The acronym ENSO represents the coupled phenomena of El Niño and which oscillation?

• A. The Southern Oscillation
• B. The Pacific Decadal Oscillation
• C. The North Atlantic Oscillation
• D. The Arctic Oscillation

Answer: (A)

ENSO is El Niño Southern Oscillation, a coupled ocean–atmosphere pattern in the tropical Pacific. The “Southern Oscillation” part refers to the swing in atmospheric pressure across the Pacific, especially the difference in sea-level pressure between the eastern and western Pacific (think Tahiti versus Darwin). This pressure difference drives the trade winds and, in turn, the distribution of warm water. When the pressure difference weakens, winds slacken, warm water shifts east, and El Niño occurs. When the difference strengthens, winds push warm water west, leading to La Niña. The other options describe different climate oscillations in other regions or timescales, not the tropical Pacific atmospheric pattern that pairs with the ocean changes to form ENSO.