What causes ENSO?

• A. Interaction between the surface layers of the ocean and the atmosphere in the tropical Pacific.
• B. Volcanic eruptions modifying global winds.
• C. Changes in Earth's orbit.
• D. Strength of solar radiation.

Answer: (A)

ENSO is driven by the close interaction between the ocean surface and the atmosphere in the tropical Pacific. When trade winds weaken, warm surface water that is normally pushed toward the western Pacific shifts eastward, warming the eastern Pacific. This change in sea-surface temperatures alters where convection and rainfall occur, which in turn feeds back to wind patterns and ocean currents, reinforcing the cycle. This ocean-atmosphere coupling on year-to-year timescales creates the El Niño and La Niña conditions that define ENSO.

Other ideas don’t drive ENSO the same way. Volcanic eruptions can slightly modify climate temporarily through aerosol effects but don’t establish the sustained tropical Pacific feedbacks. Changes in Earth’s orbit occur over tens of thousands of years and solar radiation variations on shorter timescales don’t produce the interannual ocean-atmosphere interactions that cause ENSO.