Current ENSO Information
Technical ENSO Update
16 September 2004
Current Conditions
SST conditions within
the central equatorial Pacific, averaged over August, 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 at least 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 remain below-average.
Observations from late-August to
mid-September indicate SST anomalies exceeding 0.5 degrees C from the
160E to 120W 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 at the thermocline
depth near in the central Pacific, between 180-120W.
The positive anomalies in the eastern-central
Pacific are
the result of a 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 the last several
weeks. That combined with their relative placement, 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 wind activity does
not
continue there could be only a smaller and more temporary SST increase.
Presently, anomalous convection near the dateline is weak, suggesting
that the atmosphere is not entirely committed to the
above average SSTs in the central Pacific. The equatorial (and
standard) Southern
Oscillation
Index (SOI) has been near zero since July 2003 when smoothed with a
running
mean of 3-months or greater, although individual months have shown
noteworthy excursions in either direction. 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. Presently,
however, there is a tendency toward El Niño conditions possible
in the
upcoming months in response to low-level zonal wind anomalies acting in
the Western Pacific.
Expected Conditions
Presently in mid-September the potential for El Niño conditions
for
the Sep-Oct-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
continued neutral conditions. During the period of September 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
September has considerable tendency to persist
throughout the remainder
of
the calendar year (it "sets the stage" for the September-through-March
ENSO
condition),
the enhanced chances of El Niño carries
through
to the 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.
There is some variation among ENSO model forecasts for the coming
several seasons. The number of models that are forecasting El
Niño conditions to be occurring in the Oct-Nov-Dec period of
2004 is 13 out of a total of 19, or 68%. The number of models that
predict La Niña conditions is 0 (0%). 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, 11
out of 17 (65%) call for SSTs of at least a minimum El Niño
level for the Oct-Nov-Dec period, while 0 (0%) predict SSTs cold enough
to be considered a La Niña. (Note
1). Overall, a majority of the models indicate
that the current above-average SST conditions in the NINO3.4 region may
remain warm enough to represent at least a weak El
Niño. For the longer lead forecast for Jan-Feb-Mar 2005, 14
of the 18 models (78%) forecast El Niño conditions, 4 models
(22%) forecast neutral conditions, and no models (0%) forecast La
Niña conditions. If only the 16 models that use subsurface ocean
temperature data are included, these figures become 12 (75%), 4 (25%),
and 0 (0%). This would imply an increasing probability for El
Niño development toward the end of the calendar year. 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 indicate an enhanced probability of El Niño
that varies between 55% and 65% through the end of the 2004 calendar
year, and rises to 65-75% in late northern winter and spring 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
average or 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 to moderate.
See also:
Note 1 - Only models
that
produce a new ENSO forecast every month are included in the above
statement.
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