Current ENSO Information
Technical ENSO Update
14 July 2004
Current Conditions
SST conditions are near average, and therefore ENSO-neutral, over much
of
the central and eastern tropical Pacific. Observations from late-June
to
mid-July indicate warmer than average SST anomalies from the
east-central Pacific through
the western tropical Pacific, near average SSTs (e.g. within 0.5
degrees C from average) from about 135W to 145W longitude (i.e. in the
western part
of the NINO3 region), and nonuniformly below normal SST from 135W
eastward to the South American coast. Positive anomalies of 2 to
4 degrees C are observed at the thermocline depth (approximately 150
meters) near and just east of the dateline, while weaker negative
subsurface anomalies reside farther east. The positive anomalies are
the result of a fairly strong westerly wind event that occurred during
June in the western Pacific and now continues at a moderated intensity
in
an area that has extended east of the dateline. These low-level
westerly wind anomalies, if continued, could induce SST increases to
the east of the dateline that could allow the El Nino threshold to be
exceeded in the NINO3.4 region. If this occurs, it would represent a
seasonally rather late onset of El nino conditions, reminiscent of the
1986-87 El Nino. 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 a
lack of ocean-atmosphere coupling despite SSTs at the dateline being
above average. 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. In June the SOI indices were
fairly strongly negative. Thus, overall, conditions
have not consistently indicated a
tendency
for either El Niño or La Niña development. Presently,
however, there is a tendency toward El Nino in terms of the SOI and
low-level zonal wind anomalies.
Expected Conditions
Presently in mid-July the potential for El Niño for the
July-Aug-Sep
period is believed to be close to its historical,
climatological
probability
of 25%. There is virtually a 0% probability for La Niña and a
20%
probability
for El Niño, leaving an approximately 80% probability for
continued neutral conditions. During the period of August through
the
remainder of 2004,
there is a somewhat enhanced probability for development of El
Niño
conditions (approximately 40% from northern autumn through
early
2005),
compared with the average probability of 25%. Because the ENSO state in
August,
and even to some extent in June-July, has some tendency to persist
throughout the remainder
of
the calendar year (it "sets the stage" for the July-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 Aug-Sep-Oct period of 2004 is 8 out
of
a total of 20, or 40%. The number of models that predict La Niña
conditions
is 1 (5%). 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, 7 out of 18 (39%) call for SSTs of at least a minimum El
Niño
level
for the Aug-Sep-Oct period, while 1 (6%) predict SSTs cold enough to be
considered
a La Niña. (Note
1). Overall, a weak majority of the models indicate that the now
average SST
conditions
in the NINO3.4 region may stay somewhat warm, but not rise to levels
sufficient
to represent a weak El Niño. For the longer lead forecast for
Nov-Dec-Jan
2004-05, 6 of the 19 models (32%) forecast El Niño conditions,
12
models
(68%) forecast neutral conditions, and no models (0%) forecast La
Niña conditions. If only the 17 models that use subsurface ocean
temperature data
are included, these figures become 5 (29%), 12 (71%), 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 rises beyond 35% near the end of the 2004 calendar
year, and maximizes at near 45% during the seasons of DJF and JFM of
2004-05. 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 neutral conditions
throughout the
outlook
period, and chances for La Niña that are less than for an
average year
through the remainder of 2004 and into early 2005. It calls for chances
for El Niño
onset slightly
higher than that of
an
average year from northern autumn season onward, but never attaining
the 50% level. This suggests that the greatest likelihood is for
continuation of the currently near
neutral
conditions over the coming several months and throughout the rest of
2004 and early 2005.
See also:
Note 1 - Only models
that
produce a new ENSO forecast every month are included in the above
statement.
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