2014 has been a rough year for El Nino forecasting.
During Winter and Spring, an extraordinarily strong Kelvin wave rocketed across the Pacific. Containing heat anomalies in excess of 6 C above average, this flood of trans-Pacific warmth hit the ocean surface, dumping an extraordinary amount of heat into the atmosphere. The heat helped drive global sea surface temperatures for May, June, and July to all-time record values.
Many forecasters believed that this heat would lead to a moderate to strong El Nino event starting this summer. And, by June, NOAA was predicting that El Nino was 80% likely to emerge some time this year.
But the initial oceanic heat pulse was crushed by a failure of atmospheric feedbacks. The trans-Pacific trade winds, with a few visible exceptions, remained strong enough to suppress El Nino formation. And so it appeared that, by late July, the initial powerful heat pulse providing potential for El Nino had almost entirely fizzled.
Then, a second warm Kelvin Wave began to form even as Southern Oscillation values started to fall.
(Second warm Kelvin Wave running across Pacific has resurrected the potential for a weak to moderate late 2014 El Nino. Image source: Climate Prediction Center.)
This second Kelvin Wave contains a broad swath of +2 to +5 C anomaly values and is rapidly propagating toward the surface zones of the Central and Eastern Pacific. And though not as strong as the Kelvin Wave that formed earlier this year, the current Kelvin Wave is occurring in conjunction with what appears to be a somewhat more robust atmospheric feedback.
The Southern Oscillation Index, a measure of pressure differences between Tahiti and Darwin, is an indicator of Nino related atmospheric conditions. At consistent values below -8, weather variables tend to favor El Nino formation. And, for the past twelve days, 30 day averages have been below the -8 threshold. If these values extend for much longer, the coincident warm Kelvin Wave and atmospheric conditions favorable for El Nino may well set off this long-predicted event.
Model runs still show a 60-65% chance of El Nino formation before the end of this year and NOAA’s forecast continues to call for a weak El Nino forming some time in late 2014:
(Model Forecast shows 60-65 percent chance of El Nino by November through January. Image source: CPC/IRI.)
It is worth noting that this second warm Kelvin Wave is providing the last chance for El Nino in 2014. So if atmospheric feedbacks fade and sea surface temperatures remain just on the high side of ENSO neutral, then 2014 will close without the incidence of this wide-scale Pacific Ocean and atmospheric warming event.
With weak El Nino, however, there is still a likelihood that 2014 will tie or exceed hottest ever global surface temperature values. A failure for El Nino to form will probably result in 2014 closing as one of the five hottest years on record, given current trends.