Current ENSO Information
Technical ENSO Update
15 April 2004
Current Conditions
SST conditions are near average, and therefore ENSO-neutral,
over much of the central and eastern tropical Pacific. Observations
from late-March to
mid-April indicate warmer
than average SST
anomalies
from the dateline through the western tropical Pacific, and near
average SSTs (e.g. between -0.5 and 0.5 degrees C from
average) in most of the east-central and eastern part of the
basin. SST anomalies in the east-central part of the basin had attained
the
0.5 C level during October and November, but since then have returned
to average. 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 western equatorial
Pacific. The
equatorial (and
standard) Southern Oscillation Index
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. Trade winds across much of the
basin have been near normal over the last 6 months, with some spells of
mildly stronger than normal trades this month following weaker than
normal
trades two to three months ago. However, during the last few weeks a
strong anomalously westerly wind event has been occurring in the
western Pacific from 140 to 170E longitude. This comes amid a general
lack of large-scale
support from either low
level equatorial winds or sub-surface sea
temperature anomalies over the last 2 months for development of either
El Niño or La
Niña.
Expected Conditions
Presently in mid-April the potential for El Niño for the
Apr-May-Jun 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 May 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 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 May-Jun-Jul 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 May-Jun-Jul 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 Aug-Sep-Oct 2004, 8 of the 18 models
(44%) forecasts El Niño conditions, 7 models (56%) 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 8 (53%), 7 (47%), 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 45% 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 highest probabilities for
neutral conditions at the beginning of the outlook period, decreasing
to near its climatologically expected 50% around northern summer, and
then rising again slightly at the longer leads. 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 late northern
spring 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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