El Niño–Southern Oscillation (ENSO)
We spent the better part of 2014 (and the first part of 2015) waiting in anticipation for an El Niño event that was initially forecast to be one of the stronger events on record. By early 2015 the event in question had not yet materialized, and some questioned whether El Niño would ever arrive. Eventually it did, and has been going strong for months, with most forecasts indicating that it will remain a strong event through the winter. As this event unfolds, there are numerous impacts we might expect to see across the southwest over the course of our cool season (approximately Oct - Mar). In the coming months, CLIMAS will aggregate news, information, and commentary about the possible and expected impacts of El Niño, from the perspective of what is most relevant and applicable to the Southwest. This will include what we have learned from past events, and what forecasting and models can tell us about planning for this event.
Please visit www.climas.arizona.edu/sw-climate/el-niño-southern-oscillation for more information. We will use that page as a central repository for our ENSO related outreach materials, and will update this information hub with timely and relevant information about El Niño throughout the winter.
El Niño–Southern Oscillation (ENSO)
El Niño and La Niña are part of the El Niño-Southern Oscillation (ENSO), a natural fluctuation in oceanic sea surface temperatures (SSTs) and surface air pressure between the east and west tropical Pacific Ocean. During an El Niño event, easterly trade winds weaken, allowing warmer surface water from the western tropical Pacific Ocean to flow eastward. During a La Niña event, these trade winds intensify, preventing warmer water in the west from moving east, and stacking warm surface water in the west. Large areas of lower surface air pressure and convective precipitation follow the warmer water as it migrates across the tropical Pacific Ocean, altering broad-scale atmospheric circulation patterns (e.g., the Walker Circulation), which can influence weather around the world (Fig. 1).
The Effect of ENSO on SW Weather
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 (Fig. 2a), 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 (Fig. 2b). 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.