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.
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