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ENSO Quick Look
Technical ENSO Update21 October 2004
Current ConditionsSST conditions within the central and east-central equatorial Pacific, averaged over September, were above-average and of a magnitude indicative of weak El Niño conditions. Since the last week of July, SSTs in the NINO3.4 region have been approximately three-quarters of a degree C above average. However, in order to be considered an El Niño "event", these above-average temperatures must persist for several more months. The SSTs in the far eastern equatorial Pacific have warmed from below-average to near average over the last month. Observations from late-September to mid-October indicate SST anomalies exceeding 0.5 degrees C from 160E to 130W with a maximum anomaly on the equator of approximately 1.5 degrees C in the vicinity of 175W. Positive anomalies of 2 to 4 degrees C are observed in the sub-surface ocean near the thermocline depth in the central Pacific, within 140-90W. The positive anomalies in the eastern-central Pacific are the result of fairly strong westerly wind events that occurred during June and August in the western Pacific. Low-level westerly wind anomalies have shown some persistence in late August and the first half of September, and have continued at a somewhat weaker intensity since mid-September. Their relative location, slightly west of the maximum SST anomalies, is characteristic of evolving El Niño conditions. If this evolution continues, it would represent a seasonally rather late onset of El Niño conditions, reminiscent of the 1986-87 El Niño. On the other hand, if the anomalous westerly wind activity does not continue there could be only a smaller and shorter-lived positive SST anomaly. Presently, anomalous convection near and east of the dateline is weak, suggesting that the atmosphere is not strongly committed to the above average SSTs in the central Pacific. The equatorial (and standard) Southern Oscillation Index (SOI) has been negative since June 2003, but has fluctuated considerably and some of the last four months, including September, have had only weakly negative SOI. Thus the coupled air-sea system in the tropical Pacific basin is tending toward an El Niño event, but some features typical of an evolving event are not yet manifest. However, a tendency toward El Niño conditions is quite possible in the upcoming months in response to low-level zonal wind anomalies acting in the Western Pacific.Expected ConditionsPresently in mid-October the potential for El Niño conditions for the Oct-Nov-Nov period carries a probability of 60%, which represents more than a doubling of the odds relative to the historical climatological likelihood. There is virtually a 0% probability for La Niña, leaving an approximately 40% probability for neutral conditions. During the period of October through early 2005, there is an enhanced probability for existence of El Niño conditions (approximately 60% from northern autumn increasing to 70% in early 2005), compared with the climatological probability of 25%. Because the ENSO state in early October has a strong tendency to persist throughout the remainder of the calendar year (it "sets the stage" for the October-through-March ENSO condition), the enhanced chances of El Niño carries through to the somewhat longer leads of the current forecast.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 55% and 65% through the end of the 2004 calendar
year, and rises to 75-80% in the very early months of 2005.
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 neutral conditions and
for
La Niña. This suggests that the greatest likelihood is for
El Niño over the coming several months and throughout the rest
of
2004 and early 2005. The magnitude of the event, however, is expected
to be only weak.
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