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As it is currently used, the term El Niño refers to a strong and lengthy disruption to the normal pattern of ocean currents, water temperatures, and winds in the Pacific. A. An El Niño typically begins when the Humboldt Current causes upwelling as it travels toward the equator along the coast of Peru and Ecuador. B. El Niños are preceded by the reversal of the usual east-to-west pressure gradient in the Pacific, the weakening or reversal of the trade winds, and the movement of warm water eastward. C. Comparisons of historical records with recent past events show that El Niños are becoming more frequent and stronger. D. In an El Niño, warm surface currents replace the Humboldt Current for many months, raising ocean temperatures far from the coast. E. Scientists discovered the Southern Oscillation by taking surface-pressure readings at weather stations on both sides of the Pacific. F. In recent decades, El Niños have begun to occur north of the equator and thereby affect weather conditions in the United States.