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El Niño – Southern Oscillation
El Niño and La Niña are words frequently tossed around to explain weather conditions in the Southwest. The sibling events, born thousands of miles away in the air over the tropical Pacific Ocean and in its waters, can deliver copious rain and snow to the region or cause widespread drought.
El Niño and La Niña are part of the El Niño–Southern Oscillation (ENSO), a natural see-saw in oceanic sea surface temperatures and surface air pressure between the east and west tropical Pacific Ocean. During El Niño events, the trade winds blowing from the east slacken, enabling an eastward migration of warm water. The center of rain follows, moving east to the middle of the Pacific Ocean near Tahiti. La Niña events behave in the opposite way; the trade winds intensify and stack the warm surface water in the west even more than in normal years—the waters near Australia are often five feet higher than the ocean surface in the east during La Niña episodes. The area of intense rainfall is dragged back toward Australia.
The effect of ENSO on weather in the Southwest
El Niño and La Niña episodes tend to develop between April and June and peak between December and January, when sea surface temperatures reach their warmest and coldest states, respectively. As a result, changes to atmospheric circulation, and therefore weather, are most prominent in the winter.
The ENSO fingerprint on the Southwest is principally caused by the shifting jet streams. During El Niño events, the Pacific jet stream is straighter and is pulled south (Figure 1), and storms form in the Pacific Ocean just west of California, in part because waters are warmer than average in this region during El Niño events. The combination of the jet stream and storms often results in a wet winter and increased rain and snow across California and the southern United States.
La Niña events, on the other hand, often bring dry conditions to Arizona and New Mexico. In La Niña winters, the jet streams take a more serpentine path (Figure 2). The Pacific jet stream usually carves north and enters North America through the northwestern US, bringing wetter-than-average conditions to that region and diverting storms away from the Southwest.
The effect El Niño and La Niña events have on the weather is nuanced. An El Niño does not always cause wet winters, nor does a La Niña consistently deliver dry conditions. Between 1896 and 2002 in Arizona, for example, about 50 percent of the winters experiencing an El Niño event received more than 115 percent of average precipitation, while roughly 25 percent of the winters received less than 85 percent of the average.
The amount of precipitation during ENSO events also changes by region in Arizona and New Mexico and beyond. During an El Niño the southern regions of both states often receive more winter rain and snow than northern regions.
Critical winter precipitation for the Southwest also falls as snow in the Upper Colorado River Basin (UCRB), the headwaters of Arizona’s most important river, the Colorado. About 70 percent of that water originates in the mountains of Utah, Wyoming, and Colorado. However, when the southern regions of the Southwest are wet, precipitation in the UCBR is often average or below average.
The impact of ENSO on summer weather is not as clear-cut as the changes that occur in winter. El Niño events often are associated with two phenomena that have opposite effects on precipitation in the Southwest. On one hand, an El Niño can stifle summer rains in Arizona and New Mexico because they can weaken and reposition the subtropical high that guides moisture into the Southwest. On the other hand, El Niño events also can foment a higher number of tropical storms, some of which deliver copious summer and fall rains to the region.