CDS (II) 2022 General Knowledge · Question 44 of 120

Correct — C, Gravitational pull by Sun and Moon. The other three options are lifted straight from the standard four-item list of primary forces that drive ocean currents. NCERT's chapter on the movements of ocean water puts it in one sentence: 'The primary forces that influence the currents are: (i) heating by solar energy; (ii) wind; (iii) gravity; (iv) coriolis force.' The paper has taken that list, kept three items and replaced the fourth, plain 'gravity', with 'gravitational pull by Sun and Moon' — and those are two different things. The gravity in the list is the Earth's own, acting on water that solar heating has piled up: warming makes the ocean expand, so near the equator the surface stands about eight centimetres higher than in the middle latitudes, and gravity pulls that pile down the slope it has created. The gravitational pull of the Sun and the Moon is the tide-generating force, and NCERT deals with it in a separate section: 'The moon's gravitational pull to a great extent and to a lesser extent the sun's gravitational pull, are the major causes for the occurrence of tides.' Tides, not currents.

• (a)Heating by solar energy — First on the standard list. Uneven heating makes water expand and stand higher near the equator than in middle latitudes, creating the gradient that starts the movement.
• (b)Wind — Second on the list, and the dominant driver of surface circulation. Friction between the blowing wind and the water surface drags the top layers along, which is why the great gyres follow the pattern of the prevailing winds.
• (d)Coriolis force — Fourth on the list. The Earth's rotation deflects moving water to the right in the northern hemisphere and to the left in the southern, turning what would be straight flows into the large circular gyres that fill every ocean basin.

Ocean currents are separated into primary forces, which set the water moving, and secondary forces, which shape how it then flows. The four primary ones are solar heating, wind, gravity and the Coriolis force. Surface currents, the top four hundred metres or so, hold about ten per cent of the ocean's water and are essentially wind-driven and Coriolis-turned; the remaining ninety per cent moves as deep-water currents driven by differences in density, water sinking in the high latitudes where it is cold enough and salty enough to become heavy. Currents are further classified by temperature into warm and cold — cold currents typically on the western coasts of continents in low and middle latitudes, warm currents on the eastern coasts in the same belt.

This item is really a check on whether a student has read the NCERT list closely enough to notice that one word has been changed, and it is fair to record the tension it creates rather than to pretend there is none. The gravitational pull of the Moon and the Sun is not wholly innocent of moving water: it produces tides, and NCERT itself notes that when a tide is channelled between islands or into bays and estuaries the result is called a tidal current. So on a strict physical reading lunisolar gravity does move ocean water. What the key is doing — and what the exam convention has been across several papers — is treating tides as a distinct phenomenon from the ocean circulation this question is about, and asking which of four named forces belongs on the circulation list. On that reading only option (c) is off the list, and that is the answer authored to. Note also the printed grammar, 'Which one of the following do not influence' — a singular subject with a plural verb. It is the paper's own slip and does not change what is being asked.

• NCERT lists four primary forces influencing ocean currents: heating by solar energy, wind, gravity, and the Coriolis force.
• The 'gravity' in that list is the Earth's own, pulling water down the gradient that solar heating creates; near the equator the ocean surface stands about eight centimetres higher than in the middle latitudes.
• The gravitational pull of the Moon, and to a lesser extent of the Sun, is the tide-generating force, not a driver of the ocean's general circulation.
• Surface currents make up about ten per cent of ocean water and lie in the upper four hundred metres; deep-water currents make up the other ninety per cent and are driven by density differences.
• The Coriolis force deflects moving water to the right in the northern hemisphere and to the left in the southern, producing the great gyres.
• A tide channelled between islands or into a bay or estuary is called a tidal current — the one place where lunisolar gravity does produce a current.

• Reading the 'gravity' on the NCERT list as lunisolar gravity; it is the Earth's own pull on water that has been heaped up by heating.
• Forgetting that the Earth's revolution around the Sun plays no part; it is the rotation that matters, through the Coriolis force.
• Treating tidal currents as ordinary ocean currents when a question is about the general circulation.

As a which-does-not-belong item on the drivers of ocean currents, or as a select-the-correct-factors list.

• practice — not a real PYQ

  The great circular systems of surface ocean currents known as gyres are produced chiefly by the combination of

  • (a)tides and salinity differences
  • (b)prevailing winds and the Coriolis force
  • (c)the Earth's revolution and ocean-floor relief
  • (d)rainfall and river discharge

  Answer(b) prevailing winds and the Coriolis force — the winds drag the surface water and the Earth's rotation curves the flow into closed circulations.

• practice — not a real PYQ

  Deep-water ocean currents, which account for about ninety per cent of ocean water, are driven mainly by

  • (a)surface winds
  • (b)the gravitational pull of the Moon
  • (c)differences in the density of sea water
  • (d)earthquakes on the ocean floor

  Answer(c) differences in the density of sea water — cold, salty water at high latitudes becomes heavy enough to sink into the deep basins.