Current Information
Technical ENSO Update
16 September 2002
Current Conditions
El Niño conditions continued during late August and early September.
During this time, equatorial SSTs from 180E to 120W longitude remained
at or above1 degree C above normal, and anomalies of 1.5 degrees C or more
stretched from 175W to 155W. SST anomalies sometimes reached 2 degrees
C locally within this region. This warming, which includes much of the
NINO4 and NINO3.4 regions and the western part of the NINO3 region, was
able to sustain itself through the large-scale westerly wind anomalies
created by its own existence, through its resulting anomalous convection
that has allowed for coupling to the atmospheric circulation patterns.
Thus, episodes of westerly wind anomalies in association with Madden-Julian
Oscillation (MJO) activity have become no longer necessary to maintain
the SST anomalies, and this activity has in fact become weak over the last
month. Consistent with the large-scale coupling, the Southern Oscillation
Index (SOI) continued to be below normal. The SST, along with the full
participation of the atmosphere, provides near-certainty that we are in
the first half of a basin-wide El Niño episode. Due to the eastward-shifted
convection and the associated zonal circulation changes, above normal SST
that had dominated the far western tropical Pacific has weakened and below
normal SST has now appeared over parts of Indonesia.
As a result of the westerly wind anomalies along the equator, the equatorial
thermocline has become flatter from west to east. In early September the
themocline remained deeper than normal in the central and east-central
part of the basin. Sub-surface positive temperature anomalies of 2 to 5
degrees C in portions of the mixed layer accompany the deepened thermocline.
These temperature anomalies at depth will tend to reinforce the currently
warm SSTs for at least the next month. In the far eastern tropical Pacific,
from 80W to 95W, somewhat below normal SSTs and a slightly shallower than
normal thermocline that developed during July have become limited to a
smaller longitudinal range during the last month. This feature is not considered
at odds with the continued development and maturation of the El Niño,
and is expected to warm as the episode continues to evolve.
There is only a negligible possibility that the above normal SST in
the central equatorial Pacific could substantially weaken over the next
three months. The continued presence of a large area of positive SST anomalies
of greater than 1 degree C, the persistence of westerly wind anomalies
in the central Pacific, continued below normal SOI, the eastward-shifted
convection, and the presence of positive subsurface temperature anomalies
all favor the continuation of the current SST conditions through the end
of the year.
Expected Conditions
For purposes of this discussion, neutral SST conditions are defined
as the sea surface temperatures (SSTs) in the NINO3.4 region averaged over
the Nov-Dec-Jan period (for which model forecasts are highlighted here)
being more than -0.6 and less than 0.6 degrees C, while SSTs of at least
this magnitude would be indicative of La Niña (if negative) or El
Niño (if positive). This definition allows about half of all years
to be classified as neutral. Although there is some variation among
ENSO model forecasts for the coming 8 months, the spread among the forecasts
is not large. Compared to one month ago, there has been a slight increase
in the already high number of models that are forecasting El Niño
conditions continuing through the remainder of 2002 and into early 2003.
This month, 93% of the 15 models forecast a continuation of at least weak
El Niño conditions. 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, 100% are calling for SSTs to maintain at least
a minimum El Niño level between November 2002 and January 2003.
(Note
1) Overall, the ensemble of the available model forecasts implies a
high likelihood for El Niño conditions to continue through the remainder
of 2002. More models are calling for a moderate El Niño (anomalies
exceeding 1 degree C) than a weak one. If El Niño conditions are
present during the November to January period, a continuation of such conditions
would be expected through at least March of 2003, as El Niño events
tend to reach their peak magnitude near the end of the year but often persist
into several months of the following year. Out of the 14 ENSO forecast
models examined, 12 of them forecast at least a 0.6 degree C positive anomaly
in the NINO3.4 region for the beginning of 2003 (Jan-Feb-Mar), while the
remaining 2 predict neutral SST anomalies. If we omit the models that do
not explicitly use subsurface sea temperature data, the counts become 10
out of 10 warm, versus 0 out of 10 neutral. Because of the persistence
of the warm SST observed in the NINO3.4 region since late May, our assessment
of the probability of an El Niño continuing into early 2003 is the
same as that reflected collectively in the models that use subsurface sea
temperature data—i.e., near 100%. The onset of complete participation of
the atmosphere is indicative of El Niño maturation. It appears very
likely that the El Niño SST conditions seen over the last 3 to 4
months will remain in place for the next 5 to 7 months. Confidence in the
forecasts is high, as the statistics of historical ENSO data indicate that
the observations during late northern summer usually set the stage for
what is to be expected for the remainder of the year and into the beginning
of the following year.
ENSO forecast models have a relatively modest level of skill for forecasts
made during the February to April period. Skills are considerably higher
for forecasts made in September. Caution is still advised, however, in
interpreting the distribution of model forecasts as the actual probabilities
for the ENSO conditions expected for the later parts of this year. 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.
See also:
Requirements for Basin-wide El Niño Formation
Now Fulfilled
The development of warm ENSO conditions has taken place over
the last four months, as the equatorial SSTs from west of the dateline
to the west portion of the NINO3 region have exceeded levels that rank
in the top 25% of the historical climatological distribution. When such
SST levels are reached during northern spring or early summer, El Niño
conditions become attainable, as the Pacific ocean-atmosphere system is
most conducive to ENSO phase changes in that season. The two major atmospheric
behaviors required in parallel with El Niño-associated SST warming
have now both taken place: (1) Easterly trade winds in the central tropical
Pacific have weakened with repeated westerly wind anomaly events (also
reflected in sea level pressure as negative SOI values); (2) Deep convection
that is normally limited to the western tropical Pacific has migrated eastward
toward the central Pacific. The substantial area and magnitude of the warm
water, and the associated atmospheric changes, occur when the ocean/atmosphere
dynamical feedbacks are mutually enhancing as they became in late July.
Weakened easterlies allows more warm water in the western Pacific to move
eastward, and enhanced convection and heating over the anomalously warm
water east of the dateline serves to reduce trade winds further, allowing
still more warm water to move eastward. This positive feedback cycle may
continue until negative Delayed Oscillator feedbacks begin to take effect
more than 6 months later. This El Niño is not yet fully developed,
as the spatial extent of the warmed water does not encompass the far eastern
portion of the basin, east of 95W. The positive feedbacks described above
will likely allow development of warm SST along the South American coast
during northern autumn 2002. The reduced variation among the different
models’ forecasts reflects that ocean-atmosphere coupling conducive to
El Niño is occurring. The models also agree that the strength of
this episode will likely be much weaker than the episodes of 1982-83 and
1997-98, and perhaps more like those of 1965-66 or 1986-87.
The Probability of an El Niño in 2002
IRI's assessment is that there is a 99% probability of an El Niño
this year, very likely lasting into early 2003. This assessment is based
on the collective forecasts of many computer models of various types, as
well as on the experience of the several oceanographers and atmospheric
scientists familiar with ENSO. This is a very small increase relative to
the IRI's August statement in which the probability was estimated at 97%.
There is still some uncertainty regarding the strength of this El Niño
as it fully matures over the coming few months. It now appears more likely
to be moderate than to be weak. The probability of an El Niño of
high strength (e.g. 1997-98) is low. The magnitude of the NINO3.4 anomaly
at maturity is likely to be between 1 and 2 degrees C, placing it into
the moderate range. It would likely persist through March or April 2003.
Note 1 - Only models that produce
a new ENSO forecast every month are included in the above statement.
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