Normally, winds blow west across the tropical Pacific Ocean, away from Central and South America. As wind-driven warm water moves over the ocean, it piles up in Indonesia and elsewhere in the western Pacific. Warm air rises offshore, causing rain to fall. Meanwhile, cold water comes up from the bottom off the coast of South America. This flow allows a richness of life to flourish near the coast, and it helps maintain predictable weather patterns from season to season.
Every 5 to 10 years or so, though, the wind dies down. As a result, the surface of the Pacific Ocean gets warmer. Rainfall then tends to fall further to the east. Such a change in weather causes, among other things, floods in Peru and droughts in Australia and Indonesia. This new weather pattern is known as El Niño.
An opposite cascade of events happens during the weather pattern called La Niña, when Pacific surface temperatures cool down. Both El Niño and La Niña, when they happen, usually last for 2 to 4 years.
But it’s not just El Niño and La Niña that explain prolonged periods of rain or of dry weather, leading to droughts. For instance, in the last few years, scientists from the U.S. Geological Survey (USGS) have begun to link precipitation on the Colorado Plateau to temperature shifts both in the Pacific Ocean and in the Atlantic Ocean.
A recent statistical study by USGS researchers found that less moisture falls on the United States when surface temperatures in the North Atlantic are warmer than normal. These conditions prevailed during a number of droughts over the past century.
The study also found a correlation between warm water in the central North Pacific and drought in the southwestern and northern plains of the United States. When water is warm in both the North Atlantic and the North Pacific at the same time, conditions can get mighty dry in the American West.