ENSO Quick Look |
IRI ENSO Update / Forecast
Technical ENSO Update
IRI Plume-Based ENSO Forecast
CPC/IRI Consensus Probabilistic ENSO Forecast
ENSO Prediction Plume
Technical ENSO Update
19 January 2011
Recent and Current ConditionsWeak La Niña conditions emerged in early August 2011, and grew to weak/moderate strength during northern autumn 2011. As of mid-January 2012, SST anomalies remain in weak to moderate La Niña territory in the central and eastern equatorial Pacific. For December the SST anomaly in the NINO3.4 region was -1.04 C, indicative of weak to moderate La Niña conditions, and for the October-December season the anomaly was -1.02 C. Starting in December 2011, the IRI's definition of El Niño conditions follows that of NOAA/Climate Prediction Center, in which the SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) exceeds 0.45 C. Similarly, for La Niña, the anomaly must be -0.45 C or less. The most recent weekly SST anomaly in the NINO3.4 region is -1.0 C, indicating weak to moderate La Niña conditions in the tropical Pacific; this is about the same as the -1.04 C level observed in December.
What is the outlook for the ENSO status going forward? The official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI. Now, in the middle of January, a new set of model ENSO predictions is available as shown in the IRI/CPC ENSO prediction plume, to be discussed below. The current La Niña SSTs have continued to be below average, and subsurface temperatures across the equatorial Pacific imply a strong likelihood of maintaining at weak to moderate La Niña conditions in the short term, with a good chance for continuing at approximately the current weak to moderate strength for another month or more before beginning to weaken in late February and March.
As of mid-January, most of the dynamical and statistical models
predict La Niña conditions for the Jan-Mar season, with weakening
beginning toward the end of that period and continuing into the
subsequent seasons in early 2012. For the Jan-Mar season,
92% of the models indicate La Niña conditions, and 8% indicate neutral conditions.
For Feb-Apr, these figures become 77% and 23%, respectively.
At lead times of 4 or more months into the
future, statistical and dynamical models that incorporate
information about the ocean's observed subsurface thermal
structure generally exhibit higher predictive skill than those that do
not. Among models that do use subsurface temperature information,
70% predict ENSO-neutral SSTs for the May-Jul 2012 season,
25% predict La Niña conditions, and 5% predict
El Niño conditions.
Caution is advised in interpreting the distribution of model predictions as the
actual probabilities. At longer leads, the skill of the models
degrades, and skill uncertainty must be convolved with the
uncertainties from initial conditions and differing model physics,
leading to more climatological probabilities in the long-lead ENSO
Outlook than might be suggested by the suite of models.
Furthermore, the expected skill
of one model versus another has not been established using uniform
validation procedures, which may cause a difference in the true
probability distribution from that taken verbatim from the raw
An alternative way to assess the probabilities of the three
possible ENSO conditions is more quantitatively precise
and less vulnerable to sampling errors than the categorical
counting method used above. This alternative method uses
the mean of the predictions of all models on the plume, equally weighted,
and constructs a standard error function centered on that
mean. The standard error is Gaussian in shape, and has its
width determined by an estimate of overall expected model skill for the
season of the year and the lead time. Higher skill results in a
relatively narrower error distribution, while low skill results in
an error distribution with width approaching that of the historical
observed distribution. This method shows probabilities for La
Niña at 97% for Jan-Mar, decreasing to 81% for Feb-Apr,
54% for Mar-May, and down to 37% by Apr-Jun.
Model probabilities for El Niño are near
0% for Jan-Mar through Mar-May and remain below 10% through Apr-Jun 2012.
Probabilities for ENSO-neutral
conditions are 3% for Jan-Mar and rise to 19% by Feb-Apr and
more quickly thereafter, exceeding 50% beginning in Apr-Jun.
A plot of the probabilities
generated from this most recent IRI/CPC ENSO
prediction plume using the multi-model mean and the Gaussian standard
error method summarizes the model consensus out to about 3 seasons into the future.
The same cautions mentioned above for the distributional
count of model predictions apply to this Gaussian standard
error method of inferring probabilities, due to differing model
biases and skills. In particular, this approach considers only the
mean of the predictions, and not the total range across the models, nor the
ensemble range within individual models.
The probabilities derived from the more than 20 models on the IRI/CPC plume describe a dissipation of the La Niña during the northern spring 2012. Past experience has shown that some models are biased in the direction of prolonging ENSO episodes for somewhat too long a period at the end of the typical ENSO cycle. If such a bias exists in the present case, this weak to moderate La Niña may end in the February to April timeframe rather than the March to May timeframe as indicated by the models collectively. Factors such as known model biases and recent changes that the models may have missed will be taken into account in the official outlook generated by IRI/CPC to be issued at the beginning of the February.
See also:Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.