Current ENSO Information
Technical ENSO Update
18 March 2004
Current Conditions
SST conditions are near average, and therefore ENSO-neutral,
over much of the central and eastern tropical Pacific. Observations
from late-February to
mid-March have shown 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
approximately 1 degree C are
observed in the sub-surface temperature (or equivalently themocline
depth anomalies of about 20m) in the far eastern equatorial Pacific,
associated with a weakening eastward-moving Kelvin wave stemming from
an
anomalously westerly wind episode near the dateline in early January.
This Kelvin wave is about to reach the
South American coast, and may counteract the recently strengthening
negative SST anomalies along the
coast of Ecuador and northern Peru.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. There has been a lack of large-scale
support from either low
level equatorial winds or sub-surface sea
temperature anomalies for development of either El Niño or La
Niña. The overall state of the tropical
Pacific climate had leaned slightly more in the direction of El
Niño
than La Niña two to four months ago. This leaning has not
continued during the last month.
Expected Conditions
Presently in mid-March the potential for El Niño for the
Mar-Apr-May period is believed
to be lower than its historical, climatological
probability of 25%. There is a 5% probability for
La Niña and a 10% probability for El Niño, leaving
an approximately 85% probability for continued
neutral conditions. During the period of May through about October
2004, there is a very slightly enhanced probability
(30%) for development of El Niño conditions, 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 very 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 Apr-May-Jun period of
2004 is 2 out of a total of 20, or 10%. 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, 2
out of 17 (12%) call for SSTs of at least a minimum El Niño
level for the Apr-May-Jun 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 to
slightly
above average SST conditions in the NINO3.4 region will continue, but
not rise to levels sufficient to represent a weak El Niño. For
the longer lead forecast for Jul-Aug-Sep 2004, 6 of the 18 models
(33%) forecasts El Niño conditions, 12 models (67%) 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 6 (40%), 9 (60%), 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 JJA and JAS,
thereafter dropping
to near 30%. From JJA onward, the probability for neutral
conditions is near its climatological value of 50%, and for all periods
the probability for La Niña is slightly 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% from northern summer
onward. The chances for La Niña onset are slightly less
likely than for an average year through all of 2004, and for El
Niño
onset slightly higher than that of an average year from JJA onward.
Still, chances for El
Niño never exceed 30%. 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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