Under normal conditions in the tropical Pacific, the trade winds
Answer: A
The warm pool sits over Indonesia and cold upwelled water lies off South America. This west to east temperature contrast is what drives the Walker circulation. Upwelling keeps the eastern side cold. The warm pool sits over Indonesia.
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Assertion (A): Sea level in the western Pacific is normally higher than in the east.
Reason (R): The trade winds push surface water westward and it accumulates there.
Answer: B
Both are true and the reason gives the mechanism, and the difference of about half a metre is one of the quantities that changes markedly during an El Nino event. The trades do the piling. Half a metre of difference is typical.
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The Walker circulation is
Answer: C
Air rises over the warm western Pacific, flows east aloft, sinks over the cold eastern Pacific and returns as the trades. It is a zonal cell rather than a meridional one like the Hadley. Warm water anchors the ascending branch.
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During an El Nino event the trade winds
Answer: D
The warm pool migrates east, upwelling off South America is suppressed and the whole Walker cell weakens. La Nina is the opposite phase, with stronger trades and colder eastern water. The Walker cell weakens through the event.
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Match each phase of the oscillation with its Pacific conditions.
List I
A. El Nino
B. La Nina
C. Neutral
D. Southern Oscillation
List II
1. trades and temperature gradient near their average
2. weak trades and a warm eastern Pacific
3. the atmospheric pressure seesaw that accompanies them
4. strong trades and a cold eastern Pacific
Answer: A
The ocean and atmosphere components are two faces of one phenomenon, which is why the combined term ENSO is used. Neither can be understood on its own. The two phases are opposite in every respect. The combined term reflects the coupling.
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Assertion (A): El Nino and the Southern Oscillation are treated as a single phenomenon.
Reason (R): The oceanic warming and the atmospheric pressure seesaw are coupled and occur together.
Answer: B
Both are true and the reason gives the justification for the combined name, and the coupling between them is what allows the event to develop and to persist. Ocean and air act together. Neither half of the phenomenon stands alone.
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The Southern Oscillation Index is based on the pressure difference between
Answer: C
Correct answer: Tahiti and Darwin
The Southern Oscillation Index is the standardised difference in sea level pressure between Tahiti in the central Pacific and Darwin in northern Australia. Those two stations sit at opposite ends of the Walker circulation, so the difference between them tracks the see-saw in pressure that gives the oscillation its name - and both have long, continuous records, which is why they were chosen.
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Consider the following statements about the Southern Oscillation Index.
1. A strongly negative index accompanies El Nino.
2. A strongly positive index accompanies La Nina.
3. The index is computed from sea surface temperature.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: a strongly negative index means unusually low pressure at Tahiti and high at Darwin, the weakened gradient that accompanies El Nino, and a strongly positive index accompanies La Nina. Statement 3 confuses the two families of index - the Southern Oscillation Index is computed from pressure, while the Nino indices are computed from sea surface temperature.
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The Nino 3.4 index measures
Answer: A
Correct answer: sea surface temperature in a defined region
Nino 3.4 is the average sea surface temperature anomaly in a box spanning 5 degrees north to 5 south and 170 to 120 degrees west, in the central and east-central equatorial Pacific. It has become the standard index because that region is where the coupling between ocean and atmosphere is strongest, so it correlates best with the global teleconnections.
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Assertion (A): The Bjerknes feedback explains how an El Nino event amplifies itself.
Reason (R): Weaker trades warm the east, which weakens the trades further.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason states the loop. Weakened trades reduce the upwelling and let warm water spread east; the warmer eastern Pacific reduces the west-to-east temperature contrast that drives the trades, so they weaken further. That positive feedback is the Bjerknes mechanism, and it is why a small initial anomaly can grow into a full event - and why the same loop run in reverse produces La Nina.
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