CDS (II) 2024 General Knowledge · Question 79 of 120

Correct — C, Statement 1 is correct, but statement 2 is false. The first statement is straight physical oceanography and NCERT puts it in almost the same words: wind blowing over the sea drags the surface water along, the Coriolis force turns that water to the right in the northern hemisphere and to the left in the southern, and the water piles up in great mounds whose circulation around them is called a gyre — one in each ocean basin, driven by the global pattern of surface winds. The second statement breaks on a single adjective. Equatorial currents do run westward, pushed by the trades, and the water does turn poleward along the east coasts of continents as a western boundary current — the Gulf Stream off North America, the Kuroshio off Japan, the Agulhas off south-east Africa, the Brazil Current off South America. But that water is warm, not cold. It has been sitting in the tropics gaining heat, and carrying it poleward is exactly how the ocean does its share of the planet's heat transfer. Cold currents are the other limb of the gyre, running equatorward down the western sides of continents, which is where the California, Canary, Benguela and Peru currents are found.

• (a)Both the statements are correct and statement 2 is the correct explanation of statement 1. — Cannot hold, because statement 2 is not correct. Equatorial currents carry warm water, not cold, and calling them cold reverses the ocean's role in moving heat away from the tropics.
• (b)Both the statements are correct, but statement 2 is not the correct explanation of statement 1. — Same failure. The second statement's error is factual, so no reading of the relation between the two can rescue it.
• (d)Statement 1 is false, but statement 2 is correct. — Reverses the item. Gyres driven by the global wind pattern are the textbook description of surface circulation, and it is statement 2 that fails.

Surface ocean currents are set going by four primary forces — heating by solar energy, wind, gravity and the Coriolis force. Water near the equator stands about eight centimetres higher than in the middle latitudes because it has expanded on heating, so there is a slope for gravity to pull water down; wind drags the surface along; and the Coriolis force bends the flow. The result in each basin is a closed gyre, clockwise in the northern hemisphere and anticlockwise in the southern. Each gyre has a warm western boundary current running poleward along a continent's east coast and a cool eastern boundary current running back towards the equator along a west coast, which is why coastal climates on the two sides of an ocean at the same latitude are so different.

Assertion-style items on currents almost always turn on one adjective, so read for the temperature word. The rule to hold is short: warm currents are found on the east coasts of continents in low and middle latitudes, cold currents on the west coasts in the same latitudes, and the sense flips in the higher latitudes of the northern hemisphere. Statement 2 gets the direction of flow right and the coastline right, and then attaches the wrong temperature — a common trap because a candidate who checks the geography stops checking. The consequence is worth remembering as well: where a warm current meets a cold one the mixing brings up nutrients and the world's best fishing grounds are found there, off Newfoundland and off northern Japan.

• NCERT lists heating by solar energy, wind, gravity and the Coriolis force as the primary forces influencing ocean currents.
• Large accumulations of water and the flow around them are called gyres, and they produce large circular currents in every ocean basin.
• Warm currents bring warm water into cold water areas and are usually found on the east coasts of continents in the low and middle latitudes.
• Cold currents are usually found on the west coasts of continents in the low and middle latitudes, and on the east coasts in the higher latitudes of the northern hemisphere.
• Where warm and cold currents mix, oxygen is replenished and plankton flourish, which is why the best fishing grounds lie in those zones.

• Reading the coastline correctly and skipping over the temperature adjective.
• Thinking gyres turn the same way in both hemispheres; the Coriolis deflection reverses.
• Confusing the wind-driven surface gyres with the density-driven deep circulation.

As a two-statement item in which one sentence gives the correct geography with the wrong temperature, or as a straight warm-or-cold identification of a named current.

• practice — not a real PYQ

  Which of the following is a cold ocean current?

  • (a)Gulf Stream
  • (b)Kuroshio
  • (c)Benguela Current
  • (d)Brazil Current

  Answer(c) Benguela Current — an eastern boundary current flowing equatorward along the south-west coast of Africa; the other three are warm western boundary currents.

• practice — not a real PYQ

  In which direction do the surface gyres of the northern hemisphere circulate?

  • (a)Clockwise
  • (b)Anticlockwise
  • (c)East to west at all latitudes
  • (d)North to south along the mid-ocean ridge

  Answer(a) Clockwise — the Coriolis force deflects moving water to the right north of the equator, and to the left south of it.