Questions: El Niño–Southern Oscillation (ENSO)

Question 1

During an El Niño event, what happens to the thermocline depth in the eastern tropical Pacific, and what is the consequence for marine ecosystems?

• A. The thermocline shallows, enhancing cold nutrient-rich upwelling and boosting productivity
• B. The thermocline deepens, suppressing cold upwelling and reducing surface nutrient supply
• C. The thermocline disappears, mixing warm and cold water uniformly
• D. The thermocline remains unchanged; only atmospheric pressure shifts

Answer: (B)

During El Niño, eastward-propagating oceanic Kelvin waves cause the thermocline to deepen in the eastern Pacific. This traps cold, nutrient-rich water below the sunlit surface layer, suppressing the upwelling that normally supports productive fisheries off Peru and Ecuador. The collapse of the Peruvian anchovy fishery during strong El Niño events is the canonical ecological consequence.

Question 2

ENSO is primarily an oceanic phenomenon — the ocean warms, and the atmosphere responds passively.

• T. True
• F. False

Answer: False

ENSO is a coupled ocean-atmosphere mode. The Bjerknes feedback makes this clear: warm sea surface temperatures (SSTs) lower atmospheric pressure, which weakens trade winds, which allow warm water to spread east and suppress upwelling, which further warms SSTs. Neither the ocean nor the atmosphere leads in isolation — the two systems reinforce each other. This coupled positive feedback is what allows El Niño to grow from an initial perturbation into a basin-wide event.

Question 3

Describe the Bjerknes feedback loop and explain how it can amplify either an El Niño or a La Niña event from a small initial perturbation.

Model answer: The Bjerknes feedback is a positive ocean-atmosphere feedback: warmer eastern Pacific SSTs reduce the east-west temperature gradient, lowering pressure in the east, weakening the Walker circulation trade winds, allowing even more warm water to flow east and further suppressing cold upwelling. The feedback is self-reinforcing, so a small initial weakening of trade winds grows into a full El Niño. La Niña is the mirror: anomalously strong trade winds cool the eastern Pacific further, which steepens the pressure gradient, which strengthens trade winds, amplifying cooling.

The Bjerknes feedback is why ENSO events can grow rapidly once initiated. It is a positive feedback in the dynamical systems sense — deviations from neutral are amplified, not dampened. Understanding this explains why ENSO is not just a gradual warming/cooling but often has a rapid onset and a characteristic asymmetric time evolution (fast growth, slower decay).