El Niño (“little boy” in Spanish) refers to abnormal conditions in the tropical Pacific Ocean that occur, on average, every three to seven years. During El Niño, air-pressure masses over the tropical Pacific reverse. This periodic “flip” is known as the Southern Oscillation, and the event is officially called the El Niño-Southern Oscillation, or ENSO. During ENSO, high-pressure air masses that normally occur in the eastern Pacific shift to the western Pacific, and low-pressure air masses that occur in the western Pacific flip over to the eastern Pacific.

During ENSO, the rain-producing warm-water pool that normally occurs in the western Pacific shifts eastward to the central Pacific. This shift has dramatic effects on wind patterns and rainfall around the globe. This shift has been associated with floods, severe storms, and droughts in many parts of the world. Scientists know that ENSO causes drought in Australia, Indonesia, and the Philippines. These places lie near the normal location of the warm-water pool in the Pacific. When the rain-generating warm-water pool moves eastward, it takes the rain with it, and these regions experience drought. ENSO also brings drought to India (it causes the monsoons to fail) and to Central and South America. Brazil's Amazon rain forest becomes tinder dry and may burn for months. In North America, ENSO causes a high-pressure cell to form in the northern Pacific. This blocking high plants itself over the Northwest and channels moisture-rich air masses around it. The Southeast may get lots of rain, but the Northwest, Midwest, and Northeast may experience drought.

The Tropical Atmosphere Ocean (TAO) Project array is a series of equipment-laden buoys deployed along the equatorial Pacific. The buoys constantly monitor ocean and atmospheric conditions, and they are the most efficient means of predicting the onset of ENSO and the droughts that accompany it.

Droughts often occur without an ENSO event. In the United States, scientists and agricultural experts use different indexes to determine if a state or region is on its way to experiencing a full-blown drought. The Palmer Index, for example, monitors many variables, including precipitation and soil moisture content, to help predict if a severe drought is imminent U.S. government agencies use the Palmer Index to determine when drought relief may be needed in any part of the country.