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The world’s oceans also play an important role in redistributing energy around the globe.

Because oceans cover 67 per cent of the Earth's surface, they receive around two third of the Sun's energy that reaches Earth. Oceans hold onto this heat for longer than the land does and ocean currents move this heat around.

In total, ocean currents transfer about 25 per cent of the global heat budget.

Ocean currents

Ocean currents are movements of water horizontally.

Energy is moved by ocean currents from areas of strong heating to cooler areas:

• warm currents transport warmer water from lower latitudes (nearer the equator) to higher latitudes (nearer the poles)
• cold currents take colder water from higher latitudes (nearer the poles) to lower latitudes (nearer the equator).

How ocean currents work

Ocean currents are continuous movements of water in the Earth’s oceans. They are driven by both wind and thermohaline circulation interacting with each other.

Wind moves the surface waters by friction, but away from the equator the Coriolis effect means this is diverted, to the right in the northern hemisphere and to the left south of the equator

• thermo means related to heat or temperature
• haline means relating to salt or salinity

Both temperature and salinity affect the density of water:

• the colder water is, the more dense it is
• the saltier water is, the more dense it is

Different patterns of warming and cooling control the temperature of water, Precipitaion, evaporation and river runoff all affecting the salinity.

Unlike fresh water which becomes less dense close to freezing point, cold salt water continues to become denser down to freezing point.

Near Antarctica and Greenland, cold, salty surface water sinks because it is very dense. In Antarctica this is mostly because very salty water (brine) is left behind when sea ice forms. By Greenland it is because warm and salty water becomes colder and so more dense than the cold and less salty water beneath.

So horizontal ocean currents are driven by density gradients – flowing from low density to high density.

Pattern of ocean currents

Currents do not flow directly in the direction of the density gradient. They flow in circular loops or gyres. Gyres are spiral oceanic surface currents and are found in both the Northern and Southern Hemispheres.

The map below shows the pattern of currents across the world.

You can see that:

• The currents set up circular loops or gyres. Gyres are spiral oceanic surface currents and are found in both the Northern and Southern Hemispheres.
• The main subtropical gyres flow clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. These have strong, warm, polewards flows concentrated in the western edges of each ocean. For example:
  • Gulf Stream in the north-west Atlantic
  • Kuroshio Current in the north-west Pacific
  • Agulhas Current in the south-west Indian Ocean
• In the Atlantic and Pacific Oceans, the currents make a similar pattern, although there is a stronger vertical circulation in the Atlantic.
• In the Atlantic, Pacific and Indian Ocean, there can be strong warm currents towards higher latitudes as the oceans are bounded by continents on each side.
• The Southern Ocean is different as there is no land mass connecting to Antarctica. The current is stronger and deeper but goes strongly eastwards, without a poleward flow.

The video below examines if the movement of sea water can affect the climate.

Coriolis effects

Coriolis effects prevent the flows going directly down the gradients that drive them.

This is similar to how winds in the atmosphere blow along isobars around high and low pressure systems rather than directly from high to low pressure.

Coriolis effects are strong in most areas, as oceans are very large and timescales of the currents are very long. It takes decades for water to move around the horizontal currents and around 1000 years for dense water that sinks near Antarctica or Greenland to return to the surface after filling the deep ocean waters around the whole globe.

Coriolis effects act equally whichever direction the current is moving in, but are weak near the equator where there is a shift from currents being deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The video below explains the redistribution of energy.