Which phenomenon describes a southward flowing warm water current off the western coast of South America caused by breakdown of trade winds?

• A. ENSO
• B. Gulf Stream
• C. El Nino
• D. Equatorial Upwelling

Answer: (C)

El Niño is the warming phase of the Pacific ENSO cycle, triggered when the trade winds weaken or reverse. Normally these easterly winds push surface water westward, piling warm water in the western Pacific and allowing cold water to upwell along the coast of South America. When the trade winds break down, the warm surface water spreads eastward toward the Americas. This creates a pool of warm water along the western coast of South America and can flow in a alongshore, often southward, direction, reducing upwelling and altering regional weather. The other options don’t fit this mechanism: the Gulf Stream is an Atlantic current, equatorial upwelling is driven by strong trade winds and would be enhanced by their strength, not breakdown, and ENSO is the overall cycle that includes La Niña and El Niño, but the described coastal warming due to weakened winds specifically points to El Niño.