Questions: El Niño–Southern Oscillation: Mechanisms and Teleconnections

Question 1

During the early stages of an El Niño event, the trade winds weaken slightly. According to the Bjerknes feedback, what happens next?

• A. The weakened trades reduce upwelling in the western Pacific, cooling that region and ending the El Niño before it develops fully
• B. Weaker trades allow warm water to shift eastward, reducing upwelling in the eastern Pacific, which shifts convection eastward and further weakens the trades — a self-amplifying loop
• C. The atmosphere compensates by strengthening the Hadley circulation, restoring the trade winds within weeks
• D. The weakened trades cause La Niña by allowing cold deep water to reach the surface along the entire equatorial Pacific

Answer: (B)

The Bjerknes feedback is a positive feedback: each perturbation amplifies itself. Weaker trades → less upwelling in east → warmer eastern SST → convection shifts east → weaker trades. The warming and the wind weakening reinforce each other. Option A gets the geography wrong — upwelling is strongest in the eastern Pacific (near South America), not the western. Option C describes a negative feedback that does not characterize ENSO onset. Option D describes the wrong direction of causality.

Question 2

An El Niño event begins in the tropical Pacific but causes drought in Australia and Indonesia. What mechanism transmits this distant influence?

• A. Ocean currents carry warm Pacific water into the Indian Ocean, directly suppressing rainfall over Australia
• B. The shift in tropical convection eastward alters large-scale atmospheric circulation, exciting Rossby wave trains that modify jet streams and storm tracks globally
• C. El Niño increases global mean temperature, which reduces the equatorial temperature gradient and weakens the Australian monsoon directly
• D. The Southern Oscillation's pressure difference pushes weather systems away from Australia toward South America

Answer: (B)

Teleconnections are atmospheric, not oceanic. When El Niño shifts the main zone of tropical convection eastward, the altered heating pattern drives large-scale changes in atmospheric circulation — specifically Rossby wave trains that arc poleward and modify jet streams and precipitation patterns far from the Pacific. Australia experiences drought because the convection and rainfall that normally occurs over the western Pacific (fueled by warm western SST) is displaced eastward, removing the atmospheric moisture source. This is why ENSO affects regions thousands of kilometers from its oceanic origin.

Question 3

ENSO events are not strictly periodic — the interval between El Niño events varies from roughly 2 to 7 years.

• T. True
• F. False

Answer: True

Unlike the seasons or tidal cycles, ENSO does not follow a fixed clock. The interval between events, their intensity, and their spatial pattern all vary from cycle to cycle. This variability arises from the interplay of multiple mechanisms (Bjerknes feedback, ocean wave dynamics, decadal modulation) and the chaotic nature of the coupled ocean-atmosphere system. Forecasting ENSO onset is possible 6–12 months in advance, but the 2–7 year range reflects genuine unpredictability, not measurement imprecision.

Question 4

The Bjerknes positive feedback is sufficient on its own to explain why ENSO oscillates between El Niño and La Niña states rather than locking permanently into one phase.

• T. True
• F. False

Answer: False

The Bjerknes feedback is a positive feedback — it amplifies perturbations in one direction. By itself, it would tend to lock the system into whichever phase was perturbed, not produce oscillation. Oscillation requires a negative feedback or delayed mechanism that reverses the anomaly. The recharge oscillator model explains this: during El Niño, poleward ocean transport drains heat from the equatorial Pacific, eventually cooling the thermocline and initiating a return to La Niña. The delayed action oscillator model invokes reflected oceanic Rossby and Kelvin waves. These additional mechanisms — not the Bjerknes feedback — are what generate the oscillatory behavior.

Question 5

Why does the Bjerknes positive feedback alone not explain ENSO's oscillatory nature, and what additional mechanism is needed?

Model answer: The Bjerknes feedback amplifies anomalies in one direction (warm SST → weaker trades → less upwelling → warmer SST), which would lock the Pacific into a permanently warm or permanently cool state. For oscillation to occur, there must be a delayed negative feedback that eventually reverses the anomaly. The recharge oscillator model provides this: during El Niño, the weakened trade winds allow heat to discharge poleward from the equatorial Pacific through ocean transport. As heat drains, the equatorial thermocline shoals, cold water upwells more easily, and the system tips toward La Niña. The delay is set by the timescale of ocean heat transport, creating the observed 2–7 year cycle.

This is the key conceptual distinction: positive feedback explains why ENSO events grow — small perturbations amplify into significant climate anomalies. But positive feedback alone predicts runaway growth, not oscillation. The oscillation requires memory in the ocean (stored heat) that is slowly released and discharged across ENSO cycles. Understanding both elements — the Bjerknes amplification and the recharge/discharge mechanism — is necessary to explain why El Niño events end and La Niña events follow.