The chance of a noteworthy El Niño event this winter is becoming more slender, diminishing along with California’s chances for more drought relief. In its latest monthly outlook
on the state of the El Niño/Southern Oscillation (ENSO), issued this morning, NOAA's Climate Prediction Center continued its El Niño Watch but reduced the odds of El Niño conditions from the previous 65% to approximately 50–60%. Moreover, the agency now calls for ENSO-neutral conditions (neither El Niño nor La Niña) to be the most likely state of affairs from March onward. This is a significant change from NOAA’s previous monthly outlook, which had projected that El Niño conditions would likely extend into spring 2015.
NOAA considers El Niño conditions
to be in place when monthly sea-surface temperatures (SSTs) are at least 0.5°C above average in the Niño3.4 region
of the tropical Pacific. To qualify as an El Niño episode
, the SSTs in this region must remain at or above the 0.5°C threshold for five consecutive overlapping periods of three months (i.e., a total of seven months).
Using this yardstick, weak El Niño conditions have now prevailed for more than two months. After rising above 0.5°C in mid-October, the Niño3.4 SST anomalies peaked near 1.0°C in late November, then began dropping (see Figure 1). The anomalies are now at 0.5°C, barely qualifying as El Niño-worthy. Should the 0.5°C anomaly hold for a few more months (not at all a sure thing), the 2014–15 El Niño would manage to go down in history as a bona fide episode, though a rather unimpressive one. Figure 1.
Departures from average sea-surface temperature in degrees Celsius (left-hand axis) across the Niño3.4 region of the tropical Pacific Ocean. After briefly surging above the El Niño threshold (0.5°C above average) in late May 2014, sea-surface temperatures sagged back into neutral territory until autumn. Image credit: NOAA Climate Prediction Center.
Because El Niño’s impacts on California and the southwest United States are closely tied to the intensity of the Niño3.4 anomalies, there is now less confidence that El Niño will help bring beneficial rain and snow this winter to these drought-stricken parts of the nation. NOAA’s Mike Halpert includes helpful background and graphics on typical U.S. impacts from El Niño in a feature story
on the climate.gov website.Why has El Niño been so hard to predict lately?
Forecasters have been scratching their heads over the state of ENSO for almost a year. Early in 2014, several large, shallow pulses of warm water called Kelvin waves made their way across the tropical Pacific. One of these was comparable in size and strength
to the Kelvin wave that helped kick off the record-setting, high-impact El Niño of 1997–98. The behavior of the Pacific in early 2014, together with impressive projections from some coupled atmosphere-ocean models, led NOAA and other forecast groups to call for a significant chance of El Niño conditions by mid- to late 2014. These observations and outlooks gained widespread attention in the press and blogosphere, leading to some eye-catching headlines trumpeting the chance of a "super El Niño." Official forecasts were more cautious, stopping short of calling for a strong event, as Weather Underground climate blogger Ricky Rood outlined in an illuminating comparision
last August. (It's also important to keep in mind that even a 70% chance of an event like El Niño—quite high by seasonal prediction standards—means that there is a 30% chance the event won’t occur.)Figure 2.
Departures from average sea-surface height as detected by NASA's Jason satellite on December 10, 1997 (top), when a major El Niño event was in full swing, and by the Jason-2 satellite on December 20, 2014 (bottom). Since water expands as it warms, higher sea-surface heights (whites and oranges) indicate warmer waters. The bright white in the 1997 image conveys the power of that year’s record-breaking El Niño, especially when compared to the underperforming 2014. Image credit: NASA.
Although the tropical Pacific waters behaved as if a major El Niño was on tap, it seems the atmosphere didn't get the message. Throughout 2014, the atmospheric component of El Niño failed to emerge consistently even as oceanic conditions appeared favorable. For example, a developing El Niño typically sees trade winds weakening across the eastern Pacific, which facilitates the eastward spread of warm surface water. Although several bursts of westerly wind did appear in 2014, these have not translated into widespread, long-lived weakening of the easterly trade winds. Without the linkage that emerges from this kind of ocean-atmosphere interaction, it is difficult for El Niño conditions to take hold in a big way.One researcher’s take on the mystery
ENSO was on the agenda at this week's annual meeting of the American Meteorological Society, where sessions on Tuesday morning
focused on the challenges of predicting El Niño and La Niña and what’s been learned over the last 20-plus years.
Eminent ENSO researcher Michael McPhaden (NOAA Pacific Marine Environmental Laboratory) presented a surprise talk on Tuesday called "Who Killed the 2014 El Niño?" The answer remains unclear, as McPhaden emphasized by presenting a rogue’s gallery of possible culprits in the form of a police lineup. These included:
--Negative feedbacks, or interactions that work against El Niño development rather than nourishing it.
--The negative state of the Indian Ocean Dipole
, which supports rainfall in the far western tropical Pacific, as opposed to its typical eastward shift of rainfall during El Niño.
--The negative phase of the Pacific Decadal Oscillation
, which has been in place since the late 1990s. A negative PDO tends to be associated with reduced El Niño activity.
--A lack of westerly wind bursts strong enough to kick off El Niño.
--Persistently warm SSTs in the "warm pool" of the western tropical Pacific, where water temperatures normally drop below average during El Niño.
It will take time and research to figure out to what extent the demise of the anticipated 2014 El Niño was a group effort versus a solo job. However, it seems clear that the lack of the usual atmospheric involvement sealed the deal. As McPhaden put it, "The atmosphere is not engaged."