Numbers refer to the currents shown on the map.
Warm Currents flowing from the tropical regions
1. Mozambique Current
2. Agulhas Current,
3. Brazilian Current
4. Gulf Stream
5. North Atlantic Drift (a relatively warm current in the area close to North-west Europe).
6. South Equatorial Current
Cold Currents flowing from the polar regions
7. Canaries Current
8. Benguela Current
9. West Wind Drift
10. West Australian Current
11. Labrador Current
(Note : We have only included currents in the Atlantic Ocean, Indian Ocean and Southern Ocean.)
Ocean currents form as a result of the unequal heating of the oceans – warm water predominates in the tropics where solar heating is more intense, while the water nearer to poles is very cold, and, of course vast amounts of water in the polar areas remains frozen. The difference in water temperatures leads to convection currents moving colder water moving towards the Equator, and warmer water moving towards the poles. The rotation of the earth causes a general anti-clockwise circulatory pattern in the southern hemisphere, and a general clockwise circulatory pattern in the northern hemisphere.
The West Wind Drift in the Southern Ocean is generated by the constantly strong westerly winds in this area, a movement that is also influenced by the rotation of the earth.
The general direction of currents is shown on the map above.
The Effects of Ocean Currents on Shipping
Currents affect ships during their voyages. A ship steaming against a current will need to use more fuel to maintain a required speed. A ship steaming with a current will save fuel and can proceed at an increased speed.
- A warm current may keep ports ice-free in winter. This is particularly true of the North Atlantic Drift that introduces “warm” water to the north-west European coast and ports as far north as northern Russia (e.g. Murmansk) or northern Norway (e.g. Tromso) do not freeze. (The current in this area is only just above freezing!) The cold Labrador Current, however, introduces very cold water to the eastern coast of Canada where some ports freeze.
- Off the South African coast, the Agulhas Current flows at about 2 knots, and sometimes even faster. This has the effect of creating small counter currents that flow in the opposite direction. Because of the strength of the main current, ships steaming from Durban towards Port Elizabeth will keep about 12 nautical miles off the coast to benefit from the fast-flowing current and to avoid the counter currents. Ships going from Port Elizabeth towards Durban follow a course closer to the coast to avoid the strong current and to take advantage of the counter currents. However, they need to guard against being moved off course towards the coast by the counter currents. (Several ships have gone ashore because of these currents.)
- Currents also affect the climate and weather in the region, especially in the formation of fog. (See Module 10.4.2. below.)
- The south-flowing Labrador Current off the Canadian east coast is responsible for the southerly drift of icebergs from the polar region and into the shipping lane off the Canadian coast.
Teach yourself: Currents
A ship is steaming from East London to Cape Town and her Master wishes his ship to steam at 15 knots.
1. Will the Master set a course that is about three nautical miles off the coast or 12 nautical miles off the coast?
2. Explain your answer to Question 1.
3. The current is flowing at 2 knots. The Master decides not to reduce the revolutions of the ship’s propeller. What will the ship’s speed be on the voyage from East London?
4. Assume that, for the first 16 hours out of East London, the ship benefits from the current most (i.e. she will be steaming at the speed you calculated for Question 3.).
4.1. How many nautical miles will she have steamed in that time?
4.2. The distance from East London to Mossel Bay is 310 nautical miles. Will she still be steaming at this speed when she passes Mossel Bay?
5. Will the current cause her to use less or more fuel on this voyage than she would have used if the current had not been flowing?
6. Will the current help her all the way to Cape Town?