Ocean currents and the transfer of heat

Overview

Upwelling and why does it matter for the fishing industry, thermohaline circulation and what a world map of ocean currents would show.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What drives the surface ocean currents?

   Model answer

   Surface currents are driven mainly by the prevailing winds dragging on the water, so the pattern of currents largely mirrors the pattern of global winds. The Coriolis effect deflects the moving water, and the shape of the continents forces the flow into closed loops within each ocean basin. Differences in water density caused by temperature and salinity drive the deeper circulation.

2. 2

   What is a gyre, and which way does one rotate in each hemisphere?

   Model answer

   A gyre is a large closed loop of surface currents that circulates around an ocean basin. In the Northern Hemisphere gyres rotate clockwise and in the Southern Hemisphere anticlockwise, because the Coriolis effect deflects moving water to the right of its path north of the equator and to the left south of it. Each gyre therefore carries warm water polewards along the western side of the ocean and cool water equatorwards along the eastern side.

3. 3

   What is a warm ocean current and how does it affect the climate of a coast it reaches?

   Model answer

   A warm current carries water that is warmer than the average for the latitude it is flowing into, usually moving polewards along the western side of an ocean basin. Air passing over it is warmed and moistened from below, which makes the air unstable and encourages uplift, so coasts washed by warm currents have mild winters, a small annual temperature range and reliable precipitation. Examples are the Gulf Stream and North Atlantic Drift, the Kuroshio off Japan, the Brazil Current and the East Australian Current.

4. 4

   What is a cold ocean current and how does it affect the climate of a coast it reaches?

   Model answer

   A cold current carries water that is colder than the average for the latitude, typically moving equatorwards along the eastern side of an ocean basin, where upwelling of cold water from depth often reinforces it, although cold currents also run equatorwards on western margins in the high latitudes. Air passing over it is chilled from below, which makes the lower air stable and suppresses uplift, so coasts washed by cold currents are cooler than their latitude suggests, very dry, and prone to persistent sea fog. Examples are the Labrador Current, the Canary Current, the Benguela Current, the Peru or Humboldt Current and the California Current.

5. 5

   How does the North Atlantic Drift affect the climate of the British Isles?

   Model answer

   It is the north-eastward continuation of the Gulf Stream and brings water from the Gulf of Mexico across the Atlantic to north-west Europe. The prevailing south-westerly winds blow across this warm water and arrive already warmed and moistened, so British and Irish winters are several degrees milder than the average for 50 to 60 degrees North, ports stay ice-free all year, frost is uncommon on western coasts and rainfall is reliable. Its influence is strongest in winter and weakest in summer, which is one reason the annual temperature range is so small in the west.

6. 6

   Two places lie at almost the same latitude, one on the Atlantic coast of Ireland and one on the coast of Labrador in Canada. Why is January so much colder in Labrador?

   Model answer

   Ireland is washed by the warm North Atlantic Drift and receives onshore south-westerly winds that have crossed that warm water, so January means stay around 5 to 7 degrees Celsius. Labrador is washed by the cold Labrador Current flowing south from the Arctic and receives offshore winds that have crossed the frozen interior of Canada, so January means fall well below freezing and the sea ices over. The contrast shows that ocean currents and wind direction can matter more than latitude.

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