El Niño Definition & Meaning – Merriam-Webster

an irregularly recurring flow of unusually warm surface waters from the Pacific Ocean toward and along the western coast of South America that prevents upwelling of nutrient-rich cold deep water and that disrupts typical regional and global weather patterns

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Each year around Christmas time, a warm equatorial current flows southward along the coast of Peru. In the 19th century, Peruvian fisherman named that annual current “El Niño” in honor of the Christ child (el niño means “the child” in Spanish). Later, when scientists noted that in some years this warm current flow is more intense than usual, they adopted the name and applied it to that more potent but erratic climatic phenomenon. Now El Niño is used almost exclusively for the severe episodes rather than for the annual ones to which it was originally applied.