El Niño Recap & La Niña Outlook
This El Niño event was one of the strongest ever recorded (Fig. 5), and if past performance was any indication of what was expected for the Southwest, the region should have seen above-average precipitation over much of the cool season (winter and spring). The Southwest generally saw lower-than-expected precipitation totals that were much closer to average, or even below average in some cases. There are several reasons why this event did not meet expectations. One explanation is the difficulty of predicting highly variable weather events within the context of climate. In the case of the El Niño event of 2015–2016, a ridge of high pressure diverted moisture away from the Southwest during the prime time we might have expected to see increased activity (see El Niño tracker in April 2016 SWCO for details). Another reason is related to the use of analogs, past strong events, such as the El Niño event in 1982–83 or 1997–98). This El Niño event looks to be a clear outlier compared to most other observations that quantify the relationship between the ENSO index value and precipitation anomalies (Fig. 6). This analysis is hampered by limited sample size, which means making broad pronouncements about these patterns is problematic until there are more observations of strong El Niño events to compare to this event, and more data to feed into seasonal forecasts and outlooks. A final reason for the recent El Niño falling short of expectations is the relative infancy of the science of El Niño. The climate and atmospheric/oceanic science community is still developing its understanding of El Niño and the influence of the Arctic Oscillation Index or the Pacific Decadal Oscillation. Experts in these fields will certainly look to this El Niño event to determine what additional information can be gleaned from an event that, by all accounts, could have blanketed the Southwest with regular and steady winter precipitation. Instead, warm and dry conditions dominated throughout much of the cool season in the Southwest, even while the El Niño event performed closer to expectations across most of the globe (Fig. 7).
Perhaps just as importantly, this discrepancy between seasonal outlooks and forecasts and the observed weather of the cool season did not take place in a vacuum. Long-term drought has affected the Southwest throughout much of the 21st century, and the prospect of a strong El Niño event generated considerable hope and optimism for the potential positive impacts of above-average precipitation. This optimism was augmented by media-fed narratives about the potential for extreme precipitation during a strong El Niño event that did not include the necessary caveats associated with most scientific models and forecasts. Still, it was generally understood that a single strong year would not “fix” long-term drought, but that it might at least bend the curve back towards some modicum of recovery. Near-average cool-season precipitation was disappointing when held up to these expectations but was much wetter than would be expected in a typical La Niña winter, a comparison that will be all the more salient by this time next year.
A strong El Niño event is typically associated with increased precipitation across the cool season in the Southwest, but there is a practical limit to how much additional precipitation a desert environment might experience, even in a record El Niño year. Conversely, a La Niña event is associated with decreased cool-season precipitation in the Southwest, somewhat more reliably in terms of forecasts and predictions; layering a dry signal (i.e., La Niña) onto an already dry climate may produce a more consistent result compared to layering a wet signal (i.e., El Niño) onto a dry climate (Fig. 8). The correlations between ENSO status and precipitation anomalies do generally follow this pattern; La Niña events are more reliably dry than El Niño events are reliably wet, and there is considerable variability between wet and dry in ENSO-neutral years. The El Niño event of 2015–2016 was decidedly average and a likely outlier from previous (and subsequent) El Niño events. The La Niña conditions forecast for 2016–2017 are much more likely to produce drier-than-average cool season precipitation totals, with implications for long-term drought in the Southwest.