Current ENSO Information
Technical ENSO Update
20 May 2004
Current Conditions
SST conditions are near average, and therefore ENSO-neutral, over much of
the central and eastern tropical Pacific. Observations from late-April to
mid-May indicate warmer than average SST anomalies from the dateline through
the western tropical Pacific, mainly near average SSTs (e.g. between -0.5
and 0.5 degrees C from average) from the dateline to 120W longitude, and
below normal SST from 110W eastward to the South American coast. Weakly positive
anomalies of 1 to 2 degrees C are observed in the sub-surface temperature
(or equivalently themocline depth anomalies of about 20-40m) in a portion
of the east-central equatorial Pacific, with weakly negative sub-surface
anomalies farther to the east. 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. For example, a strongly negative standard
SOI was observed in April; this has already returned to near zero. Trade
winds across much of the basin have been near normal over the last 6 months,
with some spells of both weaker and stronger than normal trades on the intraseasonal
time-scale. Overall, conditions have not consistently indicated a tendency
for either El Niño or La Niña development.
Expected Conditions
Presently in mid-May the potential for El Niño for the May-Jun-July
period is believed to be lower than its historical, climatological probability
of 25%. There is a 5% probability for La Niña and a 15% probability
for El Niño, leaving an approximately 80% probability for continued
neutral conditions. During the period of July through the remainder of 2004,
there is a slightly enhanced probability for development of El Niño
conditions (approximately 40% from late northern summer to northern fall),
compared with the average probability of 25%. Because the ENSO state in July,
and even in June, has some tendency to persist throughout the remainder of
the calendar year (it "sets the stage" for the July-through-March ENSO condition),
the slight enhancement of the chances for 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 Jun-Jul-Aug period of 2004 is 6 out of
a total of 20, or 30%. 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,
6 out of 17 (35%) call for SSTs of at least a minimum El Niño level
for the Jun-Jul-Aug period, while 0 (0%) predict SSTs cold enough to be considered
a La Niña. (Note
1). Overall, most of the models indicate that the now average SST conditions
in the NINO3.4 region may warm somewhat, but not rise to levels sufficient
to represent a weak El Niño. For the longer lead forecast for Sep-Oct-Nov
2004, 4 of the 18 models (22%) forecasts El Niño conditions, 14 models
(78%) forecast neutral conditions, and no models (0%) forecast La Niña
conditions. If only the 15 models that use subsurface ocean temperature data
are included, these figures become 4 (27%), 11 (73%), and 0 (0%). 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 it's 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 enhancecd probability
of El Nino that maximizes at near 40% during the seasons of JAS and ASO, thereafter
dropping to near 35%. 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 neutral conditions throughout the outlook
period, with maximum probability for neutral between now (May) and July.
The chances for La Niña onset are less than for an average year through
all of 2004, and for El Niño onset slightly higher than that of an
average year from northern summer onward. Still, chances for El Niño
never attain 50%. This suggests continuation of the currently near neutral
conditions over the coming several months and throughout the rest of 2004.
See also:
Note 1 - Only models that
produce a new ENSO forecast every month are included in the above statement.
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