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Technical ENSO Update
16 June 2011
Current ConditionsAs of mid-June 2011, SST anomalies have returned to average, reflecting ENSO-neutral conditions in the central and eastern equatorial Pacific. For May the SST anomaly in the NINO3.4 region was -0.45 C, indicative of borderline La Niña conditions, and for the February-April season the anomaly was -0.73 C. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Jun-Jul-Aug and the Jul-Aug-Sep seasons are approximately (-0.50C, 0.45) and (-0.50, 0.45), respectively.
Expected ConditionsThe most recent weekly SST anomaly in the NINO3.4 region is -0.0 C, indicating ENSO-neutral conditions in the tropical Pacific; this is warmer than the -0.45 C level observed in May. The moderate to strong La Niña episode ended during early to middle May as the SST anomaly warmed beyond the -0.5C level and the Southern Oscillation Index (SOI) dropped from near-record levels in April to the neutral range by early May. What is the outlook for the ENSO status going forward? June and July often represent a time of the year of development of a new ENSO state. The currently somewhat warmer than average subsurface temperatures across the equatorial Pacific imply fairly low probabilities of returning to La Niña conditions in the coming few months, and even a possibility for El Niño development. However, because of the strong and long-lasting atmospheric component of the now decayed La Niña, some of the climate effects associated with La Niña may continue to linger through June. Whether the somewhat positive subsurface sea temperature anomalies will surface in the east-central tropical Pacific and induce weak El Niño conditions in the coming few months is currently uncertain, since this is still the time of year when future ENSO evolution is difficult to predict.
Presently, the models and observations taken together indicate
probabilities of approximately 84% for maintaining ENSO-neutral conditions,
9% for returning to La Niña conditions, and near 7% for developing
El Niño conditions, during the Jun-Aug 2011 season in
progress. Probabilities for La Niña remain near 15% from Jul-Sep
through the remainder of 2011. Similarly, probabilities for El Niño
conditions increase to 15% for Jul-Sep and remain close to that level throughout the
rest of the calendar year. Hence, neutral conditions are clearly the most
likely scenario during the second half of 2011 and into early 2012, with
probabilities hovering near 70%.
The above assessment was made in part on the basis of an examination of the current predictions of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.
The majority of the dynamical and statistical models show
neutral conditions for the Jun-Aug season and thereafter through the
remainder of 2011. For the Jun-Aug season, 87% of the models indicate
neutral ENSO conditions, and 9% indicate weak El Niño conditions.
By Jul-Sep, 78% show neutral conditions, 9% show La Niña conditions,
and 13% show El Niño conditions. 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,
11 of 16 (69%) predict ENSO-neutral SSTs for the Oct-Dec season,
2 of 16 (12%) predict La Niña conditions, and 3 of 16 (19%) predict
El Niño conditions.
1). (Note that La Niña conditions for Oct-Dec require
a NINO3.4 SST anomaly of -0.75 or stronger, and El
Niño conditions require 0.75 or stronger.) 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 to use the mean of the predictions of all
models, and to construct a standard error function centered on that
mean. The standard error would be Gaussian in shape, and would have its
width determined by an estimate of overall expected model skill for the
season of the year and the lead time. Higher skill would result in a
relatively narrower error distribution, while low skill would result in
an error distribution with width approaching that of the historical
observed distribution. This method shows probabilities for La
Niña at 9% for Jun-Aug, increasing to 12% for Jul-Sep and hovering near 14%
throughout the remainder of 2011. Model probabilities for El Niño are
5% for Jun-Aug, rising to 15% for Jul-Sep and hovering near 17% through the rest
of 2011. Probabilities for ENSO-neutral conditions are 85% for Jun-Aug,
settling to near 70% throughout the remainder of 2011 and into early 2012.
The same cautions mentioned above for the distribution of model predictions apply
to this alternative 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 range across the models, nor the
ensemble range within individual models.
The IRI's probabilistic ENSO prediction takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates a 9% probability for La Niña conditions in the Jun-Aug season in progress, increasing slightly to near 15% thereafter, into early 2012. Probabilities for El Niño conditions rise from 7% in Jun-Aug to near 15% thereafter through 2011 and into early 2012. Probabilities for ENSO-neutral conditions are considerably higher than those of the other two categories for all seasons, starting at 84% for Jun-Aug and settling to near 70% thereafter, dropping just slightly by northern spring 2012.
See also:Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.