El Niño and La Niña events tend to develop between April and June and peak between December and January, which means the U.S. Southwest sees the most prominent effects of ENSO circulation changes over winter and into early spring. The influence of ENSO on weather in the Southwest is tied to its ability to change the position of the jet stream—the winds aloft that steer storm systems and dictate where areas of high and low pressure are positioned. During El Niño events, the jet stream over the Pacific Ocean becomes less wavy and splits into a strengthening subtropical jet stream near the equator and a weaker polar jet stream, and can result in a greater number of storms and above-average precipitation across the Southwest during winter and early spring. La Niña events often bring drier-than-average winter conditions to the Southwest, as the jet stream curves and shifts north, diverting storms and precipitation away from the region. El Niño does not guarantee a wet winter, just as La Niña does not consistently deliver dry conditions, but these are the patterns most associated with these events.
ENSO’s impact on summer weather is less clear, but El Niño events can delay the onset of the monsoon in Arizona and New Mexico by weakening and repositioning the subtropical high that guides moisture into the Southwest. El Niño events also influence development and strength of tropical storms in the eastern Pacific Ocean, and moisture associated with these storms has the potential to deliver above-average rain to the region, typically in late summer or early fall.
Precipitation anomalies influenced by ENSO also vary geographically, and the southern regions of Arizona and New Mexico tend to record larger positive precipitation anomalies during El Niño events when compared to northern regions. During La Niña events the general pattern is reversed, with reduced precipitation across the Southwest.
Figure: El Niño and La Niña events cause the path of the jet streams to move over the US in different locations, often causing wet winters during El Niño episodes and dry winters during La Niña events in the Southwest. Image modfied from the National Oceanic and Atmospheric Administration (NOAA).