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ENSO Quick
Look IRI ENSO Update / Forecast IRI Probabilistic ENSO Forecast Technical ENSO Update ENSO Prediction Plume Monthly archive Technical ENSO Update18 October 2006
Current ConditionsThe current conditions, as of mid-October 2006, indicate that El Niño is developing. SSTs are now approximately 1.0C above average throughout the equatorial Pacific, and exceed 1.5C near the dateline and in the far eastern equatorial Pacific. The SST anomalies in the eastern Pacific have been steadily increasing over the last several months. Consistent with this, there has been a slow evolution of the equatorial Pacific towards warmer conditions since mid-2006. Deep, or downward, perturbations exist across the equatorial thermocline, particularly in the east. These perturbations to the sub-surface temperature structure were generated by anomalous westerly winds over the western Pacific during July and August, and are now being reinforced by more recent westerly wind anomalies that have a larger magnitude and fetch than those of a few months ago. The deep thermocline anomalies, although not large in magnitude, do have a large and coherent spatial structure and have contributed to the increasing heat content. The increasing SST anomalies are also a result of the westerly wind anomalies, through zonal temperature advection, in the central Pacific. In the east the recent SST increases are related largely to the equatorial adjustment of thermocline anomalies generated by the wind anomalies.For September 2006, the SSTs in the NINO3.4 region were 0.72 degrees C above average, and for the July-August-September season were 0.50 degrees C above average. 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 magnitude of NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Sep-Oct-Nov and Oct-Nov-Dec seasons is approximately 0.75C away from average. Expected ConditionsIndications of El
Niño development exist in the surface and sub-surface
conditions. These include
the resurence of persistent westerly wind anomalies in the western
Pacific and central Pacific since late September, increasing
equatorial heat content and appearance of warm SST anomalies,
particularly in the eastern equatorial Pacific. However, the
sub-surface indicators, such as large-scale deep perturbations to the
equatorial thermocline, whose adjustment
could provide reinenforcement of surface warming, are modest. The
current westerly wind anomalies should enhance the subsurface
anomalies, and further expand the positive SST anomalies. If the
localized maxima in anomalous SST, in the eastern and central Pacific,
were to unite and form a more coherent and larger-scale pattern of SST
anomalies, it could initiate a stronger atmospheric response leading to
the positive feedback necessary for sustained growth. It should be
noted, however, that over the last several years the tropical Pacific
environment
has not readily supported coupled ocean-atmosphere growth,
even when provided with stronger perturbations to the ocean state than
what is currently observed. If that continues to be the case, then the
future variability of the tropical Pacific air-sea
system will be dominated by less deteministic atmospheric
perturbations, the effects of which are much more difficult to
foretell. Although the southern
oscillation index (SOI) is negative, the precipitation anomalies have
shown only minor response to the SST anomalies. The tropical Pacific
ocean-atmosphere system has been slowly evolving towards warmer
conditions over the last several months, and the model predictions of
El Niño have been suggesting progressively warmer outlooks.
As a result, El Niño is
the most likely outcome for the remainder of 2006 and early 2007. There
is an
estimated 80%
likelihood for El Niño, approximately 2% probability for La
Niña conditions to return, leaving just under 20% probability
for ENSO-neutral conditions. The above assessment was made in part on the basis of an examination of the current forecasts 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 0.45 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern 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. There is some variation among ENSO model forecasts, mainly ranging from neutral but warm to moderate El Niño conditions occuring near the end of the year. No model is forecasting La Niña conditions to occur through the forecast period. For the OND 2006 season, 16 out of 20 models (80%) predict El Niño conditions. At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, 12 of 13 models (92%) call for El Niño conditions extending into the Feb-Mar-Apr period; none indicate SSTs exceeding the threshold for La Niña (Note 1). The majority of the models indicate that the SST conditions in the NINO3.4 region will exhibit El Niño conditions throughout the forecast period: OND 2006 (80%) to JJA 2007 (50%). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the coming several months. The expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the above probability estimate from the true probability. An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts 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. When this method is applied to the current model forecasts, results also indicate a probability of El Niño that is higher than the climatological value (25%)-- fluctuating between 72-83% from Oct-Nov-Dec 2006 through the end of the year. For La Niña the probabilities stay below 10% throughout the forecast period. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills. The IRI's
probabilistic ENSO forecast 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 more than tripling of the odds for
El Niño conditions, lower than expected probabilities for
ENSO-neutral, and negligible probability of La Niña conditions
through the end of
2006 and into early 2007.
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