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ENSO Quick Look Technical ENSO Update16 December 2004
Current ConditionsSST conditions within the central and western-central equatorial Pacific, averaged over November, were above-average and of a magnitude indicative of weak El Niño conditions. 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. Since the last week of July, SSTs in the NINO3.4 region have been approximately three-quarters of a degree C above average. At this time of year, that magnitude of NINO3.4 just barely qualifies as El Niño conditions. In order to be considered an El Niño "event", El Niño conditions must persist for at least 5-6 months.Observations from late-November to mid-December indicate SST anomalies exceeding 0.5 degrees C from 145E to 125W with a maximum anomaly on the equator of approximately 1.5 degrees C in the vicinity of 175E. Although the pattern of above-average SST anomalies is large-scale, it is focused just west of the dateline rather than in the eastern or central-eastern Pacific, and thus differs from previous El Niños.The eastern Pacific, which is usually instrumental in the development of an El Niño event, has remained largely neutral throughout 2004. The SSTs in the far eastern equatorial Pacific had actually been below-average, but during October warmed to near to just slightly above average. Positive anomalies of 2 to 4 degrees C are observed in the sub-surface ocean near the thermocline depth in the central and east-central Pacific. The positive anomalies in SST and in the sub-surface temperature in the western-central Pacific are the result of westerly wind events that occurred during June, August, October, and presently again in mid-December in the western Pacific. These westerly wind events advect the eastern edge of the West Pacific warm pool further eastward. Low-level westerly wind anomalies have shown some persistence since May, but they have been weak and have remained mainly west of approximately 170E in the western Pacific, in contrast to previous El Niño events where the persistent westerly wind anomalies were at least twice as strong and extended to 170W or even farther east. The large-scale atmospheric changes expected during an El Niño event, such as enhanced convection in the central Pacific, have been conspicuously absent. Presently, anomalous convection near and east of the dateline is non-existent, suggesting that the atmosphere is not committed to the above average SSTs in the central Pacific. The Southern Oscillation Index (SOI) has been negative since June 2004, but has fluctuated considerably and the monthly values, including November, have typically shown only mildly to moderately negative SOI. Furthermore, the standard SOI (which is a difference between Tahiti and Darwin SLP) this year has been negative primarily because of the high pressure pattern over Indonesia and northern Australia, with little contribution from SLP changes over Tahiti. The anomalous conditions of late 2004 are likely to be categorized as an El Niño event as measured by NINO3.4 SST index, but many features typical of an evolving event are not manifest. The existence of warm SST anomalies in the central equatorial Pacific are likely to influence local climate patterns. The NINO3.4 region is implicated as the region of the tropical Pacific where SST anomalies are most associated with global ENSO teleconnections. The fact that the NINO3.4 index is (and has been since mid-2004) in the warmest 25%-ile of the historical distribution suggests that remote teleconnections are possible. However, such teleconnection responses are unlikely as long as the convection anomalies that connect SST to the overlying atmospheric circulation are absent. Expected ConditionsAssuming at least weak persistence of the low-level westerly winds in the western Pacific, the maintenance of El Niño SST conditions is quite possible in the upcoming months. The deeper than average thermocline (above-average sub-surface temperatures) in the central Pacific should contribute to the maintenance of the central-eastern Pacific SST anomalies and may also increase SST anomalies in the far eastern equatorial Pacific in the next couple months as the subsurface perturbation moves eastward. However, since the current magnitude of the overall ocean-atmosphere coupling is weak, there is some possibility that the tropical Pacific SST anomalies could edge back towards neutral conditions.Presently in mid-December the potential for maintaining El Niño SST conditions through the Jan-Feb-Mar period carries a probability of 75-80%, which represents an approximate tripling of the odds relative to the historical climatological likelihood. There is virtually a 0% probability for La Niña SST conditions, leaving an approximately 20-25% probability for neutral SST conditions. The above assessment was made in part on the basis of an
examination
of the current forecasts of ENSO prediction models. 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 0.4 degrees C in Mar-Apr-May and as high as 0.75 degrees C in
Oct-Nov-Dec. 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. 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 indicate an enhanced probability of El Niño
that varies between 80% and 90% through the Feb-Mar-Apr season,
dropping to below 50% beginning in the May-Jun-Jul season. For all
periods the probability for La Niña is well below its
climatological value of 25%. 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 higher than average probabilities for El Niño
conditions throughout the
outlook
period, and lower than average probabilities for La Niña.
Probabilities for neutral ENSO conditions are low until the end of
northern spring 2005, when they return toward their normal of
approximately 50%. This suggests that the greatest likelihood is for
continuation of El Niño conditions over the coming several
months. The magnitude of the event is expected
to continue to be weak.
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