. 

Water is at its calmest when the water level stops dropping. The tide then changes direction and "turns back". The effect of the tides is oscillating currents, known as tidal currents, which are important in coastal navigation. 

 

Differences in density of water masses caused by temperature

Temperature and density share an inverse relationship. As the temperature increases, the space between water molecules increases, which creates a decrease in density. As the temperature of the water decreases, the space between the water molecules decreases and the density increases. Cold, high-density water sinks to the bottom, and warm low-density water floats to the top. This creates a cycle.

9 major ocean currents and their locations

graphic showing the main 9 ocean currents on world's map

  1. Gulf Stream
  2. North Equatorial Current
  3. North Equatorial Counter Current
  4. South Equatorial Current
  5. South Equatorial Counter Current
  6. Kuroshio Current
  7. Alaska Current
  8. Antarctic Circumpolar Current
  9. California Current

Why are ocean currents important?

Ocean currents play an important role. If it wasn’t for the ocean currents; the earth would be a very different place. Because the Sun's energy doesn't cover the Earth's surface evenly, ocean currents play a crucial role in redistributing heat across the globe. Because the waters make up 70% of the Earth's surface, you can see why this is important. Currents from warmer area flow into cooler regions warming them up and vice versa. This creates a more even temperature across the globe, which in turn makes more regions habitable and brings life along for the ride. Warm water currents also tend to be accompanied by rain, vital to the survival of life on land. 

Ocean currents also carry a variety of organisms and nutrients with them. This is possible by up- and downwelling of the currents. 

  • Upwelling – most of the ocean's organic matter is in its depths or on the seafloor. This causes an issue because the vast majority of sea life is concentrated near the sunlit areas at the surface. When currents up-well or flow up to the surface, the currents bring vital nutrients with them. Plankton amongst other things ride these currents up to the surface and are then eaten by other creatures. 
  • Downwelling – it is just as crucial as upwelling for life to be sustained in the depths of the Ocean. It forces the ocean rich surface waters down toward the depths; this provides oxygen-rich waters for the creatures that dwell below the ocean's surface. Without this renewal of oxygen-rich water, the dissolved oxygen in the bottom sediments would quickly become used up by decaying organic matter, which would lead to a build-up of hydrogen sulphide. Only a few creatures would survive these conditions.

How do currents affect the climate?

To understand how currents affect the climate, we must first briefly look at how ocean currents affect temperature. Because oceans cover up to 70% of the Earth's surface, the majority of the solar radiation that hits the Earth is absorbed by its waters. If this heat were to remain there, these areas would become extremely hot, and areas that don't receive much solar radiation would stay extremely cold because of an uneven distribution of heat. 

Driven by a variety of factors, such as wind, gravity, and the Earth's rotation, ocean currents act like conveyor belts transporting warm water and precipitation, along with various nutrients and organisms, from the equator towards the Poles, and colder water from the Poles back to the tropics. These currents heat and cool surrounding areas resulting in a more even distribution of the solar radiation, this, in turn, effects the climate making the Earth a much more pleasant place. 

Effects of ocean currents on human life

The ocean affects human life in a multitude of ways, from stabilising temperatures to providing foods, medicines and supporting jobs and national economies. Most of this we have the ocean currents to thank. Below is a short list of a few effects ocean currents have on human life. 

  • Providing food – ocean currents help to diversities marine life, creating an abundant food source that can be consumed by humans. 
  • Regulating temperature – if it wasn't for currents, life as we know it, may not have evolved in the first place, some areas would not be habitable due to the harsh climate. 
  • Shipping and travelling via water – Ocean currents are utilised when travelling by sea as it provides a quicker route. They can also be used for navigation. 

Ocean currents - summary

  • Ocean currents are more or less a permanent, continuous directed movement of water in the Earth's oceans. 
  • Currents are generated by various forces that are acting upon the body of water, for example, the Earth's rotation or the salinity. 
  • Land masses and depth contours also have an effect on currents. 
  • Ocean currents can flow for thousands of kilometres. 
  • They are important in determining climates because they create a more even temperature distribution. 
  • Deep ocean currents are driven by water density and temperature gradients. 

Sources

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