Final answer:
El Niño was first discovered in Peru by local fishermen.
Step-by-step explanation:
El Niño, the warm phase of the El Niño Southern Oscillation (ENSO), was first discovered in Peru. Specifically, El Niño is named after the Christ child because the phenomenon usually occurs around Christmas time in the coastal waters of Peru and Ecuador.
The local fishermen noticed the periodic warming of the waters which affected their fishing yields and gave it the name El Niño, meaning 'the boy' in Spanish.