Ocean Currents

Notes

Ocean currents are continuous, regular movements of ocean water in a definite direction. A drift is slower and wind-driven; a stream is faster and more defined.

Causes

Primary: planetary winds (the major driver of surface currents), insolation (heating causes expansion; the equatorial ocean is slightly higher), Coriolis force (deflects currents to the right in the Northern Hemisphere and left in the Southern), and gravity.
Secondary: differences in temperature, salinity and density (which drive thermohaline deep-water circulation) and the shape of coasts.
Gyres: large circular systems, clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

Warm currents (flow from low to high latitudes; on east coasts of continents in the tropics and subtropics): Gulf Stream, North Atlantic Drift, Kuroshio (Japan), Brazil, East Australian, Agulhas, Florida, Alaska.

Cold currents (flow from high to low latitudes; on west coasts in the tropics and subtropics): Labrador, Humboldt (Peru), California, Canary, Benguela, West Australian, Oyashio, Falkland, Greenland. The Antarctic Circumpolar Current (West Wind Drift) is the largest.

Equatorial counter-currents flow west to east between the north and south equatorial currents in the Atlantic, Pacific and Indian oceans.

Effects

Climate: the Gulf Stream and North Atlantic Drift keep north-west Europe mild and ports ice-free (Norway); cold currents cause coastal deserts (Atacama, Namib, West Australia) and fog.
Fisheries: warm and cold currents meeting give plankton-rich waters: Grand Banks (Labrador and Gulf Stream), North Sea, off Japan (Kuroshio and Oyashio).
Navigation: icebergs carried by the Labrador current.
El Nino is linked to changes in Pacific currents.

Indian Ocean: its currents reverse seasonally with the monsoon: during the south-west monsoon they flow clockwise (the Somali current is a strong, cold current in summer); during the north-east monsoon they flow anticlockwise. The Equatorial Counter Current appears in the Indian Ocean too.

Thermohaline circulation (the "Great Ocean Conveyor Belt"): the sinking of cold, dense, salty water in the North Atlantic and Antarctic drives deep currents that connect the oceans.

Key Points

Winds, Coriolis force, temperature and salinity cause currents.
Gyres: clockwise in the NH; anticlockwise in the SH.
Warm: Gulf Stream, Kuroshio; cold: Humboldt, Benguela, Labrador, California.
Cold currents cause coastal deserts; warm currents moderate climates.
Indian Ocean currents reverse with the monsoon.

MCQs

Q1. The Humboldt (Peru) current is a:

(a) Warm current in the Atlantic
(b) Cold current in the Pacific
(c) Warm current in the Pacific
(d) Cold current in the Indian Ocean
Ans: (b).

Q2. The Atacama desert is associated with the:

(a) Benguela current
(b) Humboldt current
(c) California current
(d) Canary current
Ans: (b).

Q3. The largest ocean current system is the:

(a) Gulf Stream
(b) Kuroshio
(c) Antarctic Circumpolar Current
(d) Labrador current
Ans: (c).