El Niño and La Niña

The name El Niño comes from nineteenth century Peruvian and Ecuadorian fishermen. They noticed that each year, within a few months of the Christmas holiday, the seawater off the South American coast became warmer, the nearshore ocean currents assumed new patterns, and the fishing became poorer. Every few years, the changes were strong enough to wipe out a fishing season, and to bring significant, long-lasting changes in the weather. For example, normally dry areas on shore could receive abundant rain, turning deserts into lush grasslands for as long as these strong El Niños lasted. Because the phenomenon happened close to Christmas each year, the fishermen dubbed it El Niño, Spanish for "the boy child," after the Christ child. Only in the 1960s did scientists begin to realize that the strong El Niño events were more than a local South American phenomenon, and were rather one half of a multi-year atmospheric-oceanic cycle that affects the entire tropical Pacific Ocean. The other half of the ENSO cycle has been named La Niña, the girl child, or, less commonly, El Viejo, the old man.

The Southern Oscillation was detected, and named, in 1923 by Sir Gilbert Walker. Walker was the director of observatories in India, and was trying to understand the variations in the summer monsoons (rainy seasons) of India by studying the way atmospheric pressure changed over the Pacific Ocean. Based on meteorologists' previous pressure observations from many stations in the southern Pacific and Indian oceans, Walker established that, over the years, atmospheric pressure seesawed back and forth across the ocean. In some years, pressure was highest in the Indian Ocean near northern Australia, and lowest over the southeastern Pacific, near the island of Tahiti. In other years, the pattern was reversed. He also recognized that each pressure pattern had a specific related weather pattern, and the change from one phase to the other could mean the shift from rainfall to drought, or from good harvests to famine. In the late 1960s, Jacob Bjerknes, a professor at the University of California, first proposed that the Southern Oscillation and the strong El Niño sea warming were two aspects of the same atmosphere-ocean cycle, and explained the ENSO phenomenon in terms of physical mechanisms.