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Technical ENSO Update
20 January 2011
Current ConditionsAs of mid-January 2011, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For December the SST anomaly in the NINO3.4 region was -1.53 C, indicative of moderate to strong La Niña conditions, and for the October-December season the anomaly was -1.52 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 Jan-Feb-Mar and the Feb-Mar-Apr seasons are approximately (-0.55C, 0.50) and (-0.45, 0.40), respectively.
Expected ConditionsThe most recent weekly SST anomaly in the NINO3.4 region is -1.7 C, indicating moderate to strong La Niña conditions in the tropical Pacific; this is just slightly cooler than the -1.53 C level observed in December. What is the outlook for the ENSO status going forward? January is a time of the year during which the observed ENSO state is often beginning to move toward weaker anomaly values, particularly if an ENSO episode has been occurring. One might ask whether the current La Niña condition should therefore be expected to weaken, and if so, at what rate. In the current case, negative subsurface sea temperature anomalies have continued to occupy the central and eastern equatorial Pacific, and have shown little tendency to weaken even in the last one to two months as the event has been in the process of discharging at the surface. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following few months, as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies indicate much enhanced trades, especially in the western and west-central portion of the tropical Pacific, and very strongly positive traditional and equatorial SOI indices. These atmospheric features imply above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming two or more months. The continued surfacing of below-average subsurface waters implies a likelihood for short-term (at least one month) continuation of the strength of the currently moderate to strong La Niña conditions, despite that the seasonal cycle of ENSO suggests a typical decline in strength at this time of year. Above-average subsurface waters exist in the western tropical Pacific, and although they have been edging eastward during the last few months they do not appear poised to displace the large volume of below-average water to their east for at least two or more months, and possibly longer.
Presently, the models and observations taken together indicate
probabilities of approximately 98% for maintaining La Niña conditions, near
2% for returning to ENSO-neutral conditions, and nearly 0% for developing
El Niño conditions during the Jan-Mar 2011 season in
progress. Probabilities for La Niña decrease slightly to 88% for
Feb-Apr, and to 67% for Mar-May. In late northern spring
the probabilities for La Niña weaken
at a faster rate, declining to 46% for Apr-Jun and to 27% for Jun-Aug and
for the following several seasons.
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 models show unanimous agreement regarding the continuation of
La Niña conditions into part of the second quarter of
2011, but vary somewhat in their predictions of the rate of decreasing
strength starting from the Jan-Mar season.
Most statistical and dynamical models call for at least moderate La
Niña strength (stronger than -1C) through the Feb-Apr season.
For the Jan-Mar, Feb-Apr and Mar-May seasons, 100% of the models
are predicting La Niña conditions, while none predicts ENSO-neutral
conditions. Following Mar-May, some models begin indicating
a return to neutral ENSO conditions, but not to the point of being
a majority until the May-Jul season. By Jul-Sep,
two of the 23 models call for weak 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,
6 of 13 (46%) predict ENSO-neutral SSTs for the Jun-Aug seasons,
6 of 13 (46%) predict La Niña conditions, and 1 of 13 (8%) predict
1). (Note that La Niña conditions for Jun-Aug require
a NINO3.4 SST anomaly of -0.50 or stronger, and El
Niño conditions require 0.45 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 near 100% for Jan-Mar and Feb-Apr, declining to
95% for Mar-May and 69% for Apr-Jun.
(Note that the threshold for La Niña
weakens from approximately -0.55C to -0.45C between Jan-Mar
and Apr-Jun, due to the seasonality of the interannual variance).
The forecasters believe, however, that the northern spring model-based
probabilities for La Niña may be higher than it should be, due to a
common model bias of persisting
ENSO episodes for too long a duration at the end of their typical seasonal cycle. Model
probabilities for La Niña are 46% for May-Jul, and decrease to
less than 40% for Jul-Sep and beyond.
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.
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 98% probability for La Niña
conditions in the Jan-Mar season in progress, and only slightly
less (88%) for Feb-Apr.
Probabilities for neutral ENSO conditions rise from 2% in Jan-Mar
to 11% in Feb-Apr, and to 44% by Apr-Jun.
The probabilities for a return to El Niño conditions remain at
very low levels from the present through middle northern spring 2011, not
reaching 10% until Apr-Jun and rising to 23% for Jun-Aug and for the
following several seasons.
See also:Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.