Current Information
Technical ENSO Update
16 October 2002
Current Conditions
El Niño conditions continued during late September and early October.
During this time, equatorial SSTs from 180E to 115W longitude remained
at or above 1 degree C above normal, and anomalies of 1.5 degrees C or
more stretched from 175W to 145W. In early October, SST anomalies reached
near 2.5 degrees C locally, near 160W. This warming, which includes much
of the NINO4 and NINO3.4 regions and the western part of the NINO3 region,
is now sustaining itself through the large-scale westerly wind anomalies
created by its own existence, through its resulting anomalous convection
near and east of the dateline. The heating caused by this convection enables
a coupling between the SST anomaly and the atmospheric circulation patterns.
Episodes of westerly wind anomalies in association with Madden-Julian Oscillation
(MJO) activity are no longer necessary to maintain the SST anomalies, and
this activity has in fact become weak over the last two months. 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,
indicates that we are in a basin-wide El Niño episode that is approaching
maturity. Due to the eastward-shifted convection and the associated zonal
circulation changes, above normal SST that had dominated the far western
tropical Pacific through July has been replaced by below normal SST over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 October 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, even with normally strong trades
in the eastern portion of the basin. In the far eastern tropical Pacific,
from 80W to 95W, slightly below normal SSTs have returned to near average
during the last month, and SSTs along the immediate coasts of Ecuador and
northern Peru have risen to somewhat above normal. The far eastern tropical
Pacific is expected to warm further as the episode continues to mature,
as has been shown to occur in nearly all previous El Niño episodes.
There is nearly no 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, 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 and into the
early part of 2003.
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 Dec-Jan-Feb period (for which model forecasts are highlighted
here) being more than -0.7 and less than 0.7 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 no change
in the 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 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 December 2002 and February 2003. (Note
1) Overall, the ensemble of the available model forecasts implies a
high likelihood for El Niño conditions to continue into early 2003.
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 December to February period, a continuation of such conditions would
be expected through at least March or April of 2003, as El Niño
events tend to reach their peak magnitude near the end of the year and
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 early 2003 (Feb-Mar-Apr),
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 a maturing of the El Niño. It appears
nearly certain 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 October. Caution is still advised, however, in interpreting
the distribution of model forecasts as the actual probabilities for the
ENSO conditions expected 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.
See also:
Requirements for Basin-wide El Niño Formation
Now Fulfilled
The development of warm ENSO conditions has taken place over
the last five months, as the equatorial SSTs from west of the dateline
to most 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 feedbacks associated with the Delayed Oscillator
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. Such warming in the
far eastern basin is already being observed. 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.
Other possible analog episodes would be 1994-95 and, on the strong side,
1991-92.
The Probability of an El Niño in 2002
IRI's assessment is that there is a greater than 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 virtually no change relative to
the IRI's September statement in which the probability was estimated at
99%. There is still some uncertainty regarding the strength of this El
Niño as it fully matures over the coming few months. It now appears
most likely to be moderate. The probability for a strong El Niño
(e.g. 1997-98 or 1982-83) is low, and of a low strength El Niño
(e.g. 1977-78 or 1979-80) is also 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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