CDS (I) 2021 General Knowledge · Question 20 of 120

Correct — D, California Current. It is a cold-water Pacific current that moves southward along the western coast of North America, forming the eastern limb of the North Pacific gyre. Two features of the description settle its temperature without any memorising. It flows from higher latitudes towards the equator, so it carries cool water into warmer seas. And it runs down the eastern side of the ocean basin, where the wind pushes surface water away from the coast and colder water rises from below to replace it — the California Current is one of the world's great upwelling systems, which is why its waters are cold, nutrient-rich and among the most productive fishing grounds on earth. The same combination explains the fog that hangs over the coast in summer and the aridity of the land behind it.

• (a)Brazilian Current — A warm current, and the mirror image of the correct answer. It flows poleward down the eastern coast of South America as the western-boundary limb of the South Atlantic gyre, carrying tropical water southwards until it meets the cold Falkland Current.
• (b)Gulf Stream — The best-known warm current in the world. It carries tropical Atlantic water north-eastward past the eastern seaboard of the United States and continues as the North Atlantic Drift, which is why north-western Europe is far milder than its latitude would suggest.
• (c)North Equatorial Current — A warm current by position. It is a westward drift in the low latitudes, driven by the trade winds, moving water that has been heated in the tropics — it is the source of warmth that western-boundary currents such as the Gulf Stream then carry poleward.

Surface currents are organised into gyres, the great circular systems driven by the prevailing winds and turned by the earth's rotation — clockwise in the northern hemisphere, anticlockwise in the southern. Within a gyre the limbs behave differently. The western-boundary current runs poleward, is narrow, deep and fast, and is warm. The eastern-boundary current returns towards the equator, is broad, shallow and slow, and is cold, usually with coastal upwelling alongside it. The equatorial currents that close the loop on the tropical side are warm.

Rather than memorising lists of currents, learn the rule that generates them: a current flowing away from the equator is warm, and one flowing towards the equator is cold. Applied here, the Brazil Current runs south from the tropics and is warm, the Gulf Stream runs north-east from the tropics and is warm, the North Equatorial Current sits in the tropics and is warm, and only the California Current is moving equatorward. The paper's spelling, 'Brazilian Current', is a variant of the usual 'Brazil Current' and does not change what is meant. Cold eastern-boundary currents matter beyond the classroom — the Peru, Benguela, Canary and California currents all support major fisheries and all lie beside coastal deserts.

• The California Current is a cold Pacific current flowing southward along the western coast of North America.
• It is the eastern-boundary limb of the North Pacific gyre and one of the major coastal upwelling systems.
• Upwelling brings nutrient-rich water to the surface, supporting large fisheries and abundant seabird and whale populations.
• The Gulf Stream and the Brazil Current are warm western-boundary currents, of the North Atlantic and South Atlantic gyres.
• The North Equatorial Current is a warm, trade-wind-driven westward drift in low latitudes.
• General rule: currents moving away from the equator are warm, those moving towards it are cold.

• Assuming any current in the northern hemisphere must be warm; direction relative to the equator is what matters.
• Mixing up the Falkland and Brazil currents, which run against each other off South America.
• Reading the North Equatorial Current as cold because it flows westward; low-latitude water is warm regardless.

As a pick-the-cold-current item, or as a statements or match question sorting several named currents into warm and cold.

• practice — not a real PYQ

  Which one of the following pairs of currents meets off the coast of Argentina, producing a rich fishing ground?

  • (a)Gulf Stream and Labrador Current
  • (b)Brazil Current and Falkland Current
  • (c)Kuroshio and Oyashio
  • (d)Benguela Current and Agulhas Current

  Answer(b) Brazil Current and Falkland Current — the warm Brazil Current running south meets the cold Falkland Current running north off Argentina.

• practice — not a real PYQ

  Cold eastern-boundary currents such as the California, Peru, Benguela and Canary currents are typically associated with

  • (a)heavy rainfall on the adjacent coast
  • (b)coastal upwelling and productive fisheries
  • (c)the formation of tropical cyclones
  • (d)the absence of any marine life

  Answer(b) coastal upwelling and productive fisheries — offshore-moving surface water is replaced from below by cold, nutrient-rich water, and the coasts alongside are usually arid.