El Niño events are among the most studied of Earth's weather phenomena. Though their origins may be particular to tropical latitudes, they can shape climate on a global scale. To understand this anomaly, it may be helpful to review what normally drives weather cycles in tropical latitudes.

Typically, high pressure predominates over the eastern Pacific Ocean and low pressure over the western Pacific Ocean. Denser, cooler air rushes in to fill low-pressure voids left by rising warm air, producing trade winds and a corresponding current of water that move from east to west between South America and Australia. The flowing surface water, which is warmed by the heat of the Sun to a depth of about 90 meters (300 feet), raises sea level in the western Pacific by about 40 centimeters (16 inches). To equalize the loss of water in the east, cooler water rises from the depths to the surface near the South American coast, bringing with it nutrients that attract fish stocks like anchovies, an important staple in the global fish supply.

Around late December each year, a warm ocean current flows south along the coasts of Ecuador and Peru. In some years, that current is warmer and stronger, and it flows farther than usual. At the same time, heavy rains fall in normally dry areas along the coasts of North, Central, and South America. These unusual conditions, called El Niño because they occur during the Christmas season, are thought to be caused by a slackening, and then a complete reversal, of the normal wind patterns and ocean currents. When trade winds weaken, they can no longer buttress the additional mass of warm water in the west, warmed now to a depth of 150 meters (500 feet), so it surges back eastward due to the force of gravity. The winds change direction, and all of the rainmaking that normally happens in the west now happens in the east. The deeper pool of warm water blocks the normal upwelling of nutrients, thus also devastating fish stocks.

After a period of months, or as much as a year or more, the Pacific waters cool and conditions return to normal. More frequently, however, the conditions turn to the opposite extreme. All the warm surface water that moved east returns west as abnormally cold water. This reversal is called La Niña. Scientists cannot be certain of the precise cause of El Niño events, but the documented increase in the frequency and power of these events has led some to look for a connection with global warming, and in particular whether or not accumulating greenhouse gases are overheating Pacific waters.

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