Download Surface currents: See the map in your main notes for the surface

Surface currents:
See the map in your main notes for the surface currents. Note the strong equatorial and
circumpolar currents. The prevailing winds, the spin of the Earth (Coriolis effect) and the
shape of the ocean basins mean that the surface currents circulate in giant loops or circuits
called “gyres”.
Surface ocean currents:
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Transfer water and heat from tropical to polar regions
Influence weather and climate
Distribute nutrients and scatter organisms
How surface currents are formed:
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10% of the ocean flows in currents on the surface of the ocean.
Water flows horizontally in the upper 400 metres of the ocean, mostly above the
pycnocline.
The largest surface currents are organized into huge circuits known as gyres.
Some are rapid and like rivers, with well-defined boundaries; others are slow and
wide.
Four forces form the ocean’s surface currents:
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The Sun’s heat: The sun warms the Earth's surface and the atmosphere. Warm air
rises, becoming less dense. Dense cool air moves in and replaces the rising warm air,
forming winds. Different amounts of solar energy are absorbed at different latitudes.
The tropics receive direct sun; the rest of Earth receives indirect sun.
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Surface winds: These are formed when hot, less dense air rises and cooler, denser
air rushes in to replace it. The easterly trade winds and the westerlies move the
water in the direction the wind is blowing, forming surface currents.
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Wind friction: Friction between the wind and the water surface causes the water to
move.
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The Coriolis Effect: the Earth's rotation causes moving air or water in the Northern
Hemisphere to curve to the right and in the Southern Hemisphere to the left. The
Coriolis Effect is stronger at the equator and weakest at the poles.
Ekman spiral in the Southern Hemisphere: The force due to the wind and wind friction is
balanced by the Coriolis force. In the SH this is 900 to the left. See diagram in notes.
Ekman spiral in the Northern Hemisphere: The force due to the wind and wind friction is
balanced by the Coriolis force. In the NH this is 900 to the right. See diagram in notes.
The Ekman spiral is only formed in the mixed layer and doesn’t go through the pycnocline.
Ekman transport piles up surface water in some areas and removes water from other areas,
producing variations in the height of the sea surface, causing it to gradually slope.
Coastal upwelling and downwelling:
See diagram in notes.
Upwelling: Where water from the deep sea travels up to the surface, from a depth of about
200-300m. Upwelling often happens where wind blows along a coastline and Ekman
transport causes the water at the ocean surface to move away from the coast. Water from
deeper in the ocean rises up and takes the place of the moved away water. Upwelling areas
are full of marine life.
Downwelling: the opposite of upwelling - surface waters push down into deeper areas of
the ocean. This happens when winds cause Ekman transport to push water towards a coast
and then down into the ocean
There are 4 types of surface currents:
Western boundary currents: narrow, deep, fast currents found at the western boundaries
of ocean basins. They carry water and heat from the equator towards the poles and help
moderate our climate.
Eastern boundary currents: cold, shallow and broad, and their boundaries are not well
defined. They transport water from the pole to the equator.
Transverse currents: these currents close the tops or bottoms of gyres. At the equator
they are called the North and South Equatorial Currents. The Antarctic Circumpolar current
(ACC) is the strongest surface current and carries the largest volume of water per second.
Counter-currents - are another type of transverse current. The subtropical gyres in each
hemisphere are separated by narrow eastward-flowing equatorial counter-currents flowing
opposite to the equatorial currents.
Transport of heat by the currents:
The currents transport:
 huge masses of water
 enormous amounts of heat around the globe. Once water is heated up by the sun it
retains the heat and takes months or years to release the heat.
 transport this heat for thousands of kilometres
 In the tropics and subtropics, 37o either side of the equator, more heat arrives at the
Earth’s surface, on average, than it releases.
 In the higher latitudes less heat arrives than is released. The ocean releases heat on
an immense scale.
 The atmosphere and the oceans transport energy north and south from the equator
to compensate for the imbalance.
 The Atlantic and Pacific Oceans each carry heat northward from the tropics and subtropics. However, the Indian Ocean moves very little heat.
 The THC transports more heat than wind driven surface currents.